AE#61 Repair Of A Tektronix TDS 340 Oscilloscope.

[Music] greetings this is already from artifact electronics and today's episode we'll have a look at a Tektronix TDS 340 hundred meg digital scope I picked this up recently and most or many of you asked usually what do you pay for instruments and I usually oblige and tell you but I run into an interesting situation where in order to keep my channel going and Finance what I do is I don't keep all of the instruments I feature around for too long so over myself I don't I do not need 8 oscilloscopes on my bench but then you inadvertently run into the situation where people see the channel you sell this thing and a particular thing and people go well how much do you want for it I go I want hundred dollars for it because they're going for even more than that on eBay but wait a minute you just told us that you paid $35 for it so you're overcharging us well yes if I were to buy and sell something then maybe I should keep a 5% Commission on it but I do spend a bit of time fixing these things obviously I mean going by and finding than fixing them and all of that but people just don't understand or don't want to understand that and it gets into an unpleasant situation so on certain things are most things that I'm going to repair because of the aforementioned reason I'm no longer going to state how much I paid for it if you want to see what something is worth I still advise you to have a look on eBay about this from the University salvage outlet here and they do have a test bench I dragged it over to the test unfortunately they don't have a very a con the testbench but I plugged it in and to test it and it didn't work and let's see what that looked like we got it plugged in right now so here's our switch it can hear a click and another click when you let go which kind of sounds to me like the jet there is some sort of a powerup really that's pulling in when you push it I hear nothing no fan no activity whatsoever and then when you let go the button the relay seems to disengage again so there's no power coming into this but I couldn't resist because cosmetically it's in really nice shape so let's do the obvious and check the fumes to get out the few should normally be an easy thing and even if you look on the service manual for this it still tells you that there's a round fuse plug located approximately here that you just unscrew and check the fuse but this one has a little sliding drawer underneath the IAC connector and there's a slot here that you basically have to get a small screwdriver and to pry the fuse holder assembly out which I've done and it's made of plastics are gonna be careful to not mark it too much you know it doesn't even want to come out the rest of the way and here we go so let's get out the fuse and can't really see that but looking at it here it does look it looks fine even though we know the pitfalls of using just looking at it and figuring out if it's good by doing it that way which is not what you should be doing it says three amps of 250 volts and what it tells us here that once a 3 amp slow blow so let us fetch a meter and check check the fuse so it does tell us that the fuse is good and and is the correct value I always do a quick short check by touching the Nutri on the hot inputs even though that's not not going to tell us a whole lot right now we can confuse this guy so let's put the fuse back in now we can test okay there's no short and if we actually do a resistance test it shows 130 K so there's no short at least not on the primary side notice that this device has no options installed even though it does have all of the stickers here to indicate what could be installed in there it's kind of but too bad because it does have a VGA output or has an option for a VGA output which is kind of cool but of course this one doesn't have anything this is the base model with no options ordered one thing I realized when I bought it was this scope was awfully light compared to the others for instance compared to a tds 460 and other scopes in its class it just seemed very very light to me and maybe that's because it's devoid of any any options and also people go to the front it doesn't even have the floppy drive it does have a blank panel in it so I do I was kind of afraid that or I am kind of afraid that when I open this thing up it's been completely cannibalized but then I weighed it and it was just under 15 pounds and then I looked in the user manual and it states but the Scopes supposed to be 15 pounds dry so I guess it is the proper weight but there's only one way to find out so let's dig in you leave the t-50 take out four screws on this panel here and once the end panel comes off we have to pull off the case and then we're in so that gets off the rear plate easy enough but if don't start yanking on the case yet like I have done before because most Tektronix scopes of this evening have another screw in front holding the case to the chassis and it's easy to overlook that and of course the second thing you need to do is swing the handle out of the way because you won't be able to pull the case back because the handle is on the case I'll just get stuck on the front panel so let's see if I can get this I always been opinion to get chances of oops but we're in and looks awfully empty and isn't it smoothly this is where all of the expansion cards fit in but this one not when there's no connections running in here so I guess if you buy an expansion card it will come with the cabling to connect it but as we can see I mean it doesn't look cannibalize it just looks like it never had anything of course you know we got the fan the power supply mounted here the video driver and CRT front panel which we can glimpse through here and then the part that gave me a bit of a solace looking at the back because at first glance you go looks like the whole analog board is missing but after checking some pictures online that is not the case they basically dumped everything on this board has the inputs over here and then a bunch of a6 and that gets connected to power and the display that's pretty much all there is in there so there you have it they've put everything on a single board does that the expansion connector here but it doesn't look like anything was ever installed over here and yes as I said online that's what it looks like there really isn't anything else in there so let's see why she isn't firing up just luring it back in and pressing power I can hear that same click and the power supply sounds