The BEST VTVM? You Decide! HP 410B Restoration and Alignment

hi everyone and welcome to another episode of mr carlson's lab today we're going to take a look at possibly one of the finest or one of the best vacuum 2 volt meters ever created and we're going to restore this and make it perform the way it did when it was brand new so let's get started this is the hp 410b hewlett packard vacuum tube volt meter and as you can see it's in really nice condition it has the usual scratches and nicks for something of its age but all in all it looks to be pretty nice looking still as you can see it has the old handle on the top now word of advice if you find something like this at an auction or flea market avoid the temptation to grab the handle and pick it up because much of the time these things break right off and you know what's gonna happen this is gonna fall right to the floor right in front of you and then it's just going to be destroyed i've seen this happen at swap meets so whenever you find something old like this don't grab it by the handle always pick it up by the case just to be safe it came with the probes which is nice so this is just a little connector that screws onto the bottom here and attaches the probes into the unit you can remove this and all of the probes as you see here so this one is the ac probe it has a vacuum tube inside it this one here is the dc probe this is the ohms probe and then there is a commoner ground clip right here so there's four wires that come out of this and as you can see they're listed right here so ohms is the red wire and as you can see you could follow ac you can follow this lead to the appropriate probe and then dc you'd follow this to the appropriate probe which ends up here and then the common which is right here now you can remove this and put this in the back of the unit for storage or if you're going to take this you know on the road with you but most of the time i find that these things sit on the bench all the time so it's a good idea once you put this on just leave it on you don't want to put that extra time on the unit by putting you know taking this off and putting it on and taking it off it's just hard on connections and everything like that so i find that i leave them all connected on the back side you see the line cord coming out of the back you can store the probes in the back if needed down in here with the line cord it's a little bit tight but everything fits in there so all the usual adjustments that you need to calibrate the unit you see a tube right here and they've listed the tubes right here on the outside so they're behind this so hewlett packard made this relatively easy to service which is very very nice so there are a lot of perks about this vtvm aside being you know very very accurate i should say aside from being very very accurate so great meter to own if you ever have the chance to own an hp 410b i would say jump on it if you need a vtvm this is the thing to have now you might be saying well isn't a digital meter just as good as a vacuum tube voltmeter like this absolutely not so i've recently posted a video on patreon talking about uh how this meter differs from a modern meter and i show that where a standard dmm you know would even like the this test equipment over here a regular dmm bench style dmm will load circuitry so it'll actually you know drag the voltage rate off of an agc line whereas this thing it's invisible to the circuit so the input is such a high resistance or impedance if you like that this thing here will you know it remains invisible to most circuitry so i'll explain that once i get this thing running and maybe i'll even include a small little clip of that video just giving you an example of how important this is when you're working on very high impedance or high resistance circuitry depending on whether it's an ac or a dc circuit right so anyways i'll get this thing out of the case we'll take a look inside and we'll get moving with the restoration let's make this thing perform the way it did when it rolled off the line getting the hp 410b apart is really quite easy so the first thing that we're going to want to do to service this is get rid of the probes so we'll remove this and remove this here now usually the screws have got little stoppers on the top to stop them from coming out so i just go back and forth because a lot of the times they unwind and there they are so that stops the screws from coming out you'll find a lot of the times these things will unwind looks like this one has come off just a little bit but at any rate that removes the entire probe assembly isn't that nice so you can get this thing right out of the way get rid of all of these cables just like that so i'll just put this over here and the next thing is the back you just press this down get this off of here like so now of course this is going to have to stay on the line cord because the plug won't fit through that hole but not a big deal see these two screws right here remove those and we are in just like that very easy to get inside this so remove this here that one and this one here everything in real time right so there we go and i'll give myself a little bit of cord cause i'm gonna lift this up look at that and it looks like it's in really nice condition it has the antique dust included look at that everything is there that's a good sign and i don't see any exploded electrolytic capacitors so that's pretty good right here and it's looking really nice this capacitor is looking like it's in fairly nice condition so nothing really bad has happened at any rate the rectifier is on the bottom now it looks like that's okay as well so our switch is here and lots of resistors to check and calibration points and everything so that's what we're going to do next that's what i'll end up doing is i'll get this thing all set up i'll move the camera so we can take a closer look at things and we'll start checking some of the components together here's a closer look inside the hp 410b and as you can see there's a nice row of resistors here to test now here's a little trick with vacuum