DR #10 - Power Supply Repair - HP E3612A

hey everyone welcome to another duality repair video this is Nick tonight I'm gonna be repairing a benchtop power supply so believe it or not I don't have a benchtop power supply every electronic technician should so I bought this off eBay for parts and assuming I can fix it I'm gonna keep this one so it's obviously not in the best cosmetic condition a few dings on the cover some tape adhesive residue there but all that should be able to come off so this should look pretty clean if I can get it repaired I'll clean it up so let's just jump straight into the symptoms so I'll power it up obviously I have my multimeter hooked up and I started off with the voltage and current set points all the way counterclockwise so it should be at zero and so we have our voltage output here and you can see my meter here it's reading it's tracking very well and then of course the zero milliamps so let's see what happens when I turn up the voltage here and I should have mentioned the specs 0-2 volts excuse me zero to sixty volts DC at 0.5 amps that's range one range to a zero to 120 volts at 0.25 amps so not a high-powered device but certainly a really nice voltage range so again you can see the voltage tracking with my meter here let's start cranking it up a little bit so I am on range 1 the 0.5 amp range which is 0 to 60 volts so I'll full turn of this pot excuse me this is a 10 turn pot so a full you know the full cycle of this should bring me up to 60 volts DC and so let's watch again tracking well we're at 0.9 volts for at 1 volt and 1 volt that's looking good you can't tell how much I'm turning it but I am turning it a lot but I'm very near the range already and I should be a much higher of a voltage so this is going to be there it is so I'm at the very end and actually I turned it on and tested it earlier and it was getting to eight volts so this is just crazy so obviously it's not able to reach the setpoint voltage at all and the other thing that this thing has is so it's got content constant voltage mode which is just you know whatever your voltage is set to it's gonna keep that voltage maintained regardless of the load assuming it doesn't go past your 0.5 or 0.25 amp rating and then it's also got a constant current mode which is set by this button here so you should just be able to hold it in and adjust the current knob for your desired maximum current and so if you were to adjust this to say 100 milliamps the voltage would it would put out whatever your supply is whatever you set point is unless it hits that 200 million mark at that point it would do whatever it has to do to keep the current at 200 milliamps or less so if it has to drop the voltage it'll drop the voltage and so I should just be able to hold this button and turn it and it should go all the way up to 500 milliamps as a as a constant current setpoint and it's not it's hardly doing anything you can see it move just barely so none of this is working at all this is actually behaving worse than it did earlier so something might be getting worse I'm gonna power this off now that we've seen symptoms let's open it up I do have the schematic but let's open it up and take a look at the internals I have no idea what we're gonna find in there here's the supply with the cover off and it does pass the smell test but it does not pass the visual test so they look for any any signs of heat damage anything like that I didn't find anything like that I didn't find any bulging capacitors but what I did find was this one and you can't pick it up too well on the camera but this one right here has definitely leaked out of the bottom so you can maybe pick up this residue if you look on this chip see if I can clean some of it off a little bit so you can see I'm scraping it off there's a bunch of residue all around this area and it's emanating from this capacitor the base of this capacitor I can see tons of a really dark looking sticky liquid and that's that's gonna be a blown capacitor more than likely so let's take a look at the schematic I guess can look around the supply really quick so obviously we have the main transformer coming in huge only 30 Watts 30 watt power supply so it's a pretty big transformer for that we have a transistor here and here those are going to be our drive transistors and obviously just a couple of capacitors here this one's gonna be our main filter capacitor from the from the supply to ground so you got that mainboard on the bottom with all your power capabilities and then you have the front panel board here that's got all your controls on it you can see it's got a few chips on there for your control there and really there's not that much to it so take a look at the schematic here so this is the silkscreen tells me this is capacitor c7 so let's take a look at where that's at so first of all the schematic in general we'll start off here on the Left power coming in AC power coming into the main transformer here and it's got three legs three outputs so we have one supply up here at the top one in the middle and one on the bottom we'll start at the top one at the top is the as its labeled the display power supply so it comes in through a fuse has its own bridge rectifier has its own regulator and then it spits out five volts and so that's what's going to be going to the display so we're gonna have among other things some communication or some other signals we're gonna have five volts from this board to the display second part of the Transformers output here is