What Can Old Test Gear Teach Us About Electronics? Find Out.

hi there and welcome to another episode of mr. Carlson's lab today we're going to open up some older test gear and see what we can learn through their design so basically what we're going to do is open this thing up and read it like a book so we can get an idea of what the engineers were thinking when they designed this thing and sometimes we get to encounter people's modifications throughout time sometimes it's for the better and much of the times it's for the worse so anyways you never know what you're gonna find in these older pieces of gear so let's get started this is the older test gear that we're going to be taking a look at today and yes it does look pretty ugly you're probably asking yourself why did you buy these things well the price was right five dollars a piece at a local ham radio swap meet and if you really look at it the parts and pieces inside these oscilloscopes definitely exceeds five dollars a piece now when I bought both of these I'm thinking you know I could probably make one really nice one out of the two and convert it into a really useful piece of test gear it's a lot of these older oscilloscopes you know it can still prove their worth by being converted into something useful nowadays and that's what's gonna happen to one of these scopes when it gets fixed up I haven't determined which one I'm going to do yet pretty sure it's gonna be this one here because this one looks like it's been sitting at the bottom of a lake look at this there's mud on the knobs so it's gonna be very interesting to see inside this oscilloscope and see what lurks behind the behind the case here inside the case look at the graticule you know looks like somebody tried to tape it into place and as you can see they're pretty trashed looking this graticule and this one as well luckily what I'm going to convert this into doesn't require the use of a graticule so I can completely remove that the rubber is still good around the CRT so that can be cleaned up I imagine this one here this tape is probably made its way I might be able to be removed a lot of the time when you put certain tapes onto rubber it almost becomes part of the rubber this stuff might actually come off at any rate this one here is in nice shape and all the knobs are complete so everything's played on this scope and complete on this scope so basically I really have to complete devices it's basically just choosing which one would be the easier restoration and maybe the one that isn't so trashed inside who knows what's inside this you know it could be anything inside here at this point see this one here is handles just a boat coming off whereas this one is in quite a bit better condition looking at the case the case on this is a little bit rusty and this one here looks quite a bit cleaner but this has got you know a really bad dent here and some other issues with it so it might be easier just to clean the case repaint the case on this one and put that on maybe this one here or we'll have to see as we go along we'll take a look inside both of these scopes and see which one is the better restoration candidate again each one of these is going to have a very interesting story to tell and we can also get an idea of what the engineers we're thinking of when they put these things together originally and maybe even come across some modifications as well so what I'm going to do is move the camera around and this one here is the really ugly one so this is the interesting one let's take a look in the side this one here first and then we'll take a look inside this one here and determine what's the best course of action in order to get inside this oscilloscope it looks like I have to remove this screw this one here is already missing and on this side looks like there's only one screw here as well and on the backside looks like the cord would probably come up to this point here I imagine during disassembly the grommet you'd be able to push this back into the case and then the cord itself would probably go right through so obviously if the grommets in here that you know the plug won't fit through the grommet so you'd have to push the grommet out rubber is pretty solid on here I'll see if I can press that through and get this thing out of the case so that's what we'll do right now so just remove those two screws and then I imagine this should slide forward I've removed the screws on the side here and it looks like that's all that's holding this together this thing is missing screws all over the place including some on the bottom as well so what I'm gonna do is just put my hand on the face here and then tip it on its face and hopefully this is going to try to come out of the cabinet when I lift this yeah that still feels like it's pretty solid so the screws on the bottom here have been removed it's it's screw here and looks like there is another one just in behind there I know if you can see that right in there looks like it's right there's a little threaded hole in there so and this here is kind of missing something over here so it looks like it's still stuck in there so let's see the top of it wants to come out it's just the bottom this is loose on this side but it seems to be pretty solid down here so I'll just move this over here again this is looks like that might be threaded it is they're hiding a screw inside that little foot there so it'll come off now let's see that big wax capacitor nothing but trouble written all over at that one nice little circuit board on the top a bunch of capacitors here standing up so what I'm going to do is get rid of the case here and get the camera situated a little bit better so we can take a nice close look at this all right so what do we have here looks like we've definitely got a lot of dust he's rub my finger on here already lots of dust inside and cobwebs and things like that the circuit board itself looks to be in very nice condition on the top side and it really doesn't look like there's a whole lot of tampering so this thing might have just been thrown in a puddle for a whole bunch of a bunch of time who knows so see what the tubes are here see some numbers there 1287 it says and on this one here it's also a 1287 you see that there the capacitors are made by American Radeon ik company incorporated interesting capacitors well the band ends are towards the circuit board here interesting to know if they followed the rules back then these are a wax no it feels like that's an epoxy or some form of glue in the top there so they're not filled with wax it'll also be very interesting to see if these are leaky or not we have some know another one here American radio on it here Sarah cap down here let's take a look at the other side let's take a look at the tubes here what do we have four tubes in there six vr8 a this one here you see the numbers there and the one in the back is a 6b k7b don't you see that numbers on the tube there in the camera 6b k7b this one right here more American Radeon accompanied capacitors here looks to be in very nice condition inside anyways aside from the outside looking like the things been thrown into a puddle or something filter capacitors most likely are definitely toast this is the vertical vertical range right here a bunch of adjustment capacitors and some accurate resistors here another sarah cap here let's take a look at the other side and another two tubes here I can tell what these are just by looking at them you know you've been around tubes too long when you can look through the glass see the internal structure and know what they are so this is gonna be a 64 and this is gonna be a 6 X 4 now in a lot of oscilloscopes they use 6 c4 tubes as a high voltage rectifier it's a triode and they just use them as rectifiers this is going to be the 6 X 4 here which will be the low voltage rectifier now in today's speak it's definitely not low voltage when they say low voltage they mean 450 volts are around there right up to 450 the high voltage rectifier here would be up to a thousand somewhere I would imagine just due to the rating on this capacitor here we can tell that this is the high voltage capacitor just because it's 1,000 working volts DC and it's a mighty big capacitor 0.5 micro farad so half of a micro farad so easy chassis here we have the band end here tied to the chassis it's installed correctly even though this is a negative supply and see up here wire goes here right to the CRT and then if we follow the purple wire it goes right to the high voltage rectifier so I'm tempted