tube circuitry vacuum tubes are relatively high impedance devices and when they're off they basically look like an open circuit so it's very easy to test most resistors in vacuum tube circuitry right in the circuit now there is the odd case where a resistor will be parallel with something you'll have to lift one end of the resistor to accurately check it but in most cases you can just go and test most resistors right there so for example i'll just choose one this area has been hot here you can see it's kind of discolored so let's test this 10k carbon composition resistor right here 10.8 k it's a little out of focus because the focus is up here i should move that over to the meter there so 10.8 k that's going to be most likely okay 7.4 so this is a 6.8 k resistor and it's gone up to 7.4 so depending on where in the circuitry this particular resistor is that may not matter so if you understand the circuitry that basically surrounds the vacuum tubes that might cause just a very slight change in voltage from say what it originally was measured at it a lot of the times won't affect the circuitry again it just depends where it is in the circuitry so as i go along and explain other vacuum tube circuits here i'll talk a lot more about that how to understand if the values really need to be that accurate a lot of the times in this old vacuum tube gear they didn't even have a fourth band which means that they were 20 tolerance all right so if this isn't a circ a portion of the circuit that needs to be you know relatively accurate i will replace this resistor but i'll take some measurements because these are all aged and chances are they're not going to move very much now if they are okay they'll just leave them in now you'll see that there are some with gold bands here and i would expect these to be very accurate and these are most likely in a portion of the circuit where the accuracy really matters since this is discolored here this is probably just you know maybe in the power supply portion well you can almost see it right here there's a capacitor and you know this is most likely part of the power supply so that little bit of difference is really not going to make a whole lot in vacuum tube circuitry now i know that may sound a little bit contrary to what a lot of people believe because people are used to solid state stuff and in a solid state circuit where you're dealing with low impedance devices yes small changes in resistance values do mean quite a bit we're dealing with vacuum tubes here different story again i will go over more of this and how to recognize this and go okay you know this is 6.8 k but 7.4 is absolutely fine i'll just leave it i'll explain all of that in in many of my future videos so let's uh let's see what else can we test here so this hasn't been hot it doesn't look like this uh 2.2 k resistor let's just test it so you can see it's kind of darkened over here so these have gotten warm so let's test the 2.2 k and see what it this says no problems 2.1 k actually and as you can see how it's slowly climbing it went down and out slowly climbing up that usually indicates that there's a capacitor somewhere in circuit so no problems so that's 3.9 meg ohms let's see what that is going to do not bad so most of the resistors here are most likely going to be okay again i will look in the circuit find out where these things are and uh if they need changing they'll get changed i have lots of carbon composition resistors so over here we have two 12a u7's which are dual triodes over here we have a ballast tube and this ballast tube is used for the probe so they have this in line with the filament of the actual tube inside the probe to act as a form of regulation here's another regulator tube this one here i'll just get the focus on that right there this here is a regulator tube for the b plus so the 6x4 will be the high voltage rectifier this will be a regular regulator circuit to make that nice and stable and down here we have a very nice looking rectifier look at that what a nice color and it looks like it's in pristine condition here all the resistors that make this thing very accurate so what you're reading with your probe equals what you're seeing on the meter face and yeah just it's a you know pretty standard surprisingly simple design is what i would say for something that is so incredibly useful honestly this is just these are just wonderful you know vacuum two volt meters so what i'm going to do now is i'm going to just go through and test all the other resistors and uh this capacitor is going to get changed it's a really low value four micro farad or something like that yeah four micro farad at 450 volts dc so i'll end up testing this too as well just to for the fun of it to see what condition that is in and we'll pretty much power the thing up and see if the thing works there really isn't a whole lot to this there isn't a whole lot to a restoration like this so if you're interested in getting into vacuum tube equipment restorations this is a very very simple restoration to start off with of course know that all of these devices have high voltage inside them right because vacuum tubes work with elevated voltages so if you're unfamiliar with working on a device like this that has high voltage in it don't do it right you know do your research and when you feel that you're qualified to do so do so then so if you're following along you're definitely doing so at your own risk just be very very careful all right i'll get some components changed and we'll turn the thing on and see how it works and i'll show you some of the bad components as well that i find all right let's test this capacitor here and see how well it tests out still maybe good may not be so i think by the condition of this thing it looks like it is rather low time but depending on the facility or wherever this thing was used there's a really good chance that um they just took really good care of it so what i did is i just cut the there's a small ears that i call them what they are is they they take the wire put it through and fold it over and it makes what