like yeah it's making continuous clicking noises different than the relay noises so looks like the thing is trying to start up but it can't I let go of the button and everything goes away so looking at it you can kind of see the relay over here and it's just clicking from somewhere in here don't know if you can hear it but it's the characteristic sound of I switched power supply trying to start up but I'm able to do so oh this time when I let go the power switch it continued running sure that means something but I have no idea what that means so let's take a power supply I made some quick tests by this disconnecting everything that's coming out of the power supply to see if any of the external devices the front panel or the display are loading it down I have a defect and that I'm plugging the display didn't yield anything I'm plugging the mainboard you saw on the other side that doesn't yield anything either because you can't even you can't turn it on I guess the this power switch it's a soft power switch and it runs to the processor board and if you unplug the power board guess what then you can't turn it on at all and the clicking no matter what I disconnected other than of course the processor board continued the clicking noises also the fan I unplugged the fan which is probably just a 12 volt connection here that didn't make any difference either so yeah the Bolong we gotta take it out the power supply that is a lot of people have a knee-jerk reaction to when it comes to busted the power switching power supplies and that is all it's it's a bad gap it's about capped that is not necessarily true I've had two instances on video where I fixed the small sony tv that had a bats which are in it and it wasn't it was a Zener diode and I had something else I can't recall now that also wasn't the capacitors but there was a recent one where I fixed the RadioShack model three computer and that one actually the power supply failed and it did have a bad capacitor which fixed it but should do a visual inspection on the RadioShack a computer I did a visual inspection and I could see that the capacitor was bulging on top and that there was liquid seeping out fluid sleeping at the bottom so the electrolyte was coming out know is clearly an indicator on a capacitor but in most cases I've seen it is usually something else now I started out by doing a visual inspection on all of the electrolytic here none of the pops are bulging and there's also nothing seeping out of them I did using the SR meter on them while in circuit and the ESR was good on all of them of course being in circuits some of them some of them won't measure the capacitance properly because of other components but there was no immediate indication of anything gone wrong in here so what I'm suspecting is that it's something else the service manual for this does not give you a component level troubleshooting it's basically do this and that men replaced the board so we're kind of on our own in where to go and check for possible problems and from experience I guess I'm going to start checking via heatsink mounted transistors and diodes first and see if Y if I luck out and find anything so I poked around for a while and I may have found something that looks suspicious in circuit so let's get it out and see how it behaves outside this is a dual transit a dual diode housed in a to-220 Schottky diodes ultra fast rated at 8 ohms each has getting strange values out of it so let's take it out and see what it says on the bench did I just say eight homes I meant eight amps per diode but you knew that already great so here's the device let's set the meter to diode and I think the tab is connected to the common cathode on this but let's make sure of that and it is so first checking the left-side diode gives us a good voltage reading let's check the check it in reverse and it still shows over loads of us open now to the other side the right side diode and lo and behold that shows the dead short let's check it in the reverse direction still shows with that short well yeah if we may have found a the culprit or at least one of the culprits here so let me go and tear through my parts bins and see what I can come up with so after a lot of searching I could not find an exact replacement part but I found this guy does look used looks like I pulled it off some other switcher sometime in the past but I checked the specs online and they all meet or exceed the primary specs such as the current handling capability max voltage and the switching speed so let's see what this one has to say so the left side is good can't use a tab on this one because it's plastic yeah that makes it more difficult to put patience all right so the left side diode measures okay and so that's the right side one so yes eventually I'll go out and get myself an exact replacement which which are available but for now I'm going to give this one a try to see if that solves the problem or if we have to go and dig further one of the problems is since that one came the replacement came out of somewhere else you can see that the appends I've been cutting are significantly shorter than the ones on the original so let's see how well I can get that into place and get the and get it to make good contact with the heatsink it did fit in you can see it over here and with enough of the pins poking through to make a good solder connection the only difference is that the clip in this case pushes against the pushes against the tab whereas if we look at its next door neighbor which is the same part we can see that the because of the length of the elites the clip actually pushes against the body of the to-220 package so the only difference here really is that because there's more distance here this clip is probably pushing a little harder than this one but there's a big heat transfer sticker behind it so we should be okay with this well plan to put it back in and see the fruits of our labor he thinks back in hopefully I have all of the internal collections plugged in correctly it's got an AC connection so let's go ahead and test it the fan starts up it's a very good sign lights come on which probably means you're real a click and it's up but what does that screen say [Applause] oh we missed the startup which probably tells you whether it passed so let's power cycle it and see what the startup says this is great came up screen looked really nice there is no magic smoke come on you can do it power on self-check past push clear menu to perceive and there you go there's