this section and as you can see it's labeled reference and by supply and so same thing as up here we come in through a fuse and actually two fuses and we have a bridge rectifier again and and so we're gonna come in through this upper portion through this regulator here and we're gonna generate 12 volts as well as five volts so this regulator is actually going to generate the 12 volts and this Zener right here is gonna regulate or create the 5 volts so these are going to be reference signals that we'll see you later on down on the circuit and then down here on the bottom leg we have another regulator which is going to output our negative 12 volts and then you have your bottom portion of the circuit which again has its own fuse now doesn't have a fuse but this is our current selection switch so either 0.5 or 0.25 right here and again it's got its own bridge rectifier and we come in here's that c2 that's this big capacitor right there c2 that's the one right across the output of the bridge rectifier and so this is going to be our actual power output our power supply right here and so it's got this part labeled into several sections we have the series pass transistors right here okay and so this is what's gonna be driving the output and we have our actual driver which is going to drive the transistors and over here on the right we have our voltage error amplifier so it's going to have our voltage control which is just our voltage control knob on the front and this is gonna feed an input to this op amp and the op amp is gonna pull in it's gonna sample the output and compare it to what our voltage control knob says it should be and output accordingly and so that's going to come out here through the bottom to this driver transistor q2 which is going to drive the series pass transistors and so you'll see here down here in the bottom and the voltage error amplifier we have +5 volts that's reference voltage from up here so very very important this is very necessary we have plus 12 and minus 12 which is from the reference down here so plus 12 and minus 12 in the drivers circuit and then we've got these other circuits as well as other portions of the circuit so we have a current error amplifier which is just like the voltage error amplifier but when set in current constant current mode this is going to be in effect and this if it so this is going to sample the current at your output and if it exceeds the constant current rating whatever you set it to this is actually going to cause the driver to reduce the output voltage and then we have a few other tiny circuits here parts of the circuit the meter sir meter input circuit so this is what's measuring the current and the voltage and then we have our CV or constant voltage CC constant current indicator circuit so this is literally just going to tell us if we're in constant current or constant voltage mode and so these are just getting feedback from the circuit telling you and they'll they'll display and they'll illuminate an LED when certain conditions are met so now let's take a look at again this was c7 the one that's leaking where is c7 well I found it it's up here in the reference supply and so c7 is right here and you can see it it's filtering this this output to this regulator right here which is feeding our 12 volts and our 5 volts so extremely important now where does the 5 volts used again it's used down in this voltage error amplifier circuit if this 5 volts is all messed up if it's unfiltered we're gonna get really goofy signals coming into the op-amp and it's not going to know how to drive based on this reference signal so this makes sense for why we're getting a very strange output voltage it's also again part of the positive 12 circuit you know what else uses that constant current the current error amplifier circuit uses plus 12 and so does the driver and so if both of these voltages have some issues with them these are not these circuits are not going to behave normally so it all kind of makes sense I didn't I took a quick look around I didn't see anything else wrong with the board obviously these are the same capacitors here this is c-13 let's take a look at where c-13 owes c-13 is just below c7 so here's c7 here's c-13 c-13 is gonna help excuse me it's gonna help filter out though negative 12 supply so they are they're actually they're not quite the same this is 330 at 35 volts and this is for 70 50 volts so they're different ratings but they're they're they basically have the same importance so this one didn't appear to leak but if I have a spare I'm gonna replace that as well but first things first I'm gonna take this out clean the board up replace this and see if that helps I'd say that things toasted that's nasty so let's go ahead and get that replaced so this is a 330 330 micro farad's at 35 volts is an itchy con so it's a good brand and 105 C I think I have a direct replacement one extra from an amplifier that I was every campaign a while ago and 330 at 35 at 105 C I do the package size is definitely different it's narrower and longer so the new ones on top they're much smaller radius but shouldn't be a problem there's plenty of there's no restrictions on height here because the highest thing in here is the tallest thing and here's the that main filter cap and the transformer this is nowhere near that so this is going to be good the SOL this will help so I'm not sure how II didn't notice this before but maybe you did you can see the the copper right here the copper trace is exposed you can see where the