looks like an ear and what you do is you just clip those little loops off and it'll allow you to very easily desolder something like this so what i'm going to do is i'm just going to position it like this so i can see the wires from my angle and what i'll do is i'll just heat this up like so as you can see they just come right out so let's move that out of the way move that out of the way and let's test this for capacitance so see if i can get this all in here turn this on so it should be around four microfarads something like that so the ground of this looks to be isolated from the chassis they've got the two wires coming to the ground here of the capacitor and then they have an insulating you can see that this little insulating wafer here so something to keep in mind if you're going to try and replace something like this a lot of them have those little tabs that twist or bend out in this case the capacitors negative is isolated from the chassis okay so see if this will tell me anything all right oh so you can definitely tell that um this it's a real overachiever of a capacitor or or it's definitely you know on its way out for sure so let's see a lot of these older capacitors were over achievers just the way they were so what i'll do is i'll grab the other tester here this is a leakage tester and what this is going to determine is whether this is electrically leaky inside so whether this is beginning to look more like a resistor than a capacitor so one of the big tricks or one of the clues that these things are getting leaky is this was rated at four micro farad and we saw this capacitor tester indicating about nine or over nine microfarad why is that well these capacitor testers use math to determine what the capacitance of the capacitor is so what it's doing is it's measuring the amount of time it's taking to charge this capacitor and then converting it into a number here in microfarad so what's going to cause this to read higher well if you add a resistor across this it's going to take longer for this to charge so we would put a resistor in parallel with a capacitor it would take this longer to charge up and then this is going to indicate a higher capacitance so the rate there is a clue that this thing is going to be leaky all right so to really determine that use a dedicated leakage tester so what i'll do is i will put this on here this is one of the little devices i've designed and this is available on patreon with all the plans and everything so if you want to build one of these things for yourself it's all up there schematics alignment procedure plans the whole everything whole bottle of wax so you've probably heard of esr equivalent series resistance right this is testing for parallel resistance so completely different test than an esr tester all right so i'll move the focus over here so that we can see it a little bit better so what i'll do this is on the electrolytic position right now and i'll put this that's discharge right there and this is test already i can tell that this is very leaky and still in the red scale yeah very leaky okay so what i'll do so i'll shut this off now this is reading roughly 9 or 10 micro farad so i have a 10 micro for red capacitor let's compare it to a really good capacitor all right so if this is four microfarad and this is ten microfarads so say this was four right it was actually reading at four this should take twice as long to charge so you'd see this move down a lot slower than this one right here all right this is a newer capsule let's check it out okay so let's put that onto the side here like so or here where we can see it and go on to discharge and here we go look at how good that is just zip right through that second scale so no problems with the new cap so you can see how bad this is right you see how quickly that moved down the scale and this is why you avoid the temptation to plug these things in when you first get them and just to see if they work because a lot of the times these capacitors are turning into resistors and when they turn into resistors they put extra load on the b plus section in the unit itself and when that happens it damages components the most common thing to happen is these things will go closed completely not well available this unit here is much safer to use than the older capacitor testers that use high voltages to test caps again available on patreon if you want to build one for yourself definitely check it out so what i'll do now is i will install this because i'm going to replace this 4 micro fred with 10 anyways a little bit of extra filtering won't hurt anything so i'll add that in here and on to the next portion of the project i've installed the new capacitor right here so they're so small now they pretty much fit anywhere that allows me to leave the original capacitor in and maintain that original look right over here so what i've done is i've disconnected the wires from the original positive lug of the capacitor and i've installed a little standoff this is a ceramic insulator right here and i've just moved the wires from the lug up to this insulated area right here and i've put the new capacitor across this so this allows me to very easily change a capacitor down the road if i need to do that if i was to restuff the can and do all that you know it's quite a bit more maintenance to get in here now this thing is not going to be a shelf queen i'm going to end up using this thing all the time so this is a piece of equipment that i you know plan to have on the bench and use and you know display its functionality so the new capacitor worked out well and now what i'm going to do is start cleaning all of the switches down here and i'm going to get it ready just to test the thing out let's see how it works with all of the original resistors in here all right let's try this thing out let's give it a shot and see what it does so i'll have to wait a few moments here for this to warm up so i have my power supply set to one volt and it's on the one volt scale here so i have to wait for things to stabilize here i've left the selenium rectifier in just because