DR #7 - Tektronix 2205 Oscilloscope Troubleshooting and Repair

hey everyone its Nick welcome back to another duality repair video today I have a Tektronix 2205 20 megahertz dual channel oscilloscope just got this in from ebay I bought it for the very reasonable price of $30 I believe the issue was it's got some issues with the vertical deflection so let's power it up I have not powered it up this will be the first time it's plugged in let's power it on and see what it's what's going on with this thing can we have a beam it's obviously good and yeah sure you can see that yeah so with no input we have some noise in the vertical deflection so that's what they're talking about I've never worked with one of these Tektronix 2205 so bear with me as they figure out where all the buttons and knobs are I'm not crazy about the focus so this is channel 1 let's see what channel 2 produces let's go to the same volts per division right now I'm at one it's got it too so if we go to both yeah they have the same thing mmm so obviously there's not a problem with a single channel but something that's common to both channels so that's what we're gonna have to figure out let me change the horizontal here keeps in line pretty well and see it all the way up curious what it will look like if I am if I have something hooked up so I hook it up to the Cal signal maybe when I go back to channel 1 let's go to channel 1 and where is it let's just put a probe in there and hook it up to the Cal signal and see if see what we get curious you can see so that was the ground DC and AC both look like garbage I doubt I'm gonna see anything on this yeah it doesn't look good DC not some it just looks bad doesn't look good my channel too quick how much going on there see that change a little bit well it's just adding it to that noise great ugly though obviously some noise going on we'll have to pull up the service manual take a look at what could possibly be causing this first guess would be capacitor a capacitor related issue on the vertical deflection circuit or maybe the power supply can be power supply itself with some noise not sure yet so let's pull up the schematics and take a look before we start digging into the schematics and looking at the vertical amplification circuit I pulled this up from the service manual it's got the power supply test measurements here so you can see this little table here on the right and it's got six voltages to measure negative eight point six positive five positive eight point six positive 102 positive 2 O 5 and positive 22 unregulated five of these six are on the main board and I've pulled their locations up here service manual and of course this is a scanned copy that's what was available online so it's very difficult to read the read everything they can legible at all so I I wrote down each position you can see the positions here they pretty much line up w-9 $89.99 91979 92007 the middle here so I should be able to find those pretty easily this sixth one 22 ma related we find on a time base 10 you meter board and that's right at the top so that should be fairly easy to find depending on where these boards are located so we'll go after these first we will make sure that they are within specifications and then we'll see if there's any ripple on any of these specifically I believe the vertical board uses negative and positive eight point six and the positive five so these three we're on pay particular attention to might as well check the other three as well because they're they're all critical so I will get this thing open and we'll take a look at these voltages all right I was able to remove the unit from the case and I haven't flipped over on its side exposing the main board and so let's just go through these test points one by one starting with 989 this should be negative eight point six volts and we'll check DC first so 989 is up here looks pretty good negative eight point six it's 291 which should be positive five nine ninety one is here 5.06 good 987 should be positive eight point six nine eighty sevens here it's pretty close eight point seven expect for that is two eight point eight six so that's good okay let's go to nine eighty four which is 102 volts so I need to change my scale here 984 102 point nine hundred and three volts the spec is plus or minus 3 volts so that one's good nine eighty five should be plus two oh five volts yeah change skills again nine eighty five to ten to eleven that's the upper end of its spec upper end is to 11 point one so we may want to adjust that one it's probably for a CRT voltage there and that final voltage is on the attenuator board so we'll have to look that one later that one's unregulated anyway and I don't believe this has any effect on the vertical so I may skip that for now but let's go back and check AC on all these test points that's really what we're curious about right so let's check AC starting with w 989 the negative eight point six points 989 checking for AC not a good 991 positive 5 volts very critical nothing 987 should be + 8.6 chekc on that one nothing and then we'll check 102.9 84 which is 102 might be a little bit on that nope nothing and 9 85 here practically nothing okay mm-hmm I think it's safe to say that we don't have a power