capacitor leaked and it leaked out over this trace and adjuster at eating the trace the cover of the trace here so the trace is it looks to be intact but it's no longer coated so that's that's interesting I'm definitely going to examine the that path and make sure it does have continuity I'm not aware of any process to cover that up again so I you know may just have to live with an exposed trace like that it's unfortunate definitely I want to make sure I clean up clean all of that up because any remaining electrolyte there will eat up any other trace around it I can see a link over here this is just the mount for the heat sink for this that's one of the rectifiers here so it's kind of starting to eat that away too so I'm gonna clean up as best I can I may even have to remove it just to clean it up really well but yeah that's bad not just eight right he heats straight through to the trace there so interesting okay we keep moving here keep cleaning all right I cleaned the board up install the new capacitor and I powered it up so let's see what happens here no no change so that's probably half that's range right there it should be at 30 volts by now let's change the range doesn't do anything should just double it anything on the current currents that nope that doesn't change either mm-hmm so we have another issue that's okay that capacitor is obviously bad let's look at it one more time you know that nasty so what's next let's just take a look at the schematic again and I saw some test points on there so we'll check voltages check the reference supply voltages some other test point voltages and just keep moving forward we'll figure it out okay first place I'm going to check is between test point one and test point five so that's the positive 12 and positive five volts of the reference supply since those are the two voltages that are directly in line with this capacitor that was leaking I'm gonna start with those and you know supplying the reference voltage is extremely critical especially for a voltage air amplification circuit so let's check that out I have the jumper here from test point one which is positive 12 so I'm gonna go to test point five which is positive five so we should have a seven volt signal here test point five is right here and nothing it's no good so again both of those are driven by that same regulator one the twelve volts is driven directly by the regulator and the five volts by de Zener but hmm so that's not getting voltage so maybe there's something wrong with this this is the the regulator here I'm gonna go back this has the fuses right here f2 and f3 excuse me f1 and f2 let's check those out how are the cement off here let's jump between the fuses f2 and f3 are don't zoom in you don't need to see the meter or count anybody checks see you f2 and f3 they're certainly expected okay well you know what that's all that leaking capacitor right there and who knows what happened let's take a look okay f2 is open and f1 is open sorry you can't see that so I checked these two fuses here they're both open and let's make sure my meter is working correctly and I'm not going crazy here let's check out three f3 is the fuse for the display power supply which we know is working because the display displays so that's right here yeah so that's fine so these two fuses are blown I found some Pico fuses in a spare board that I had laying around they were 2 amps so I'm a little worried about that but you know it's not terrible I'll turn it on and if I smell anything here anything funny I'll turn it off right away so let's give it a shot though let's plugged in let's power it on interesting so we're in constant current oh wow okay so this is this is different we never had this before we're in constant current mode I believe because our current knob is all the way at the bottom so let's see if we can it's not gonna let us adjust the voltage let's see if we can adjust the constant current setpoint yes we can the only thing I don't like that's definitely off scaling is off so you see we start at 242 milliamps that's all the way counterclockwise and we hit 500 and I can go well beyond that 726 to 236 so it's about 500 so something's a little off there maybe it's just the pot needs to be just a little bit I don't know it's definitely different but let's tweak this up to 400 just so we can see if we can get an output okay so it at least is reading zero milliamps output let's see if we get a voltage much better I barely turned that and I'm getting an output voltage here two volts three volts still not registering anything on the current output which is good it's not tripping anything no shorts and I maybe turned it one or two full revolutions and we're at 20 volts that's good probably good time to plug in the meter and see if it's accurate so let's do that I'm gonna part off just while I do that and since it's been on I don't haven't smelled anything so that's good things seem to be behaving better anyway of minus the constant current setpoint is obviously off let's put these up to the meter see if what the supply is reading is accurate or at least as accurate as my meter is which you know this isn't the bus but it does do the trick put the meter in there so I'll starting in the 20 volt range and I'll turn it all the way down again it's powered on okay set it to 5 volts 5.0 5.0 4 volts on my meter that's good let's go to 15 15.0 for good and go up let's go to 50 see if we can get up that high still not reading much on the current is there a point where one to two