it's fine and not only that there's hardly any load across it it's not really in a high voltage application so it should be absolutely fine to be left alone now if this was in a b plus application that blue selenium i'll show you that here in a little bit again i would end up replacing that with a diode but again low current not a big deal it should last the term okay i'll just zero this off so what i'm going to do is i'm going to take the dc probe and i'm going to short it to ground here and i will zero this up close enough for now okay let's see what it does at one volt pretty close 0.9 a little bit under 0.9 of a volt now keep in mind this is also sitting on its back and i haven't zeroed the movement out so these meters they like to sit straight up and down so not bad really without anything really done so not too bad at all so let's see you can see it sits up a little bit here i take the probe off you see how sensitive that is it moves all over the place just me moving my hand around it is just very very very sensitive sensitive meter so let's try a resistor okay so this should be fun go to ohms remove this from the power supply and i have to use the red probe now just wrapped around everything on the bench get that one out of the way and this these are the two now so what i want to do is i want to adjust this for infinity right so it's basically the last line you can see the infinity symbol the last line right there so i'll bring that up and then what i'm going to do is short these like so and then i have to zero this out you have to go back and forth a few times with these things sometimes so this is on the lowest scale too so it's going to be real picky so i'll open this up and now we need to adjust this for infinity again or the last line and i'll short this again and adjust for zero again i am on the low ohm scale so this is a really picky adjustment and i'll open this up again and i'll adjust that right up there short this like so that's getting close just bump that up to zero all right let that stabilize one more time and i'm just about there look at that just about there just a touch i want to get this about as accurate as possible so we can get a really good idea of how well this thing's working okay not bad so rummage around over here i think i have a lower ohms resistor here somewhere they do this is a 3.9 yeah 3r9 so it should be 3.9 ohms okay so let's see should be right around the 4 if this is uh accurate let's see what happens we still on that line we are look at that no problems there no problems at all so let's see what else should we try here um let's go to the next scale here i have a 220 ohm resistor sitting off to the side now you can see how different this is so again i'll just for infinity this here now i haven't tested any of the tubes uh it's these meters are very sensitive with the 12 au7s so if you can match the triodes in a 12a u7 it's the best thing for these meters so i'm going to design a tube matcher for triodes and i will release that on patreon so anyways uh just another thing planned okay so uh let's short these out and go here zero this again and then i'll open the probes and bring this to infinity and then short them getting close make sure there's a good connection there very very close i'm getting kind of picky now and let's see if it goes to infinity okay so 220 ohms here we go attach this and here look at that it's right on the line so it's 220 240 260 280 300 so that's 220 that line right there okay i'll zoom on in to make things a little bit easier to see look at that it's pretty good not bad no alignment yet so nothing has happened with this thing as i said pretty nice meters all right now what i'm going to do is test some of the tubes in here and if i find a weak one i'll let you know and we'll proceed and move on and try and do one volt calibration doing the one volt here i'll just back this up because you can't see what i'm pointing at doing the one volt dc calibration calibrates the entire dc scale uh the ac scale needs to be calibrated by multiple variable resistors inside so so far the ohms is spot on and the dc is not far off so what i'm going to do is test the tubes and i'll be back all right i tested the tubes and i put them back in so i can show you what i'm about to do here so this tube right here has seen better days so what i'm going to do is just remove the retainer like so and replace this one here so this one here didn't test very good at all so i was looking through my pile of stuff and look what i came across hewlett packard and it tests wonderful 12a u7 so i will put in the hewlett packard tube into the hewlett packard meter just by doing that this should magically work right okay so now let's turn this back this way and i'll put these under here to steady things it's right at the top so let's see if we can get any closer right now put this back into my isolation transformer current limited variax supply turn this on i'll attach this to my dc supply we're still set at one volt and i'll let everything settle off here things need to warm up let's prove over here and i'll short this here you can see how far off this is right so i'll just move this up if you recall when i was zeroing the meter for dc was almost right over to the stop now this is way up almost centered actually okay so i'll just bring that to zero we're on positive dc volts and it is one volt again now keep in mind this isn't warm so let's try this out i don't know we're on the wrong scale on the three volt scale here i'll do that again i'll zero that up still from the uh testing the resistor i didn't move that back okay so here we are there you go so you know that there's there's no no pre-testing before i do this it was still on the scale i was using for resistance loss okay so here we go and i've zeroed it up and as you can see again there's you know lots of room it's pretty much centered right so zero that there okay one volt let's see what happens look at that so now it's a little over so i could use a touch on the cal there so again i'll short this out make sure everything's zeroed properly yep