one trace and pushed channel 2 I think they're superimposed right now yes they are there's a second trace does look kind of noisy but these digital scopes tend to look like that so what happens if we switch it to ground doesn't really make a difference all right let's give it a signal and see if the channels actually work [Applause] okay let me let me get a BNC cable be right back now one thing I noticed that may be a problem here is it does have the battery back RAM and real time clock module in here that and the battery is inside this package and it's not replaceable well not easily anything can be done but I've looked into them and then they're bonded inside it's it's near impossible to change that battery you have to get a new one and I don't even know if this keeps all of the calibration data in there and if it's live and if it's not live am i you know what do I have to go through to calibrate it but well I guess we can turn it on and see if there's a way to display the clock and see what that says well obviously at some point somebody turned on the channel one frequency measuring part it looked like it retained that so let's go in and see if we can display the clock and read up options display time [Music] set to OFF just turn that on and back out of here and it's a few minutes off but it's kept the time I don't know how long this thing's been dead and unused and not powered up but looks like the battery in the RTC module is still good another thing I did while I was poking around the board before repairing it was just to make sure I took out the fan and plugged it directly in to 12 volts to make sure it works and it did work but it was pretty noisy made a kind of a light grinding noise but then I got distracted I did something else while the fan was still running and when I came back about 45 minutes later the fan was quiet so it must have had dust in the hub and it just eventually blew it out cause it's a normal sounding Tektronix fan now well let's check signals now [Music] so far so nothing oh you see if turning down here and there you go there is there's your sign display let's turn off channel 2 and just look at channel 1 make sure it's actually triggering well it must be triggering off channel 1 but the yes and it is and there you go we got a sine wave let's switch to square nice square but notice the frequency is pretty low and of course triangle very nice display let me bump up the multipliers on the function generator and we use the where is the auto set button there it is running at about ten kilo Hertz and going faster again rock stable well that's only one megahertz keep in mind this is a this is a hundred megahertz scope but that's not too bad if I turning up the function generator to max I guess the function generator goes to three three megahertz and causes an unstable histogram but there you go the less channel one so now let's go to the channel 2 they turn off channel 1 and we center channel 2 and it's Otto said it and there's your triangle now this one you know Matt three Meg's why did the frequency go away the clocks still up but the frequency oh there you go so it's still just above three megahertz looks like BIA edges yeah the tops and bottoms are a little bit rounded on here but it's a pretty nice-looking display looks very characteristic of the earlier TDS series so I think we have success things are back together again and it still works one thing that remains to be done is want to go in and run the Diagnostics from the menu so it gives us a detailed report of what got tested and what the results were so we go to Diagnostics and okay so go ahead and run the Diagnostics doesn't take or shouldn't take very long some relays chatter and it pretty much not pretty much it passed everything and the next thing to do would be to go back into the utility menu and let it calibrate itself so let's do that so it's calibrating the signal paths this is going to take a while and we'll report on it when it's done so it successfully did the signal path compensation and what we need to do now is push clear menu and that's it as the last thing let's check the compensation signal now the calibration and there it is so what remains to be done is not much I mean one of the things was this thing was completely clean normally stuff out of the university has stickers all over at calibration sticker as inventory stickers all that sort of stuff this one was completely clean so it looks like somebody had this in their own personal lab for a while and removed all traces of where it came from but when I decided to not work anymore it ended up at the University Salvage again of course what some of you may be thinking is I left out something big that needs to be done and that is to go through and recap the whole thing and eventually that needs to be done especially in the power supply but for now everything is running just fine and as I mentioned before I usually won't go in and wholesale replace everything after visual inspection and checking in with and checking the caps with the meter doesn't reveal any obvious problems I leave it the way it is a final word is there's a lot of help that can be found online but if you look at the keyword clicking switch small power suppliers so the first thing that 90% of the people say is go in and replace all other caps my advice is check the caps first if you see any caps that look suspicious take them off the board and check him off the board and if you don't see anything bad start checking the other components which is essentially what I did here and in this case obviously found a bad component that prevented the power supply from starting up yet didn't cause a short for it to blow it's fused so get suggestions get suggestions off the internet but in the end use your own head and use a logical approach to fix these things so nice skull pads off the Tektronix they know how to build scopes even though this thing is probably a mid to late 90s model so it's pretty old but it runs nice and at 100 100 Meg bandwidth it's more than enough to check vintage a lot of vintage little digital equipment and also audio signals so please support the channel subscribe thumbs up and we'll see you next time depending on what I can dig out to repair and or look at for then have a good one

Channel: Artifact Electronics

Views: 9,571

Rating: 4.9649124 out of 5

Keywords: artifact electronics, repair, diagnose, oscilloscope, tektronix, TDS340

Id: ro5VE7caF8c

Length: 36min 11sec (2171 seconds)

Published: Sun Sep 09 2018