just to plug this into my current limited variac supply and just bring it up slowly and see if I can get a trace on this thing so it's current limited what can go wrong we can release some smoke here our transformers I don't know kind of looks interesting like somebody's maybe taking the cap off or something on here what this is some sort of a sealant or paper or something like that this is the horizontal sweep selector as the capacitors here more American radionics company incorporated capacitors interesting to see if these things leak or not whether they are paper some interesting combination of elements they've changed capacitor technology so much in these days so Sprague was changing to all sorts of different kind of capacitor compounds dye film capacitors they called them and they still actually show very low leakage today so maybe there's some form of a dye foam I really don't know at this point so what I'm going to do is check these out with you guys so what I'll do is I'll do solder one of these things first and we'll check it for leakage and see these if these are leakers or not let's remove one of the capacitors from the board so one of the capacitors is right across these two areas right here so we'll remove the solder off of this and off of this little pin right here as you can see the circuit board is in very nice condition as well they didn't remove all the flux residue but other than that looks really nice nice and clean again not a whole lot of tampering it looks like it's pretty clean here so in order to get my desoldering tool in here I remove the two tubes as you can see down there I'm just gonna push this wire out of the way here it'll burn the wire and I'll get my desoldering tool in here now these older circuit boards are a little finicky so you don't want to really spend a whole lot of time with your desoldering tool on here solder wick will pretty much destroy these things just do it at the dwell time and there it is came out like that so point one at two hundred working volts DC so I'll get my capacitor tester let's see if this thing leaks or not all right let's see if this capacitor is leaky so I have to have the brightness down here so that the I tube shows up on the camera so I just have this little area lit up down here so now before I do anything I want to make sure that this is on three volts and I want to make sure that this is on discharge because this can stay in the leakage position if I have this up at 600 volts and I leave that in a leakage position and I touch these leads I'm in for a nasty surprise so in leakage starts at 3 volts needs to be paper and maika etc so what I'm going to do is attach the leads here to the capacitor see that I'll just put this off to the side here where it's out of the way and click on leakage here now if this eye closes that means that this capacitor is leaky and it should be good up to about 200 volts yeah starting to show leakage slowly creeping open they're put up to 200 and see what it does it's taking a long time to charge so chances are this capacitor is leaky you can see it's trying to open up just about tried there again so we definitely know there it is so chances are they're leaky they're good rate tip while this is good rate to its maximum voltage which is 200 volts and as you can see the eye isn't completely opening and it does show signs of you know like it's kind of flickering up and down now my soldiering iron is going off and on on the bench so the voltage is probably going up and down a little bit and that's why you're seeing that happen just shut my other iron off here and see if that settles off a little bit turn the iron back on see what happens see the honor definitely affects it so very sensitive so what I'll do is I'll get a good point one and I'll show you the difference be right back all right so here is a good point one capacitor this is a modern one right here made by Illinois capacitor good brand so it'll do is I'll just attach the leads up here put this off to the side and again as you can see it's on discharge and down at three volts so this capacitor is rated at six hundred and thirty volts all right so this only goes to 600 so what I'll do is I'll turn that right up to 600 volts I'll click this to leakage and watch how quickly that eye opens with a capacitor that doesn't leak that's a good capacitor so discharge again see how quickly that charges no problems there so now when this is in the discharge position it also discharges the capacitor it's always a good idea to never take the chance so before you go vote removing a capacitor from a jig like this just in case that the unit itself this thing here is not discharging the capacitor it's always a good idea just to short it so I've got a jumper here and everything is fine so I'll get that one out of the way and I'll show you the other one again to show you the difference so you saw how quickly that I opened so here we go go to here again and to here I'll put this up to 200 volts it's maximum will be nice - it will go 150 all right now watch the leakage see how long that took to open so indefinitely tell it's leaking so all of those capacitors in there are most likely going to be very leaky including that really ugly wax capacitor that point 5 micro farad 1000 volt one that one's gonna be a real big problem but I'll still put it on my current limited supply and bring it up slowly here and see if there's any life in the oscilloscope I have the oscilloscope attached to my current limited variac in isolation transformer and I'm gonna start this off at about 60 volts so I'll just move my very act here to the 60 volt mark about there and I'm going to turn on them ac supply and if I'm lucky this oscilloscope will come to life it looks like the dial light or the indicator light is coming on little red lamp down here on the bottom is lighting up slightly you can see the reflection of the bulb and behind here and it also looks like the tubes are lighting up so I'll just zoom on into that so that's a good sign the filament winding in the transformer is working so far the current limiting is absolutely fine no problems so nothing is going wrong so let's see if we can get anything on the screen here it's at about 60 volts and nothing is going wrong so what I'm gonna do is I'm just gonna take this up to about 85 help things out just a little bit see the tubes get a little bit brighter in the background there okay so first thing we want to do is turn the brightness right to the maximum here and that should help us so it's always a good idea to try and Center everything Center the vertical position and horizontal position and everything on the front of the scope here so I'll move the vertical position around this is rated it's max look at that there it is I'm actually pretty amazed that this thing is working it's out of focus so this is the focus up here that's working as well well the scope that looks like it's been sitting in a puddle is working see if there's any vertical sensitivity there is well that's kind of a horseshoe moment well the oscilloscope is working makes me wonder the really ugly looking scope is working maybe it's the nice one that doesn't work I better shut this thing off well well everything's working so that's a good indication that the CRT is good it was at 85 volts or it's not even up to the standard 120 all the tubes are lighting up lots of brightness and focuses so really this oscilloscope could very easily be resurrected and turned into a working device or turned into a nice piece of test gear all right well you know that this one's working let's grab the other oscilloscope and see if it's going to work all right so here's the nicer looking oscilloscope cords and everything all wrapped around and it's been floppy jacked on the bottom there so this looks like the same deal is the other one it's missing screws all over the place so let's take this screw out here nicely driven in on an angle see this one here is out of the way and in order to get this out of the case line all the cords and everything on here and see what's there's any screws on the bottom looks like this one here somebody had put the foot on the bottom of the other scope in this hole right here so that's what's stopping the face from coming up there's empty right here and there's some tape and some lint or fuzz stuck on there so I think this one should just fall out get rid of this cable on here just lean this on its face up with my hand here like so so that it doesn't fall out as I'm doing this in order to completely remove the back