and right there no problem so i wonder if i stand this straight up if it'll just read right on wouldn't that be something again meter movement right i don't know if you can't really see this so i'll move this out of the way move this back here maybe you can well you'll get a slight view of it anyways let's see if i can prop this up on something i think this may hold it okay a little bit unsteady there put this one under this side and it won't go crash bang all right so there we are let's see if i can zero this up grab that dc probe again that's one of the things that's tricky you end up grabbing the wrong probe all the time so i'll zero this up and it's pretty much zeroed still so i will give this one volt and yeah it's still a little bit over so not bad on the uh from this angle it looks like it's way over but looking at it straight on it's half a bar over the one so not too bad so if i adjust this here maybe if i lay it down the dc cal will be right here why do i know this well i have worked on many of these meters so here we go let's adjust that down just sit right on the one so sitting right on the one right there if i short this out it's right on the zero okay so let's see if it reads the same now when this is sitting back like it was before that would be nice if it does okay so here we go so i sort this out just about oops really trying to bury this probe into that ground there to get the most accurate zero i can let's do this very close okay let's try it now one volt look at that right on one so let's try for the fun of it 0.6 of a volt which would be right there all right 0.6.6 enter okay let's see what that does see what i mean by fantastic so far so good the roughest calibration you've ever seen on the bench meter laying backwards everything still performing good so that weak tube was definitely a part of the issue so and of course now that it has a hewlett packard tube in there it feels right at home all right so what i'm going to do is i'm going to clean the rest of the adjustment vrs here so i'll just shut this off and remove this from my isolation transformer and current limited variax supply grab the frame here so what i'm going to do is i'm going to just spray cleaner in all of these and move them around a whole bunch of times and then get this ready for a real calibration you know get the dust off of this and everything you know still pretty filthy in here right and again i was going to tell you about this i tested this down here absolutely fine right because the selenium rectifiers or any type of rectifier should be tested just because you don't know what's you know happened over the over the years i just tacked this back on just in case i was going to you know fit another diode in there but it's absolutely fine there's no problems with it and it's in again you know a low voltage portion of the circuit so and uh no not very much current demand on it or anything like that so i'd like to leave it in so you know that's personal preference if you're scared of those types of rectifiers and you want to change them you know feel absolutely free to do that all right i'll clean some vrs let's perform an alignment here and we'll also try out the ac probe as well because we need to do a complete ac alignment the the tube and the ac probe is working very well here i'll just show you something i'll show you the tube inside the probe it's very easy to change there's two different types of tubes there's an ea 53 which is a 6.3 volt version and there's a [Music] what is it 0-2 i think 2-0-1 2-0-1-c i think so when you take this apart be careful that you don't lose this because there's a little spring-loaded tip on here and that makes connection inside the top there you see that right at the top so you don't want to lose those this is a little vacuum tube right here and it just plugs in like so so this one is the ea-53 so the ea 53 is the 6.3 volt version and the other one is the 5-volt version so let's put this on here so it just depends which tube you have that back on so if you have the 2-0-1-c it may require a little bit different alignment for the filament voltage just put that on like so and wind that back on so for higher frequency use if you're going to be testing you know around the gigahertz range or something like that of course you'd want the shortest leads possible and they have this clip on here that'll allow you to you know have a pretty short lead at one gigahertz this is still you know quite the inductor but you know you would want a very very short ground and of course you know even at one gigahertz this amount of length is acting as an antenna of some sort so yeah for higher frequency you see the attachment of this probe is very specific but for most you know ac measurements if you're going to be measuring the kilohertz most people wouldn't be measuring uh you know you know any type of an ac frequency you know much beyond the audio level if you're into rf and you want to use this as an rf probe that's absolutely fine but it does take a little bit of knowledge to understand that you know lead length and you know the way that things are connected will really impact the reading so and of course you know again you know a piece of lead length like this will act as an inductor at higher frequencies so things like that need to be known again i will go over more of this stuff in the future and how to attach these things so rf gets a little bit tricky audio is a little easier to work with so that's just uh that's just the way it is so as i've said many times before uh if you're into the rf world once you understand the way the rf world works it'll make understanding the way audio works seems so much easier so it's kind of walks up the ladder i guess you could say in frequencies the first thing i'm going to do before i start the alignment is set the filament voltage of the tube in the probe so that's a very important thing to check it's very easy to do so this pink wire here that runs to the tube we'll put our ac volt meter on there and i can just touch the chassis and get an idea that's seven volts so that's pretty high