supply problem as in a filtering problem causing Ripple we traverse its outputs specifically the vertical boards so now I'll have to dig into the vertical amplification circuit and take a look see where we want to poke around there so I'll be back when I have that circuit print out here is the schematic for the vertical amplification circuit I understand it's pretty difficult to read not only it's very small but it's as I said it was scanned it's a scan version so it's it's almost eligible but we can get the general gist of what we're looking at here so you look at the top here this is the channel 1 input and the preamplifier the bottom is channel 2 input and preamplifier and then in the middle you have your channel selector switch I'm not really concerned with these areas since we have noise on both channels there's hardly any commonality between them here at all when I'm going to start where I'm going to focus is in this middle section here which is the output amplifier driver circuit and really there's not much common between them there either but there's a little bit so I have a zoomed in picture of this so we have our signals from channel 1 coming in here on the left and the top and channel 2 on the bottom each obviously is fed into a transistor q2 l 3q tool 2 and then they go their separate ways channel 1 up through this transistor and out channel 2 down through this transistor and out well where do they have commonality right here in the middle mm-hmm so they have this resistor R 2 O 4 this resistor R 2 15 and this capacitor C 2 15 I believe may be 16 so these are items of concerned specifically that capacitor all right anything that is common between the two channels is of concern where I'm going to test first is on these test points so there's a test point here and they're actually shared W 224 and W 225 I'll find I was able to find W 224 so I'll check there first so let's take a look at at that point and that's gonna be right there on the board you can see W 224 right here which is gonna be right there so I'm gonna power the unit up it's not plugged in yet I'm gonna power it up don't power it up now and we'll see if we get any noise on that mm-hmm test point if so well we can rule out portions of the circuit based on what we find we go it's powered on I'm gonna give it a second make sure we have a beam yep I have noise on that beam so let me probe the test point like that noise looks identical to what we have your not identical but that's nasty looking No okay that means we don't really have to go any further into the amplification circuit we have to go backwards and I said specifically that capacitor was of concern because that's common between the two I might check there first so let me show you that again so again this is the output amplifier driver circuit right so here's the main circuit for the entire circuit right look in here with the output amplifier driver and I'm testing here and I'm getting noise so that means I don't have to worry about looking post this test point it's not happening we're not getting noise introduced in the vertical an output amplification circuit we're getting noise introduced probably here or maybe somewhere back but I'm gonna focus still on the output amplification driver circuit which is again here again I'm testing here I have noise on these points so we have to work backwards so right now I'm gonna try and find this capacitor it's 215 or soot 216 difficult to tell and I'm gonna test on either point on either end of that capacitor and see if this capacitor is what's causing our issue so let me try and find that alright I was able to locate C 215 so I'm gonna start probing see if I get a signal on either side here nothing on that side some strange noise on that side so I'm back on the test point just to get it whoops locked in here there we go now go back to see 2:15 okay so this is my input on one side and that's what I get from the other side of that it's nasty you know the resistor that's in line with that I'm not good either so no it doesn't look identical to this when you have an input like this to the capacitor and an output like that I'm gonna say that capacitor is faulty so I flip the unit over we'll take a look at it see if see if it physically looks in bad condition or not regardless I'll likely want to replace that and see if that makes a difference pull the capacitor out and I'm definitely second-guessing myself at this point the capacitor is ceramic it's not a electrolytic I don't think that thing failed so I went back to the schematic and in here here's the capacitor in question again no noise on this side and then the noise is introduced on this side of it so I assume this capacitor was bad but I noticed that our negative 8.6 voltage input is coming in right here so now I'm thinking about the power supply again so what I'm gonna do and I know I did it earlier is I'm gonna check those test points again he was using the multimeter on the AC mode that's not necessarily gonna pick up the noise so I'm gonna try it with the scope I have the scope hooked up there I'm gonna power the unit on so let's take a look so let me run through the test points again we're gonna find the Sheep test points on here I'm gonna start at the negative eight point