on this I'll have to take this rubber grommet off I don't know if you can see this here this probably can't I'll have to push this rubber grommet through the case as well so squish it together and push it in okay is it gonna come out let's see looks like the bottom is a little different on this as well no it's stuck in there still as well maybe some of these are holding it did I miss a screw nope just doesn't want to come out maybe on this one these act as mounts as well it doesn't feel like it nice one here I don't think so I think it's just being fussy what else would be holding this thing in oh no I think it is coming out it's just it's just being fussy all right oh this one here already has caps that are a Polly style capacitor so all of these will not need to be replaced they don't work just fine this one here will have to go yeah all nice Polly style capacitors here so maybe a more modern device what I'll do is I'll reposition the camera again and we'll take a closer look at this one here okay so let's see what they've done in this if they have done anything or whether this is stock yet to be determined so again this looks like this has Polly style capacitors inside it'll be interesting to see if they've put them in the correct way as well you see all the lines here so it'll have to be compared to the other silk opal take a look at that here in just a little bit the tube here is a 6 CL 8 see that 6 CL 8 and on the board its marked 6 BR 8 or 6 FV 8 so somebody's been in here and changed those around possibly they are RCA tubes so I don't know maybe they just substituted those I'll have to take a look into that here let's see on the bottom side now this has that standard kind of a flexi looking residue left over everywhere and it really doesn't look like it's been tampered with it's looking it's looking pretty clean under here so I think that those capacitors were most likely put in when this was assembled so most likely they just upgraded the capacitors because they found that those other capacitors were leaking which would make a lot of sense so RCA tubes in here still as we can see here on the sweep selector you know it's got ya poly style capacitors there they're everywhere in here so this thing was just built with those capacitors and the electrolytic serum course would have to be changed out there all 450 so 450 volts so what is this 10 Mike's 20 20 and 40 all rated at 450 here and the other one I can't see the ratings on it but in a little bit here we'll take a look at the schematic and that'll tell us so here we go on the back here again you have this large capacitor put in in with the line side towards the chassis which is correct and yeah looks like a little bit different potentiometer here than in the other one so from what I can see this is off ously put together I guess you could say this would be a newer oscilloscope let's put it that way so you probably made some revisions over time you can really tell you mean it looks like this is just put in here factory all the tubes are all RCA so whether this one tube over here this one right here that's just something that you know would match up and worked fine and they just put that in or whether somebody swapped that in over time really don't know very clean build just like the other one one of the this one works or not yet to be determined we'll try that out here in a little bit nice little shield on the neck here you'll notice in both of these things this is a very small and compact Scylla scope so you can tell they were really thinking about the CRT here we have a shielded transformer just below the CRT now if you didn't have a shielded transformer here say had something with an open core and you didn't have this nice little pipe over the neck of the tube here what would happen is you would get probably some noise onto the trace either that or it would shift it so they're trying to you know keep this thing as clean as possible keep that trace as clean as possible on the screen that's the reason they put this here it's the only reason that they've put this here is to basically stop any stray magnetism from affecting the beam in the CRT a lot of the times they use a thing called MU metal you know the MU metal is very handy and some of the larger Scylla scopes to keep the CRTs nice and clean stop any strain magnetism from affecting it so they've gone to you know all the steps to ensure that you know they had a nice clean CRT or a nice clean trace shielded transformer here so there's a lot of things that I notice in these scopes you know a lot of interesting thinking in here and we'll take a look at all that here in just a little bit but first of all let's see if this thing works let's try scope number two the nicer-looking one so again it's hooked up to an isolation transformer and react that is current limited and all turn it on right now at about 60 volts or so and we can see the indicator lamp has come on yeah and let's see if we have any glowing happening in the tubes here zoom on into this tube right here they look pretty dull is there a little bit of glowing happening in there let's get rid of the light here for a second wait sure looking dim looks like there might be a little bit of light happening in there here you can see anything happening with those either see the doll light on though now usually the dial light is part of the filament supply so I'll give this just a little bit more see what happens there yeah I can see the filaments lighting up just a little bit yeah they'll raise the AC supply just a little bit more here and now we can really see them coming on so that's about it 110 they're lighting up back there I don't think we'll see the bottom ones here because they get her compound it's on top no you can see the little hole in the plates there where the two plates are separated right in the middle of the 6 X 4 grain about here well better the cathode glowing in there see if we have anything on the CRT I'll just give this a little bit of light so it can find focus turn up the intensity to the max nothing so far huh look at that this one's coming to life as well focus this I'm to working oscilloscopes zoom mode on that a little bit sounds come to life just like the other one see if this has any vertical going on it is vertical sections working and since there's a line there we know that there's horizontal deflection I should say the sweep section is working well I'll shut this one off as well just keeping the electrolytic sin mind both oscilloscopes are working so basically in order to make one of these an oscilloscope that would be safe to use this particular scope here since it has all those poly caps basically just need electrolytic s-- so let's take a look at both of the scopes and do some comparisons and take a look at their thought process when they were actually putting these things together or designing them alright so looking at both oscilloscopes here this being the first one we looked at we can tell by the capacitors here this being the revised version here the first thing I notice is they've put the capacitors in all the same way in both units so there was thought here you can see that the band end is here the band end is here denoted by that little piece of silver so this is the outside foil and you'll notice that on these ones here the band end is on the bottom on all three of these capacitors there's a little bit hard to see because of where they are but the band and is all on the bottom so the band is towards the back and then you can see this wire comes towards the front you can see the band is towards the back and then the other side of the capacitor is towards the front it's the same with these as well you see the wire comes forward here and the wire comes forward so they're all put in the correct way the band end on this one here is at the bottom which is at the backside and then we have this one coming forward you can see two band and here is at the back side all coming forward so there was a lot of thought they did think about orientating these capacitors correctly in both units the reason that they've gone with these obviously in this unit here is because they found that these ones here developed leakage over time and probably caused some headaches so they just got rid of them now they filled these ones here with an epoxy but that still doesn't matter because what's inside the capacitor is still decaying and that's what's causing that leakage now again when I say leakage I'm talking leaking electrically not physically these capacitors are not supposed to