it's supposed to be at 6.3 so the adjustment point is right there this one right here looks like somebody's marked a line on there it doesn't uh look to be too accurate so what i'll do is i will put my screwdriver in that adjustment point just like so and i will bring this down 6.3 i'll bring it just a touch over 6.3 just to deal with line variation so the line variation here is this line is actually pretty high at the bench so a normal liner probably right at 6.3 so that's close enough and that will definitely save the filament in that little tube so probe filament voltage adjusted the very first thing to do in this calibration procedure is set the one volt scale so that's the dc cal setting that should be done first so i have it in positive volts here i have the leads shorted on the test fixture here or the calibration fixture so i'll zero this up that's pretty close to zero to my eyes i'm looking at this straight on it might look a little bit off to you because you're on a bit of an angle it's kind of hard to get everything all in here so i'll attach this to the fixture let things settle off for just a moment and i will press positive one volt and that is so incredibly close just from the bench it's almost not worth even touching so i'll shut this off check it again so i'll take these two leads and short them that's pretty close i'll attach this back up here again and try it one more time yes that's right on the one so that's just from the bench calibration it's still spot on so i don't need to calibrate that scale which is nice and that allows me to calibrate everything else after this point the next step in the alignment is to do the one volt ac calibration that is this variable resistor right here so it's right beside that filter capacitor that's the one that needs to be adjusted and it's pretty much the same procedure for all the rest of the scale so i'll just show you how i do this one and all the rest of them are again pretty much the same thing so click it onto ac here and what i need to do now is zero the meter again so i'll take the ac probe hopefully you can see this and i'm just gonna short that there and now i just want to adjust the ac i don't want to touch the zero behind i just want to touch the ac0 move that around very touchy control probably because i'm moving the probe too so there it is that's right on the zero mark at least to my eye anyways at this angle so what i'll do now is i'll put my little jumper clip on this and what i'll do is i'll switch this to ac and then i will shut this off so it's at 60 cycles right now at 1 volt so i will attach this here like this and i'll just set the probe down very gently and i'll turn it on see where we are well that's very close so now i need to do is move that up to the one so if we can do this from this angle right about there right on the number one to my eye yep right on the number one so what i'll do is i will shut this off there and i will ground the probe out here again hook the common to the center anyways and that's right on zero so that scale is now calibrated and it's the same thing i just go up to three set this to three volts adjust it for three you know all the way up to 300 volts and adjust the 300 volt scale on the back so that's it so i'll get going with that and get that all calibrated up and i'll adjust the ohms here as well so it's not really an adjustment it's it's basically just to make the two scales somewhat align at infinity i'll explain that when i get to that point i should mention to lesson alignment confusion the one volt ac and the three volt ac scales are the red numbers and you have to align them to those red numbers now the one volt upper scale and the one volt scale here which is the ac scale they align with each other so you can pretty much just use the needle and the needle will rest on top of both of those markers but you'll see how the three volt stops down here so i'll give you an example i'll turn on the alignment fixture here you can see how it's stopping right there on the number three so it doesn't go right to the end and that's very important for your alignment accuracy i'm over on the 300 volt scale now and the thing just aligns up so incredibly nice so i'll give you an example here so the calibration fixture set to 300 volts i'll turn that on it's at 60 cycles again as you can see you'll see it'll go right over to the 3 here that noise is just warning you that it's putting out high voltage right on the number three so i'll turn that off go down to 200 so it should be right on the two there in fact you know what we'll zoom in just a little more you can really see what's going on focus off of that so there we go all right so i'm at 200 volts now so it should be right at that two on the second scale down so here we go [Music] look at that right on the number two 200 volts i'll turn that off and i'll go down to 100 volts and we'll try it again should be right there look at that right on top of the number one so very pleased with this so far again this is just a preliminary alignment as soon as i put this in the case and let it warm up for about 45 minutes i'll go through it and do another alignment on this completely all over again just to make sure that everything is spot on so the reason i'm doing this open is so that you can see this there's also something else that i'll show you in here there's a bunch of different versions of the 410b and they have different cards on the side and they don't mark some of the variable resistors and it's kind of hard to find the information on that so i'll share that information with you here on the other bench here's some information that may benefit you if you have an hp 410b the same version i do you may have a rough time finding what these vrs do and their identification so this upper one right here is the 30 volt ac adjust this one on the bottom is the 300 volt ac adjust and then the one on the back side which is right there you can see that one that's right there that one is the 100 volt ac adjust so right down there so that's a little bit of information that sometimes is a little bit