milliamps not a big deal okay 50 on the dot okay that's great let's see if it'll go to its full range at the 0.5 amp set point of sixty volts it gets there and I'm reading 59.99 and see if it goes past it does so it's just gonna be your different current set point so we should be able to be able to get up to 120 ideally I'm cutting out there maybe I have to be in the other range and go back down to 60 and change the range okay at 60 70 80 90 100 10 15 okay it's a little bit beyond 120 what does it actually get to so all the way all the way clockwise gets 236 volts and how accurate is that fantastic 130 6.5 on my meter 130 6.9 yeah they're within 100 millivolts really of each other it's fantastic that is good so we've definitely resolved one issue the capacitor caused the the fuses to blow replace the fuses and now everything's working we have our reference voltage we have two it may not be perfect we haven't checked that yet but they're definitely working the output is accurate and is able to output the full range was there 120 volts they don't have any shorts because we're not I'm not drawing any current at any voltage output which is good there's at least still one issue remaining and that's this constant current again I'm all the way counterclockwise that I'm starting at 242 milliamps and fully clockwise I'm at seven hundred and thirty two milliamps curious what happens if I change the range let's change the range so at the 0.25 amp range our constant current setpoint goes from 245 to 490 and again at this range the 0 to 60 volt half amp range we're at 245 just 737 so that's interesting something's still potentially wrong in the current error amplifier circuit so what want to take a closer look at this circuit here so here's where you're setting constant current alright and it's gonna be reading based off these resistor divider here and then your other point comes in here this is where you're getting a current reference so we'll take a look at this concert current set circuit as well as the meter input circuit next I may have found the issue of why the constant current setpoint is reading abnormally specifically reading above zero when it should be zero so if we take a look at the schematic we're here on the right will see our constant current setpoint when I depress the switch it's in this upper position and it's measuring between this pin and positive s that's what it's measuring for that constant current so let's follow it over here and we're reading right off of the centre tap of the potentiometer r20 so when I was adjusting our 20 that's how I was able to get zero point reading here which is good but it did it just my range so my range you know and no longer went from zero to 501 from you know when I was able to get it to read zero it was like zero to 100 or 200 maybe millivolt range so that doesn't work so if we look at this specific part of the circuit here there's only a few things really that could cause this and so again it's reading between positive s and the center type of this potentiometer which is right here really so really it's dependent upon voltage drop across this right here and so what could effect that well the potentiometer itself or theoretically r40 if this is you know defective in some way that would definitely cause a difference in the voltage drop there so let's take a look at our 40 and it's way up here on the board and you probably can't see the band's but it is red blue violet orange which is 267 K and if you see there I'm in the 210 range and I'm at 42 K so that is not reading correctly let's go to a 200k range yeah 44 k not even close so what does that maybe 20% so if we go back to the schematic we go from a potential the resistance across this you know tp6 or test point six to test point seven of a little bit over 300 K hmm so what is that 317 K now with this at 44 K we're now the maximum drop is only ninety ninety four K and so the ratio between this resistance and this resistance is is almost equal now where before this would accommodate most of the voltage drop this is now accommodating most or at least half of the voltage drop and that's not what we want so I already looks for my bin I don't have a 267 K resistor I don't have anything close to that I couldn't find a pair of resistors that would get close to 267 K so really I'm just gonna have to order one and install it and we'll see I looked at other components in this part of the circuit and everything seems to be fine these potentiometers seem to be working fine our 33 is exactly what it should be and so and same as our 28 those are both what they should be the other thing to note if you look on the board here so here is the capacitor that I replaced that leaked here you can see Hova you can see the bare copper trace there worried away and then here's our R 20 and here's our 40 and some of that electrolyte although you can't really tell it it was there was a very thin layer of film kind of all around this area including over our 20 there are 40 excuse me so potentially could have got damaged due to that just to be sure I'm gonna remove it make sure it's reading 44k outside of the circuit as well highly confident it will and if so definitely an order that and replace it so let's remove that quick I remove the resistor and measured it and determined that that is not the fault it is reading as advertised said about 267 K ohms and so I reinstalled it really do the schematic and I realized what I was measuring although I was measuring across R 40 I was actually measuring