pass any direct current when they start to leak the capacitor actually turning into a resistor at the same time and that's why in that test that we did earlier when we used the capacitor tester with the I tube in it you'll notice that these capacitors here the one that we tested here takes longer for the eye to open so effectively it's taking longer for this capacitor to charge and the reason that it's taking longer for this capacitor to charge is because it's developing resistance inside it's it's turning into a resistor whereas you see the brand-new capacitor when we use that Illinois capacitor that yellow one you basically you know you click the switch on the tester and the eye opens almost immediately that's because it's not really turning into a resistor like this one here so the resistance of that particular capacitor would be extremely high the newer one so not developing leakage at this point so that's what I can see from the top you can see they use different resistors in here so the resistors in this unit are different whereas in this one here they're using more of a modern carbon film type resistor and it's a mixture with carbon composition - you can see the carbon composition resistors here and over here so you can definitely tell that this here is a newer unit so I'll just resut these and we'll take a look at the bottom side looking at the bottom side of both of these oscilloscopes here the first thing I notice again is the capacitors you can see the line side is at the front here and they've got the line side at the front here so the outside foil end is facing towards the front so if you're rebuilding one of these oscilloscopes and you're using this as a guide there you have it you'll also notice that in these oscilloscopes here you can see that there's an RCA symbol here an RCA symbol here whereas on this one here you don't see the RCA symbols but you see the the same area of writing so it tells me that the tubes down here most likely original in both of these just due to the fact that the writing is in the same spots on both of these so the RCA is hiding around the other side on these two particular tubes here you'll notice that they have a V R here that's an open-style so the carbon actual the carbon track here and the wiper are open and they've mounted them differently here whereas on this one here it has a sealed unit but it has the actual holes you see the hole here to mount this particular type of V R and here so you can see the upgrades as they're going along a little bit quicker to mount this instead of having to put you know screws and nuts in here to hold it down basically one Center nut you know right over the shaft there and this thing is mounted in place so you can definitely tell that this is the the newer model here now what I'll do is I'll flip these onto the bottom side looking at the bottom of the oscilloscopes we can see that the resistors here are different than these again they're using these carbon film style resistors here they've obviously upgraded something here because this one here is 8.2 K ohms write them back here and look at the see DAP 8.2 K whereas this one here is 6.8 K you also notice a difference in the size of the filter cans so you'll notice in the earlier version they have a smaller filter can in this version here the newer version you can see that the filter can is quite a bit taller so they may have done this either because they switch manufacturers or there was an addendum done at some particular time maybe these capacitors held out under the heat this is a pretty small unit and in a case like that chances are it's gonna get pretty warm the capacitors are both rated at 85 degrees it's just that these ones here are a little bit taller maybe just a different run you can see here that this here has this sarah cap or radeon it cap down here at this end of the shot see this here you can see the band end is here whereas on this one same thing and end is here this is this newer poly style cap so there was really great care and making sure that these things were put together properly the circuit boards on both of these look to be in very good condition this one here is a little bit darker than this one here maybe just a different run of board I noticed that the switches here are all the same you can see that they have shielding on both of them to stop the stages from interfering with one another in this unit here good plan so kind of a flimsy little metal shield there but but you know the thing isn't going to be vibrating in use or anything like that so not a big deal adjustment capacitors all look the same and wiring is looking pretty much the same in both of them you'll notice that on the sides here they did take care to bloom everything they took little pieces of scrap wire and twisted them together so somewhat nowadays like we would use a zap strap or some people like to call them tie straps you know keep the wires all nice and Illume together they've done that here so they've taken care in doing that so when you look through this you know the time was spent to make this thing you know as good as it could be you see here everything is nicely loomed AC wires are twisted together that's always very important whenever you're running any wires through a chassis or anything you always want to twist the AC wires like this and you want to tuck them as close to the chassis as possible so this is especially important whenever you're building vacuum tube audio amplifiers so you see they've done that here so you can pretty much tell where they're putting the AC lines just by the twisting you can see on the bottom here we have more twisted lines here so these are obviously AC lines here a little trick whenever you see things twisted like that so on this one here looking at the top again you'll notice that they've also ran the wires here they've tucked the wires underneath the CRT on this one here whereas on this one they've ran them over top of the CRT chances are to maybe stop this from interfering on the bottom side here maybe was interfering with some amplifier circuits or something like that so there's some reason that they put this up on the top here as you can see a little bit of extra looming over here again AC wires twisted together this is running to the on/off switch this is carrying the line up to this point right here so all in all they did a really nice job on both of these scopes they're very similar it's just they've updated them a little bit so you can see this one here I should say this one is updated a little bit more as this one here you can tell is an earlier version let's take a look at the design of this RCA wo - 33a oscilloscope we'll start with a vertical section here and then I'll work my way this way and then we'll talk about the sweep section so if you're unfamiliar with the way oscilloscopes work this should probably give you a new appreciation for the reason why these things were so incredibly expensive back in the day and some cases still are to this day now this is an analog oscilloscope this isn't a digital oscilloscope but the vertical section in this oscilloscope and in a modern oscilloscope can be basically looked at as the same thing it's basically just an amplifier circuit so in order to simplify this if you're used to vacuum tube audio amplifiers you can look at this section as just an audio amplifier really is that simple now the reason that oscilloscopes get so incredibly pricey as their frequency range goes higher so say you have a 100 megahertz oscilloscope you know that that's obviously a lot more expensive than a 10 megahertz oscilloscope reason being is this amplifier section here or in any oscilloscope its amplifier section in the vertical front end needs to be able to amplify from DC to say 100 megahertz and it takes quite a bit of design to make an amplifier section do that to make it that incredibly broad banded and you know work within parameters inside of the oscilloscope now this is a much lower frequency oscilloscope this is pretty old so you know this doesn't go much beyond the audio region but still this here does deal with a little bit of RF and it's capable of amplifying that as well now this design throughout time in different types of oscilloscopes the way that they put these particular sections together in order to make them broad banded and of course you know the larger companies like Tektronix back in the day they spent lots of time in the vertical section in order to make their oscilloscopes work up to the higher frequencies so back to this oscilloscope