hard to find especially with this version right here here's how we make the infinity align on both the r times 10 and r times one scale so of course we're in the ohms because we're measuring resistance so what i want to do is i want to adjust the infinity here to the last line just before the infinity symbol i'll show you this here so right about there and what i'll do is i'll just i'll short the test leads here to make sure that it's zeroed correctly and it is very very close on the bottom scale there could get touched up just just a bit right there it's good so i'll recheck that and yeah it's pretty much on infinity i'm being a little bit picky here here we go one more check zero yeah no problems okay again there's temperature variation while i'm doing this here it's not in its case so things will move around a little bit so now what i want to do now that i've done that i've set this on infinity that last line there when i click down to r times one it should stay at that same spot and if it doesn't i need to make an adjustment on the back and as you can see it moves so there's a variable resistor right beside the ballast tube and that one will allow me to adjust this one and bring that scale back up so what i'll do is i'll just put this in that variable resistor like so and move that up to infinity right about there very close and there we go and as you can see now it aligns on every single scale here's an example of the usefulness of a vacuum tube voltmeter when taking a look at very sensitive circuitry circuitry that doesn't have a whole lot of current behind it and this is really an example of not loading the device under test now i recently did a video about this on patreon and i'll share a clip of that video here showing you how a normal dmm digital multimeter or bench top meter will drag the voltage down in a very sensitive circuit whereas a vacuum tube voltmeter like this will remain invisible now the meter that you're going to see the vtvm that you will see in that video looks a little bit different than this but it's pretty much the same design it's just the military version of this particular meter the reason i didn't show this meter in the video is well i didn't have it restored at that point so let's take a look at that clip and i'll give you an example you can actually see how one of these dmms or multimeters any type of multimeter will drag the voltage right down whereas this will stay virtually invisible to the device under test here's a good example of circuit loading now a lot of the i guess you could say the newer techs seem to think that these meters are just as good as vacuum two volt meters right well in some cases they may be and in other cases they're not so these two one mega ohm resistors right here are these two one mega ohm resistors right here okay so we're going to be taking a look at the actual agc line so what the agc line here does is we have a path that runs all the way over here and then it runs up through the coils we have a dc path that makes it onto the control grid of the actual if section so you can see there's a if you look at the control grid here right you can see it goes through the if transformer and then we have a resistor here and it runs down to the agc line right here right so we have a capacitor on there all right to basically keep rf off into and to give it a bit of a time constant that's what the capacitors do on the agc line so uh the time constant of the agc line is basically how quickly it responds to a uh to a strong signal all right now being am it's like you tune into it and usually it is strong uh when you have skip conditions or i guess you could call it atmospheric conditions depending on the way you want to look at it if it's a fast moving condition uh you know sometimes you get a bit of overloading on a slower circuit but this seems to be just a beautiful balance it doesn't seem to ever do that this is the itube okay here's a trick all right you can watch the eye tube on here and i'm going to touch the automatic gain control line with the meter here okay so i'm gonna touch this resistor right here i'm just gonna touch the lead on it i'm gonna zoom in on this so i can give you a better view of just what's going on and uh maybe i can move the focus onto the meter a little bit here now i can't get them both at this at this distance okay so you're gonna have to deal with uh this being backed out just a little bit first of all actually here we go first of all what i'll do is i'll i'll touch the agc line with the probe and watch the eye tube okay so watch what happens to the eye tube here so i'll tell you when i'm touching this get this down into here okay so here we go see what happened so i'll remove the probe i'll touch it again you see that see how it's moving in when i'm touching my my dmm's probe to it let's get a little bit better focus on that okay so here we go that's a little better so here we go i'm going to touch it right now now i'm going to remove it see how it's affecting the shadow angle when i'm touching my probe there and why it's doing that is because this fancy dancy dmm that you see here on the bench that's supposed to you know be relatively high input resistance or impedance is loading the circuit now another way to tell that is is when you touch your probe to a circuit and the numbers slowly count down okay so you'll find that it'll slowly count down and that's that capacitor on the egc line that's the probe and the loading of this meter which is our you know relatively high impedance or high resistance meter let's move the focus there is loading it down this is like this relatively high resistance or impedance meter is pulling the circuitry down i can even pull it down with a brand new wax capacitor i have if i put a brand new out of the box the leads on it and everything wax capacitor put that on there you can actually watch the eye tube move so let me display that too okay so here we go now watch what happens to the meter i'm going to touch it to the terminal now see counting down now if i remove it and i'll touch it again see how we started at