six right here negative eight point six is test point nine eighty nine so let me check that huh so look at that good deal annoys there obviously it's not regulated very well let me test check the other one plus five volts 991 let's check that well plus five has very similar thing so this should be a straight line obviously if it's filtered properly coming into the unit let me trace back a bit I can see the traces on the board so I'm gonna look at the negative eight point six rail and go back to [Music] see so this is negative 8.6 this is the inductor that's in line with it we still have noise here a lot of noise on that ending inductor in dr. actually pulls out a lot of the noise [Music] and then here's the capacitor in line okay so this is again the test point for negative eight point six let me go to the capacitor that's in line with them so that's one leg of the capacitor and that's the other way so we're just not filtering and and it makes more sense because these are electrolytic s-- so most of the board also this main board contains ceramic capacitors power supply however power supply section just on the main board contains several [Music] so here's the mainboard you can see there's tons of capacitors but a majority of them are ceramic which aren't gonna feel very often come over the power supply section you see the main transformer there the transformer and then you have your capacitors these are electrolytic s-- these are subject to fail especially if they're original then but in this unit for 30-plus years so I think what's gonna get rid of those noise is to replace all these electrolytic mmm thankfully there's not a ton there's probably 10 in there and I think a lot of them are the same size and rating so that shouldn't be too bad so I know I don't have all of these spares so I'm gonna have to order some of these I'll get some new ones in we'll replace them and we'll see if we'll see if that makes a difference I think it will so I'm gonna order these up and I'll be back when I put them in old caps are out no caps arrived What's in stone it's good there's one mine where to go all right they're all in I'm gonna take a few minutes to make sure that I have them all in the correct orientation because none of them are actually marked on the silkscreen only one of them where is it up here it's the only one I can find with the marking on it I'm pure positive none of the rest have them for some reason so the designation is the through-hole shape is either circular or square and as you can hopefully maybe tell up here the square is positive in the circle is negative so I'll have to go around make sure it'll be easiest just to test from ground to each negative and make sure that they're all in the correct position and then verify have the correct polarity on the capacitors if those all look good I will clean the bottom of the board up and we'll power it on see what we get I verified the correct polarity the caps I clean them up a little bit clean the board up a little bit I should say and I have the scope plugged in and ready to turn it on let's see if we have any magic smoke certainly hope not all right it's on so let's take a look at that test point with my scope it was W 989 up in your upper right corner there the negative eight point six volts this one that's directly tied into the vertical circuit so let's probe that it's here we go hopefully we don't get that noise that we got before now obviously I could just look at the screen on the scope and see if the noise is there I want to check this out first hey that looks pretty good so I'm on test point W 989 and rewind just a little bit to get a refresher on what that noise used to look like and I'm checking both sides of that cap cap 989 which is filtering the negative eight point six volts and it looks good let me check some of the other test points they also had some noise on them this is the plus five point two volts let me get it on there they're a little bit oh that didn't sound good that didn't sound good something just popped there I'm gonna power that down yep something just popped oh boy well let's I would unplug it and turn it around take a look Baumer so the two caps in the middle there you can see the one on the right pretty clearly right here it's bulge that one blue and then the one just to the left of that blue as well the other eight looked to be in good shape I so this is directly both of them are directly attached to two out points I was testing so I may have inadvertently shorted something so I'm gonna replace those again will power it up and we'll see if we'll see if everything's okay it's not I'll probably to dig a little bit deeper I did verify the orientation is the polarity as well as the the capsizes and ratings themselves are correct so I'll replace those I bought some few extra spares and we'll see if that works any better all right I removed their two caps and I blew these things to smithereens you can see the one on the left is a little bit worse but they're both blown that's after being installed and powered on for maybe 30 seconds so you can see the one on the left started leaking out of the bottom tore it maybe it ran down the side but it this one just blew really bad and this one as well the other ones look