here we'll start with a vertical input this is where we feed the signal into the oscilloscope so that we can view it on the CRT screen this is a multiplier peroba pair times 10 probe will just ignore this and we'll feed the signal directly into the vertical input here so say we have a sine wave running into the vertical input here it's going to hit this point 1 micro farad capacitor the reason that this capacitor here is to block any DC so if there's any direct current on this line it's going to affect the bias of the operating point of the pentode section inside of this 6b r8 a tube and we don't want that to happen so we put a blocking capacitor there to stop any DC from changing the operating point that's the reason that when these capacitors in these older oscilloscopes or say if you're working on an older radio television piece of test gear audio amplifier when these capacitors leak they let DC through so they're turning into a resistor so you can picture a resistor jumping across this that even a very very high value resistor is going to change the bias because you can see on the grid we only have a 1 megaohm resistor tied from the grid to ground so just say that this capacitor was leaky and say it's was measuring about 1 megohm worth of resistance we would have a nice voltage divider right here so whatever you had here if you had 5 volts DC here you'd have 2.5 volts DC here so you can see that any kind of leakage in one of these capacitors is going to cause big issues within a vacuum tube again this is the reason that everything needs to be recapped from these from this particular era now luckily they were using Paulie style caps in that other oscilloscope so that eliminates a lot of the changing which is kind of nice so we have an AC signal here so it's gonna go through the cap we have an AC signal here it's gonna run down into the grid which is the control grid inside of this six bre tube now if you want to locate the grids inside of a vacuum tube it's very simple to locate the cathode it's the C will see four cathode right so you can look at this little C that's like this that's the cathode the first grid is the control grid the second grid is the screen grid the third grid is the suppressor grid and the fourth element here is the plate so this is the anode of the vacuum tube so we have cathode grid one grid 2 grid 3 and plate that's how they're marked in a lot of tubes just know that the first grid is called concurrent of the control grid the second is this screen and the third is a suppressor now the reason that they're using a pen toad in the first stage of this amplifier here is to give this a lot of gain you can see that this has a screen grid here now you can picture the screen grid as somewhat of a booster grid now the actual reason the screen grid is in here what it does in a pen toe tube is it lowers the inter electrode capacitance by creating an electrostatic shield inside of this tube when it has a positive charge on it now getting into that is a little bit more in-depth in this video here I'll explain more about pentose trials and Tet Rhodes here in the future again on different schematics there's quite a bit of explanation to do here so by putting a positive voltage on the screen here really brings this thing to life and makes it want to amplify so you can look at this tube here is basically an amplifier and the signal that's going to be present on the plate is going to be inverted one other quick thing a very quick way to troubleshoot audio amplifiers or find the signal path is keep this in mind signal in the grid then on the plate so it's you put the signal on the grid it gets amplified it's on the plate so if we want to amplify it again what are we gonna do put it back on the grid of another stage it's gonna get amplified beyond the plate will unamplified it again put it on the grid it's gonna come on the plate you can see the signal path already now this is acting somewhat of a phase inversion circuit here and this runs on a little bit different you know a little different principle but still it's a very easy way to follow the signal path here and of course you know we'll have signal present on the cathode and cathode followers and things like that but again that goes beyond the scope of what we're doing here so most of the time if you're looking at a guitar audio amplifier or lots of pieces of tester and you want to follow the signal path mm-hmm just follow the signal from the grid out the plate and that'll bring you pretty much along the way and you can scope that too and put a scope here put a very small signal here scope it you'll see a small signal on your scope put it here and it's going to be really big because this is doing its amplifying so that big signal will be present here again and then put it here and it's going to be even bigger now this stage here is going to be furnishing some Drive here because of the low resistances here and that's going to be operating this six bk7 tube here by having this resistance combination here as well also makes this tube just a little bit more broad banded and again I'll get into that here in the future so now the signal goes in to the grid so you can picture if you're looking at a sine wave say you have an oscilloscope screen you have the line in the center or eight you have a sine wave so the line being ground look at the line is being ground that'll simplify this okay so you have a signal going positive coming back down and then going negative and then coming back to the line and going positive again and going negative so that's basically a sine wave so when you feed a very small signal here with a sine wave so say the signal is going positive when the signal goes positive this tube is trying to turn on so the grids control how the plate is basically going to be you can look at it like this how the plate is going to be trying to connect to the cathode right the more positive of a voltage that you put on the grid it's going to try and connect the cathode here and the plate harder so it's basically if you were to look at this as an internal resistance of the vacuum tube as you bring the control grid positive the resistance between the plate and the cathode is going to go low so as this is turning on if the plate is going to be pulled towards the I would say we have 120 volts DC here right because this is our B+ line alright we have a hundred and twenty volts DC here what's going to happen if the tube turns on while the cathode is connected to ground through a really low resistance so this hundred and twenty volts because of all of these soft resistances in line here is going to get pulled towards ground so this might drop from say 120 volts down to 40 volts so you can see how maybe a little signal maybe one to two volts peak-to-peak will say here will call out a small signal okay one to two volts peak-to-peak will drag this from a hundred and twenty volts down to forty okay so when this starts to go negative when the sine wave here starts to go negative this is going to turn off so basically what's going to happen is it's going to shut the tube off and it's gonna try and disconnect the cathode from the plate the voltage is going to start to go high again so then we get our upswing on the sine wave as this goes right down to its off point you know this is going to be rate you know as this basically goes off with a sine wave going negative this is going to go right back up to 120 volts again and you can see how we're gonna get this sine wave recreated at the plate now a lot of people think that the vacuum tube is some sort of magic device that's you know taking it and amplifying it within the vacuum tube well no it's not all it's doing is it's taking the high voltage supply that's created by the 6x4 rectifier here and the small signal that you're feeding into the vertical inputs say you had up an mp3 player or CD player or a phonograph or something plugged into the input that audio signal that's coming in here is just turning this tube on and off as the tube is turning on and off it's taking this voltage that's being dropped through these resistors here down to 120 it's taking this voltage and causing this voltage to swing up and down so technically this tube is recreating the signal that you're feeding in here using the high voltage from the power supply and that's all the vacuum tube does thing more that's how you get amplification you're taking a higher voltage and you're swinging that up and down alright with a very small voltage vacuum tubes are voltage controlled devices much