about four volts okay so here i'll tap it again really quickly and you'll probably see that first digit go to four no that one didn't catch it that time let's see if i can maybe make this a little bit quicker by uh bringing it up a range here so a couple of ranges so here we go yeah see how it tapped four there and now it's basically almost pulling it down to three 3.3 volts probably three point two something ah it's sticking to three point three okay went to three point two nine something or other okay and i'll remove the probe again you might even be able to see the the blurry shadow angle move as i do this as well so here we go now that's because this meter is loading the circuit now you're thinking to yourself well maybe it's just this meter right maybe it's just this agilent alright so what i'll do here is i'll just uh grab this one here okay so it's another one this is the one with the remote uh head on it right here all right so uh let's see what it'll all do i'll just leave that on just unplug this plug this into this meter here okay so here we go let's try this one there we go all right so take that probe off see if i can even get that in there right at the screen you see my probe right down here so i'll poke this right now that's 4 volts and we're dragging it down to 3.3 so we're giving ourselves a false reading because the meter is loading the circuit let's try this vtvm to look at the agc circuit so i'm on the 10 volt scale which is the upper scale so 1 is 10 and this is 8 6 and 4 volts and 2 and down 3 down to 0 there all right so it might look a little bit off because it's on a bit of an angle here and this meter really doesn't like sitting on its backside it likes to sit straight up but it's uh it'll do a pretty good job of displaying what we need to see here so keep an eye on the tube and the meter and i'll touch that same point right now if i can get it in there there we go all right so remove the probe ground this out and i should probably zero that up a little bit better even on my end here it's a little bit above the old zero mark there okay let's try that again here we go look at that right where it's supposed to be 4.1 volts that second little bar that you see right up there is 4.2 volts okay all right there we go look at that no movement of the eye reading exactly what it's supposed to this is actually a shielded piece of cable right up to the probe because this is a very special type of voltmeter now i'm going to be doing the restoration on the hewlett packard version of this voltmeter here very shortly and i'm even gonna upgrade a bunch of little things inside there so i'll uh just show you the ac probe here these things are good into the gigahertz so that's a really fancy looking probe and it's nice and hot because there's a vacuum tube inside here and a very special meter again you know great for looking at rf voltage and things like that let's just put this on the bench here got to be very careful with these things somebody's dropped this and already busted this before i had it so i had to fix it and see the hairline crack here and i had to make the probe because this was missing and so this is basically just a long screw that i've sharpened on the end and i put this here to protect it so the cap you can remove the cap too and all that kind of stuff so um the probe on the hewlett packard version i like a little bit better because it's um you know a little bit smaller than this this is a really big probe for looking at rf voltages with right now keep in mind that you know what you're going to be reading depending on the frequency is going to be you know largely dependent on the size of the tip as well just because you know when you're looking at gigahertz frequencies even uhf frequencies the length of this is uh going to skew your results because this looks like inductance now i'm doing the component this series on components here on patreon and we're going to take a look at inductors and things like that and i'll explain a little bit more about that so when we start looking at different types of components in that series so i wanted to share this with you now because this is very important and i have this opportunity again because this radio is out and this is such a prime example of a you know very high resistance or high impedance circuit depending on which point you look at it it's a very very good example if you enjoyed that video clip definitely check out my patreon page there's over a hundred videos up there right now and i'm sharing many of my personal designs and inventions so there's lots of great technical information there if you want to become a great electronics technician or a great electronics engineer definitely check it out you'll enjoy it if you're enjoying my videos you can let me know by giving me a big thumbs up and hang around there'll be more videos like this coming in the near future we'll be taking a look at vacuum tube and solid state electronic devices alike so there'll be a lot of repairs restorations circuitry design all sorts of electronic related content on this channel so if you haven't subscribed now would be a good time to do that as well if you're interested in taking your electronics knowledge to the next level and learning electronics in a very different and effective way and gaining access to many of my personal electronic inventions and designs some of which you've seen in this video you're definitely going to want to check out my ongoing electronics course on patreon i'll put the link just below the video's description under the show more tab and i'll pin the link at the top of the comments section so if you click on the link it'll take you right there alright until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 55,512

Rating: 4.9533272 out of 5

Keywords: HP Vacuum Tube Volt Meter, VTVM Repair, VTVM Alignment, Test equipment repair, fix a vtvm, learn electronics, understanding electronics, Mr Carlson's Lab Video's, HP410B, HP-410B, HP 410B, Tube electronics, repair electronics

Id: FWjXgWlgl7U

Length: 60min 4sec (3604 seconds)

Published: Wed May 12 2021