fine and I'm pretty confident that these are the only two that are gonna be at fault and I'll show you why they found out what went wrong so this is a clip of the power supply circuit and this is the and i-i-i i understand that the the resolution on this drawing is is terrible but again this was a scanned copy so i have to work with what i have so this is the positive 8.6 and then negative 8.6 rails and so mm I made a mistake I don't percent my fault these two capacitors are these two capacitors and if you look at the orientation the positive is to ground in both cases and they make sense because this is the negative rail so that was a complete oversight on my on my part 100% my fault I I wasn't thinking about that negative rail when I was installing the caps I just wanted to get them installed and I tested tested each pin of the of the through holes for the negative and I put the negative leads of the capacitors in each but these two I obviously needed to switch around so I've removed these I installed two new ones with the correct orientation with positive to ground and then I'd have to be negative 8.6 rail I just hope I didn't damage anything permanently on the board or any other components it's possible that I did we'll find out so you know part of being electronics technician is making mistakes but to become a good successful electronic repair technician you have to acknowledge and learn from your mistakes so I assure you I haven't made that mistake before and I will not make it again so let's move forward with the testing do the exact same thing I did before I'll power it up we'll check for noise and I'll leave it on for a few minutes to see if I hear any fizzling I sure hope I don't again I may have fried something else we'll see I hope not so here goes nothing I'm gonna power it on right now no immediate smoke so let's take a look at these two caps let's start with 989 all right it's there's very minimal noise on there little bit that may be expected especially compared to where we were at before and then I'll check the other cap it's a bit of noise on there a little concern about that let's tech check the actual test point w 989 which is the negative 8.6 a little bit of noise little bit we definitely had a lot worse noise before change the ya know it doesn't look like it did before so we'll see check a few other test points this is the positive 5.2 that's the same so that may be a standard amount of noise Emily just powered on for a few minutes and make sure I don't hear anything sizzle and if not we'll be back to look at the look of the signals and see if that noise has gone if we have more work to do here's a 10 kilohertz signal going into both channels looking pretty good again a little touchy on a triggering so that I might need to look at that a little bit but that could be a source issue I don't have a generator at this moment let's cut that so I have a 10 kilohertz sine wave going into both channels and it's looking pretty good the triggering looks a little shaky but I mean you can see the sine wave pretty stable were there in both channels the they overlap very well so I would say there's no adjustment needed as far as uh any amplitude adjustment the gain on either channel looks pretty proportional pretty equal to each other so that's good I don't think I can go any higher as far as any higher of a frequency because I don't have a signal generator at the moment what I'm using right now is a 10 kilohertz tone file from YouTube via my laptop I'm plugging in a simple audio cable to the headphone jack and then using Oh disconnecting them using the probes to connect up to those and that's what's giving me my signal if I go to much higher I think the capabilities limitations of the computer itself show that the it's just a lot of noise on here and this is a 20 megahertz scope I highly doubt that the scope can't handle 50 kilohertz I think it's it's more of my source so I'm gonna call this one looking pretty darn good right now we have this to get rid of the noise with the caps and each signal is pulling up the each Channel is pulling up a trace very well everything seems to be working pretty well tensity gives out and maxes out focus is adjustable no can see that clearly out of focus there and there I'll have to play around with a few others the volts per division obviously working trigger means tough to trigger there and our time scale knob is working horizontal position I guess I haven't checked that yet that looks pretty good I checked both vertical positions I am an auto trigger you can put it in the norm and it lets me manually trigger and I should be able to you know feel the trigger obviously I prefer auto can change the slope so you'll see the trigger changes based on the the rise versus the fall of the slope so that's working everything seems to be working pretty well on this I'm gonna call this sculpt this 22:05 20 megahertz Tektronix scope fixed and I'm gonna get it cleaned up I might clean up these dials verify everything's working again and then clean it all up externally get it back in its case and then make room on the bench for the next scope I have a 22 13 I just got in so I'm ready to work on that as soon as I can so we'll see on the next one thanks for watching