like FETs are of course we're gonna have current here because we have resistors running down here and when we start to turn this tube on the two will draw current and the voltage will go low and that's what's happening inside this vacuum tube same thing with a triode just less grids now again the introduction of the screen grid was very important to make tubes work at higher frequencies because putting a positive charge on the screen grid lowers inter electrode capacitance effectively making a lot of the parts of the inside of the tube invisible to one another lowering capacitance means that the vacuum tube will operate at higher frequencies now we're still dealing with lead inductance from the pin of the vacuum tube to the working components or parts inside of the vacuum tube lead inductance plays another very important role in a limiting factor in the frequency response of a vacuum tube or how high in frequency that that tube can operate that's why if you look up an eight corn tube you'll see that the leads that run into the acorn tube run directly into the side of the envelope of the vacuum tube to keep the leads as short as possible to lower lead in inductance now the same kind of idea happens with the CRTs in some oscilloscopes and I'll explain that here in just a bit the actual CRT itself is a big limiting factor in how fast the oscilloscope can be itself so and that again has to do with the length of the leads so at any rate this is here the suppressor grid inside of a pentode tube the whole idea of the suppressor grid is to stop secondary emission all right so what's happening here is when you put a positive voltage on the control grid it's allowing electrons to fly from the cathode to the plate because the cap the cathode is tied to ground and the plate has a positive charge on it we haven't running from our positive supply here when electrons if say this tube is completely on when electrons are flying through these grids here and hitting the plate it very much mimics things in nature so picture a wall and you have a hose with a high-pressure nozzle on it and you're spraying a high-pressure stream of water against a wall well what's gonna happen it's gonna splatter and spray all over the place the same thing is happening with the electrons inside this vacuum tip they're gonna try and spray back into the tube again that's called secondary emission and the reason for the suppressor grid is to stop that so basically they get stopped by this grid that is tied to the cathode which is negative so that their electrons can't go flying back into the tube and creating noise within the vacuum tube that's the reason for the suppressor grid inside of a pen toad now there's tubes with lots more grids than this we have Penta grid tubes and we have octo dand all sorts of different kinds of tubes and I'll cover those here in the future in different videos when we deal with different items that have these tubes inside them so we have an amplified signal here now keep in mind that the signal is inverted at this point right because when the signal is going positive this is getting pulled towards negative so if it's good if this is going positive this is going towards ground so we have inversion so have a signal here we have another blocking capacitor to keep this DC from changing the operating point of the triode over here in the triode we have a cathode control grid and plate that's all we have this is again acting as an amplifier tube and as you can see we have with these two resistors here at this tube is going to be drawing a lot of current especially with a cathode resistor here so this is going to be furnishing quite a bit of Drive so they'll be quite a bit of Drive for this six bk7 here in the six bk7 is what's going to drive the two vertical plates within this three a qp1 CRT now this isn't a push-pull configuration so this is very much like a phase inversion circuit so very much like a phase inverter in a guitar amplifier if you look at this and you look at a long-tailed pair phase inverter you'll notice the similarities in this here of course is a little different because this has to work within an oscilloscope but you know you'll notice the similarities now you'll notice that there's blocking capacitors here point 0 5 micro farad that run to the electrostatic plates inside of the CRT this does not have a yoke this has electrostatic plates inside the CRT to cause beam deflection and I'll talk about that here in a bit you'll notice the DC which is around 250 volts is being isolated from the electrostatic plate yet we have 220 volts DC present here on each one now the reason they have these blocking capacitors here and they don't just use the tube to basically move the vertical trace up and down is because if the tube ages and say one triode pulls a little more than the other it's gonna move the trace around with the age of the vacuum tube one way to alleviate that problem is put blocking capacitors here and then use a separate supply to supply 220 volts to the deflection plates here and you can see as you bring one side closer to ground it's gonna move the beam up and down within the plate you can see here it says vertical position control that's because you're changing the DC potential on either plate so basically the potentials gonna do this and that's gonna cause the trace to move up or down inside of the tube and it's the same thing for horizontal horizontal deflection plates you'll see your horizontal position it works the same for the horizontal amplifier here this is very much the same as this up here so that's what's going on within here you can see they put quite a bit of time in here they even included an in at an astigmatism control here in order to you know create a nice sharp trace you know so they've really put the time at the time and the effort into designing the scope even though it is a low-frequency scope now these are known again as electrostatic deflection plates you'll notice in televisions they had a yoke on the the CRT and the picture tube looks looks like a whole bunch of windings of wire and it usually fit on to the the CRT like this and then the neck of the CRT would stick out there's wines you can see wires on them right at the the CRT where it goes into the neck you'll see that aperture that you know device I should say called a yoke and what that's going to do is control the how the beam is moving across the screen here okay if this was magnetically deflected and that's what a CRT is in a television older TVs have magnetically deflected CRTs this is electrostatically deflected so by putting a voltage on these plates here causes the beam to move around now since this is acting like a phase inversion circuit basically what's going to happen is you're going to have this going like this back and forth between the two plates it's the charge is going to be changing like this and of course if you want to draw a sine wave if you have a vertical plate on the top and a vertical plate on the bottom having the potential change between the upper and lower plates obviously is you're going to give us this right so on a vertical deflection plates it's you know that'll be able to draw on a sine wave but I'll get into that here in just a little bit a little bit more so we have horizontal deflection plates which are going to be doing the same thing now except instead of going vertically they're going to be going horizontally all right so now if we look here at this 1287 this here is our sweep oscillator here so this is going to be creating somewhat of a ramp looking wave like this now the cleaner that ramp you know the better it looks on the CRT again this is the magic in oscilloscopes here at analog oscilloscope that has to work all the way up to a 100 megahertz has to have a very nice designed sweep section in order to create to create that perfect ramp so if you start to get a ramp that starts to go like this and then come down more of a sawtooth the sawtooth effect at the top is capacitance and when you start to deal with capacitance as this is slowing down and lingering you know it's going to affect the speed of the trace as it gets to that lingering part up closer to the top the trace will be fast and then it'll slow down as it gets to the end and then be fast and then slow down because it's more time in this area here as it's getting to the end right so you want that as steep as possible and the to come back down and then as steep as possible now what's happening here is as this sawtooth is being created that sawtooth will be present at this