around the circuit here through all these resistors back to the other side simply because it's the path of least resistance so if you add all these resistances up it's much less than 267 K which is why that's what it was measuring it's in the 40 to 50 K ohm range and I can prove that mmm by measuring across our 40 and adjusting the current control knob here and you'll see the resistance change so let's go across our 40 we can get there there we go about 44k and I'll adjust the current control knob you'll see it go up to 48 almost 49 K and go back down and back to 44 K so that's what I was measuring there of that resistor just fine mmm what I did notice let me power back up so we'll see see set depressed AI my milliamp reading is way off and more concerning is the fluctuation it's not just a fluctuation it's a constant decrease and it starts to slow down but originally I thought it might be a capacitor simply discharging and obviously that shouldn't be happening but it's kind of what I thought but I think I know what's going on so where I'm getting voltage is right here in between our 40 and our 20 I'm reading voltage because again if we look over here a constant current set is reading between plus positive s and the center tab of our 20 mm so that's just really test point six to the center tab of our 20 which means there has to be a voltage at the base down here of our 20 so when I measure voltage here I get a voltage when I measure voltage on the other side of our 40 I get no voltage how is that possible a 5 volt rail is up here we're coming down through all these resistors and we should be dropping all the way down to the positive s a test point 6 but yet I'm getting a voltage injection in between our 14 or 20 didn't really seem possible so then what I did [Music] so I'll put my positive lead on the 12 volt rail and you can see that when I go to test point six sorry my hands coming in the way there all right I have my positive lead hooked up to test point six here and then I'm gonna probe around here so let's look at our 40 and there's our voltage around 300 volts millivolts excuse me noha volts okay and on the other side of our 40 nothing so somehow I have voltage coming in here and then drop him here does it make sense it's actually dropping across our 22 plus s so how is that voltage getting here there's no place for the to be injected here so with my probe on test point 6 I can probe around the board and that's just crazy you probably can't tell let me zoom in so let keep the voltage in rind in mind look what I'm probing that's just the surface of the board that's not a contact that's not solder that's nothing so there's a voltage between test point 6 and the look at that one start off at 12 again I'm just touching the board so what's what's happening here well the cap that bursts right here the electrolyte it didn't just kind of eat away above this trace here it there's some residue all around this area of the board and I wasn't able to clean it effectively having all these components mounted so it's it's forming a conductive path between these this especially between this exposed 12 volt rail and anything that's conductive and and so what's happening is the 12 volt rail is being bridged partially across the board via the electrolyte to this potentiometer this is our 20 and probably other components and so again if I probe this is to the board this is test point six to the board surface as I go further out you'll see I'm still touching the board but I'm at zero volt simply because I'm outside of the spill zone of the electrolyte so you can see where see it's dropping if I go further out dropping go way out must have no electrolyte there and now it's I go way over here should have nothing yeah there's no electrolyte over there so what I'll need to do is remove all the components in this area and really clean the surface of this board here to remove all of that electrolytic deposit that's the film that's just sitting on the board here again I did the best they could I cleaned off this area pretty good but I'm gonna have to clean all this up really well and I think that's gonna resolve this issue and after that we're gonna be good to go and this power supply is gonna be is gonna be ready to be put into service here on my bench so I'll be back after probably after I clean these or after I remove all of these on and I'll clean it up and we'll test it once I get back I pulled most of the components in this general area where that capacitor leaked including c-13 over here which as you can see also was leaking and you can probably see a little bit of a corrosion here eating and starting to eat away at the cover of the trace there as well so I'm going to replace this while I'm at it I clean the rest of the board up it looks much better including so you can see here if i zoom in that's about as close as it gets this is the center tap for the R 20 potentiometer and here's the 12 volt rail you can see how close they are all it takes is a little bit of electrolytic fluid and really it's this exposed trace to there so you got about I don't know four millimeters just just a few millimeters of space there where if it's coated in that electrolytic film it's gonna be conductive at least somewhat so that's I believe that's what was causing the volt voltage leakage across there so I feel pretty good about removing those cleaning it up I think this is gonna work I'm gonna go ahead and put everything back together and I'll power it up all right the board's been cleaned I