point right here if this switch is in the right selector all right you can see that it's this the horizontal amplifier tube the input is going to the external horizontal input here so if we were to move the selector switch down this is going to move down like so so what's going to end up happening is this would be connected to here so we would get our sawtooth or a ramp whatever you want to call that running into here right so we get the same action going on that we do up here between the two plates in the horizontal section so as that ramp as that that sawtooth is doing this this is voltage climbing on the screen so if you're to look at this on another oscilloscope you're going to see this action and then it drop and then this action again as it's doing this the voltage is climbing to a peak it's getting to a peak it's dropping back down to the point here again they say this is at the baseline it's dropping back down to the baseline it starts to ramp again drops back down to the baseline so as the voltage is climbing it's driving that dot across the screen horizontally and then as it gets to the top it's at one extreme end of the vacuum tube and then when it drops it goes back to the other side again and then it starts over again and we're getting this effect going across the tube so from this side to this side and then paying back to this side really fast to this side again and you know how we see a line on the oscilloscope screen well that's actually a dot moving so incredibly fast that it just looks like a line is being drawn across a CRT and of course depending on the type of phosphor that you have in the CRT in this case it's a p1 type phosphor all right so it's a very fast phosphor inside this I'll get into you know determining you know the the numbering in the coding system of vacuum tubes and CRTs here in the future as well so again p1 phosphor very very fast all right so basically it's that as fast as that dot can chase across the screen it's disappearing if you had something like a p7 phosphor or some like that as the dots chasing across the screen you're gonna see a streamer behind it because the the phosphor holds light a little longer just a different type of phosphor again I'll get into this here in the future we'll stick to keeping this simple today as simple as they can at any ring so as that dots getting driven across the stream the screen right really really fast it looks like a line so you can see how this thing is going to draw a graph on the screen and the earliest oscilloscopes were called asila graphs so what's happening here is you can see we have a whole bunch of different capacitors in the horizontal sweep selector switch so 0.2 2 micro farad capacitor rate down to 15 Pico farad now 15 Pico farad is obviously going to be the fastest that this is going to move so it's going to be creating an extremely fast sawtooth at that speed and that will be of course the highest frequency setting of this oscilloscope which isn't very high at all but it's still the highest frequency setting so now of course as you're feeding something say you're feeding for example a 60 cycle sine wave in here not a very fast signal you have to adjust the sweep speed on your oscilloscope so that when you're looking at the screen of the oscilloscope it displays that correctly so basically you can look at it as a dot moving across the screen alright so this is creating that dot going like this the vertical deflection plates are going to take that dot because this is working now and if it's 60 cycles it's gonna pull that dot towards the one deflection plate and then it's gonna go like this as it's moving along it's going to go towards the deflection plate push-pull this is a push-pull section here so it's going to go towards one deflection plate as it's moving and towards the other so you can see the faster you make this move it's obviously going to spread that out so the slower that you make this move the more displayed cycles you're going to get in there and that's basically what's happening that's very simply you know you just have a dot moving across the screen and this is deflecting it verdict all right that's how these things work very very simple and again you can see why an oscilloscope back way back in the day and even now are still worth so much money as the speed gets higher now nowadays since they're you know digital oscilloscopes you basically don't have this section doing anything in here anymore you still have a vertical amplifier section the the input to your oscilloscope is still a vertical amplifier it's just that it's working differently it doesn't directly drive any plates inside of the CRT now back to the CRT here again the CRT again is another limiting factor in how fast the oscilloscope can work because you're dealing with lead inductance again running into the actual CRT and you're also dealing with capacitance as well inside of that CRT you'll notice that say you own an older Tektronix oscilloscope if you don't I strongly suggest you get one because they're great pieces of gear to have around look at the CRT and look at the way the deflection plates are placed within that CRT you'll notice that the neck of the CRT has the connections midway up the connections are almost right at the deflection plates they're like Midway up they don't run all the way out to the pins in the bottom of the CRT like in this particular vacuum tube here they actually exit the pins right by the plates well what are they doing they're lowering lead and inductance by doing that allowing the CRT to work at much much faster frequencies again lead-in inductance and capacitance easier enemy when you're trying to make something work fast that's the hill that you're trying to climb alright so that is that that is bit the biggest portion of resistance to you whenever you're designing anything at high frequencies is that added inductance and capacitance that would be within that circuit so moving on to the power supply here we have a 64 which you can see they have the the filament section of the 64 attached directly to the cathode and they're using this as a negative rectifier so the cathode is attached straight to one end of the winding we have a negative to cheer negative 680 volts is put to the cathode which means that to this power supply the positive end of the power supply is the chassis so if you were to reverse your voltmeter leads put your positive probe on the chassis and put your negative probe at this point at the plate of this tube the chassis on your oscilloscope would read positive 680 volts why aren't you getting shocked well that's an incredible explanation within itself first of all we have isolation here in the transformer there's no connection between the primary and the secondary side and if you want to picture positive and negative supplies here I'll show you a very easy way to look at this just using two batteries the 6x4 here is a full wave rectifier this is what creates the high voltage for the rest of the the unit now this is classified as the low voltage rectifier this is classified as the high voltage rectifier when they say a low voltage they mean three hundred and twenty volts and back in vacuum tube days three hundred twenty volts is low-voltage so but nowadays three hundred twenty volts to many people scares the scares the pants off them so then of course if you did come across it it would also shock the pants off you so you got to be very careful in equipment like this there's high voltage present everywhere again you know negative 680 volts right so you'll notice in this winding here we have a full wave arrangement so one winding goes to one plate of the 6x for another portion of the winding to this and then the center tap runs to ground from the center tap to this point here is what creates our negative voltage here now in order to explain the way negative voltage works I've got two 9-volt batteries okay so this will greatly simplify things this is the positive side of the battery here and this is negative all right so we have positive negative positive negative well if we clip the battery together like that we have positive and the negative but that positive of this battery is attached here and negative is here so if you take your voltmeter probe and you do this experiment take the negative of your volt meter probe put your voltmeter on DC and then put it at this point touch the positive probe here you'll get positive nine volts touch the positive probe here and you'll get negative 9 volts same things going on here same kind of idea just to simplify things this is our troublemaking capacitor here 0.5 micro farad what's gonna happen is this thing is gonna leak so bad that it's gonna blow the bonding wire off of the cathode and we don't want to do that so this is going to start to leak which we saw those you know those really large capacitors I'll take a look at that here in just a moment again that large capacitor in the back of the oscilloscope that 0.5 1000 volt might capacitor there 0.5 micro farad that is going to end up shorting out or getting very leaky and we don't want to damage this tube so this definitely has to go this is the troublemaking capacitor inside this oscilloscope aside from changing the electrolytic s-- now these electrolytic will also end up going bad you can see they're all over the place 10 mics 20 mics 40 Mike's 20 mics there here and over here we have 20 mics 20 mics and they're all over here so we have two canned capacitors in there those two larger cans those are gonna have to be replaced or subbed out swapped out and you know basically with a modern equivalent so that you know nothing goes wrong within the oscilloscope again people like to reform these things bad idea you're gonna reform these things you're gonna leave your bench for oh 10 minutes or so and you're gonna go grab a coffee and you're gonna come back in your test room we'll be full of smoke and this transformer will be you know done at that point so it's not worth taking the chance capacitors are cheap nowadays and if you want to use this as a little oscilloscope you know for for a long ways down the road why not take care of it just not worth a risk we have a balanced filament system here so centre tap of the filament goes to chassis ground and then this goes to each side of the filaments you can see that they're using the center tap of the 1287 here you can see them there both ends are tied together 4 & 5 are tied together and the nine runs to one side so you're basically taking a 12 point 6 volt filament and using the center tap and bringing it down to 6 point 3 very common with 12 87 s 12 ax7 s 12 au sevens 12a y7 s and all of the above all of those tube you know 5750 ones all those types of vacuum tubes that are all like that right now over here we have V 1 V 2 V and V 5 which are just going to be 6.3 volt filaments they're right across here and then we have our little pilot lamp right here which is our indicator lamp and that's why when I said when we saw the pilot lamp glowing that's a good indication that the filament winding is still good over here we have a bunch of capacitors that will need to be changed these are capacitors that tie from the line to the chassis here you can see they're in dotted lines here because they weren't in all of these oscilloscopes but in some of them so these will have to be changed with a modern safety capacitor x1 y2 type capacitor and you know just you want to get rid of these because if these things leak at all or short it'll bring one side of the AC line to the chassis and that presents a very big shock hazard now some people like to put three wire cords on these oscilloscopes that's fine but it also limits the way that this oscilloscope is going to work and if you want to test things especially negative voltages and things like that you have to be very careful with that third wire so that again is personal preference if you feel safer running that third wire up to the chassis by all means do it for my own purpose I'll leave them two wires this is the oscilloscope that's going to be turned into the dedicated signature tracer and curve tracer project this is the one with the Paulie style capacitor so there's not really a whole lot of restoration to do with this particular unit basically clean it up use some contact cleaner and the potentiometers and replace the electrolytic capacitors in this capacitor here maybe given another line cord the thing is pretty much ready to be turned into that dedicated piece of test gear so this one here is the other one that needs all brand-new capacitors and I may do a restoration on this one here down the road so in order for me to begin the process of turning this thing into a signature and curve tracer I need to get rid of all the weak points and the weak points being the electrolytic capacitors and this capacitor right here now since I need this area on the other side to mount another small circuit board that small circuit board is going to be a positive and negative power supply for the curved racer what I'm going to do is move this capacitor I'm going to replace this but I'll move the replacement capacitors in this area here I'm going to free up this terminal tie strip here so I'm going to remove the line cord and move it to another area install another terminal tie strip in here all the replacement capacitors that would be all the capacitors in here will be now mounted to this terminal tie strip so the barrels will sit in here like this that'll make a nice clean install and that'll free up a lot of area on the other side nothing going to need to install a small transformer here on the other side into a circuit board so that'll get all of that out of the way so this is going to be this one particular scope is going to be a dedicated piece of test gear here and if you're interested in following along with that this will all be on patreon I'll have all the circuit board layouts all the plans and schematics and everything there as attachments you can print out the circuit board layouts directly if you have a laser printer you can do the toner transfer method everything will be sized in everything so they'll be the power supply circuit board there and the main curve tracer circuit board will be there as well this one here will also be fitted with an adapter so that it will have a digital readout that's already part of the curve tracer project so because that you can use an external digital readout with this oscilloscope you don't need that little green filter with a graticule on it so you don't need that at all just a basically a gray screen like what you would see on this here is all you would need you don't need that green filter which makes things very nice it also cleans it up and of course the CRT looks a little bit brighter it's not really that you know it isn't the brightness isn't taken down by that green filter so if you're interested in me doing a complete restoration here on YouTube of this particular oscilloscope it's quite a bit more to do on this one here let me know in the comments and in the future what I'll do is I'll do a complete restoration on this one here this one here has got lots of stuff that needs to be replaced so all of these capacitors need to go and of course it needs to be cleaned up more as this one here all the capacitors are a-okay they don't need to be changed it's a quite a bit more to do on this it should be a an in-depth restoration here for YouTube so let me know in the comments down below thanks for stopping by the lab today if you're enjoying my videos you can let me know by giving me a big thumbs up and hang around we'll be more videos coming like this in the near future we'll be talking about vacuum tube and solid-state electronics alike they'll also be lots of restorations repairs and tear downs in there as well so lots of fun stuff planned for the future if you haven't hit the subscribe button you may want to do that the oscilloscope that you've seen in this video is now going over to patreon and it's going to be converted into a very useful piece of test gear known as a signature or a curve tracer the circuit board the layout the schematics and everything have already been designed and they're available to patrons on patreon right now so if you're interested in following along a little further with where this oscilloscope is going to go you may want to check that out if you're interested in taking part in my ongoing electronics course on patreon I'll put the link just below this video in the description so if you click on the link it'll take you right there you'll learn a little bit more about patreon there as well if you do go there check out the community section there's lots of people sharing their projects there alright until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 121,840

Rating: 4.9439964 out of 5

Keywords: Mr Carlson's Lab, Understand oscilloscopes, equipment restoration, fix electronics, understand vacuum tubes, repair old electronics, fix test equipment, fix old radios

Id: 5eS8hx8lYF8

Length: 77min 47sec (4667 seconds)

Published: Fri Oct 13 2017