reinstalled all the components including a new capacitor there for the other one that was leaking so a moment of truth let's turn it on oh it's a good start so I'm holding in the CC set and I have zero displayed there see if I can get a current reading can can great goes up to five twenty and back down to zero and let's go to the other range the 0.25 amp range so reading zero and 266 just above just above a quarter nap looks great alright so that's working well let's make sure the voltage output is still working on these 60 volt range it's pretty good 62 and a half oh my does it actually go it's pretty high let's go to the other range 0 to 120 volts 120 mm great all right so it looks good last thing to do is to hook up a load to it make sure it can take that and then this thing is fixed I am gonna do some cosmetic repairs so these this one's actually busted you can see it's kind of bent in pretty good I don't know what happened there it got smashed it almost looks like it got melted on the tip there this one looks pretty worn as well this one's in good shape but I'm gonna replace all three is in three these at the same time I'll clean up the really clean up this front faceplate with some alcohol as well as this tape residue on top and the bottoms missing feet so I had those but otherwise it's it's in really good shape so let me hook up a load and they'll test that all right before I hook this up to load and test the output I thought I'd calibrate the voltage output first so service manual has you connect the DVM from the multimeter across the output adjust the voltage and low current mode until your multimeter reads 120 and then adjust our 10 on the back of this front panel until your front display also reads 120 so do that now found our 10 and let's adjust I was messing with some of these pots earlier when I was looking at the constant current setting I believe I adjusted this one as well so that's probably why it's so far off okay 120 120 so let's just go to say 90 see where we're at there ninety and ninety point one on my meter very good and let's go to higher current range see where we're at at a sixty sixty here and sixty point one on my meter very good let's go to thirty 30 and 30 excellent 15 15.0 and 15.0 let's go down to five volts see how accurate we are down there five point zero five point zero man let's go to two golds how about three point two that's a pretty common voltage zero point two three point you all right I'd say the voltage output is calibrated very nicely there is a calibration procedure to adjust or calibrate the current rating as well that requires a point 1 ohm at 1% power resistor and I don't have one of those at the moment so I'm going to leave that for now so I'll just skip to hooking up a load and we'll see where we're at there all right the only load I really have for testing at this time our for a excuse me 8 ohm 50 watt power resistors for testing audio amplifiers so I'll use those in series to make up a 32 ohm power resistor and so I'm not gonna be able to test the full voltage range but should be able to test good enough for this purpose so let's start off let's go to 5 volts that's handily just fine 154 milliamps let's go seven and a half volts still okay 227 oh yeah ten volts 300 milliamps and let's make sure that at about 500 well let's see at 528 milliamps we should cross over from constant voltage mode to constant current mode and that'll drop my voltage R T my voltage constant at that current so let's see again 528 we should cross over to constant current and my voltage will no longer increase almost there we're at 17 okay so we're there no what happens at 5:20 yeah and you can turn it all I want voltage will not increase past 17 point two okay so that's working let's check the low current range and really this should behave the same actually they'll behave a little bit differently because low current range my current limit is 270 so now I'm gonna drop out even earlier so let's make sure we drop out around 270 would cross over to constant current that's gonna be less than 10 volts and there we go at 260 we cross over and that's about eight point seven volts so it is working beautifully and this repair is done to summarize or recap no pun intended I did have to recap to these two on the power rails I see here look at the schematic c7 and c-13 both of these so these C sevens feeding me + 12 + 5 in the reference and bias supply and C 13 is feeding the minus 12 not feeding but filtering so both of those leaked c7 was by far the worst c7 was C 13 was starting to leak as well so definitely needed to be replaced and then as well as you know the leaking here caused havoc around the you know close the proximity of that circuit as well it wasn't just the capacitor itself it was the the conductive electrolyte that leaked out crossing over traces especially the exposed trace and onto the potentiometer here r20 causing the constant current read back issues so this one's fixed all done stick around for the next video I have several more coming up I'll have part two of the Tektronix 22:13 a repair coming up very soon waiting on that board and then I'm very excited I have another Tektronix tes 4:20 coming in that I'll be looking to repair as well as a lot of six I believe Agilent digital multimeters benchtop multimeters so that one will be interesting so stick around for those give me a thumbs up if you liked it questions comments in the comments and subscribe if you haven't subscribed thanks for watching we'll see in the next one you