Trying to troubleshoot a PSU - Page 1
Forum section: electronics / repair

author: diegosfb
author_rank: Contributor
timestamp: January 14, 2015, 09:11:36 pm
content: Hi guys,I am trying to troubleshoot a Bench PSU.When a set an output voltage, lets say 7V. it sets 7V on the output but a few seconds later you can hear the relays switching and that value changes to maybe 3.4, then to 12 etc. (there is no patter on what voltage it switched to).My first guess is there is some kind of problem with the feedback error amplification stage; but I am not sure what is the best way to meassure and troubleshot this kind of thing (I am no repair expert)I am attaching some schematics of the PSU. If anyone has some advice as to where and what to start measuring I will appreciate it.Best Regards,

author: RJFreeman
author_rank: Regular Contributor
timestamp: January 15, 2015, 05:02:13 am
content: looking at the schematic page one (page 2 is pretty well the same only with the switching facilities) , think of this circuit in three major parts:1) there is a power supply (yes I know, bear with me here) in the top left hand corner - this generates +12V, -12V, -6V and a +2.5V reference (off the TL431).This consists of N3, V1A, V2A, V3A, V4A, V5A, V6A, V7A and associated R's and C's and is power by Transformer windings N52) Voltage regulator circuitry below this which regulates the main output and provides current limiting and voltage regulation - this uses 2.5V off V5A as a reference voltage and the active circuitry is powered by section 1.This consists of N1 and N2 V20, V21 and V22 - V11A V12A V13A, V14A and is powered from Taps N2, N3, N4.the transformer tap is selected by section 3 - depending on the output voltage3) tap selector circuitry - this changes transformer taps for the power supply to minimise heat dissipation on the regulator pass transistors. it switches to a lower voltage tap when the output drops - either because the output voltage is set low, or because the current limit is limiting the output voltage.This consists of N4A, N4C selecting either Relay K1A or K2A via Transistors V25A or V26A.ok if the tap switcher (section 3) was changing incorrectly I would expect the output voltage to stay in regulation, so my main suspicion would be that the relays are switching because the output is changing rather than the other way around.potentiometers often get noisy so I would first suspect the pots W6, W6A, W5 and W5A - maybe W8. measure the voltage coming off these (say at the junction of R54A and C13A) see if this varies.the reference voltage could also be faulty as could the power supply rails from section 1, so I would also check the supply rails from section 1I understand this is a dual power supply (ok, triple if I include the 5V rail) I presume when running in independent mode that only one rail is playing up?

author: RJFreeman
author_rank: Regular Contributor
timestamp: January 15, 2015, 05:19:00 am
content: By the way if both sides are playing up then it is likely that your series/parallel/independent switches are noisy.It is hard to see from the pictures I can find on the internet, but if these are push-buttons mounted on a PCB then check the soldered joints to the switches as I have seen cases where soldered joints on push buttons have fractured over time.

author: diegosfb
author_rank: Contributor
timestamp: January 15, 2015, 01:34:05 pm
content: RJFreeman;Thanks for your advice. I will take it into account and tomorrow I will make those measurements you recommend.As a side note, both outputs are "jumpy", but they seem to jump independent one from another.Tomorrow I will make more measurements, if anyone has some advice in the meantime it will be very welcomeBest Regards,

author: oldway
author_rank: Super Contributor
timestamp: January 15, 2015, 04:55:57 pm
content: You have 2 led's on your PSU, one CC and one CV.What is the status of both those leds when output voltage is wrong ?What's the setting of your current potentiometer?Have you the same problem with this pot. set to max. current ?

author: diegosfb
author_rank: Contributor
timestamp: January 15, 2015, 05:01:12 pm
content: CC led is off (not current limited) and CV is on (is voltage controlled)Issue occurs in any setting of current limiter knob.Whats more the issue occurs with or without load.

author: oldway
author_rank: Super Contributor
timestamp: January 15, 2015, 05:05:47 pm
content: And in current limitation (CC), does it work correctly or not ?Check reference voltage on V5a (TL431) = 2.5VCheck trimpot W6A for bad solder and bad cursor contact.Check pot. W5 for bad solder, bad cursor contact and bad wiring.Replace C13A (1µF 50V)Check N1 (741C) for bad solder or bad contact if there is a socket.

author: RJFreeman
author_rank: Regular Contributor
timestamp: January 15, 2015, 11:49:52 pm
content: Quote from: diegosfb on January 15, 2015, 01:34:05 pmit would depend on what the switch settings are at the time.If the switches were set to parallel or series at the time then a fault in one channel would certainly affect the other (and not always in a obvious way) but if it was set to independent, then since both sides have independent circuitry, independent power supplies, regulators and tap selectors, logic would suggest:1) Coincidence - both sides have the same fault. this could come about if there was a weak or potentially dodgy component used in the manufacturing run.This could also come about from mechanical type components like connectors (internal or external), switches or potentiometers suffering physical damage from a past even or corrosion/oxidisation due to exposure to humid or less than ideal environment.2) a common shared part of the circuitry - in this case the switches are common, and I would also consider anything upstream (power transformer perhaps, although I would expect this to result in a sag rather than increase).

author: diegosfb
author_rank: Contributor
timestamp: January 16, 2015, 02:35:48 pm
content: It affects both outputs when running independently from one another.I have measured the supply voltages12V rail measured 11.85V (not as accurate as a 7812 should be but seems to be good enough)-6V rail measured -6.1V (OK)-12V rail measured -15.33V (seems to be way off, although it wouldn't explain any of the issues. Maybe V7 is not very accurate)I checked the capacitor C13, it is not bulged, or damaged in any way.I resoldered all joints on W5, W6, W9, SK2, SK1, etc.The next steps I plan for tomorrow are measuring pin3 of N1 (741 opamp)Measuring the voltage at R45Measuring the output of N1 (I am not shure what I expect to get here )If anyone has any other idea it will be welcomeThanks for all your help guys.

author: RJFreeman
author_rank: Regular Contributor
timestamp: January 16, 2015, 11:20:45 pm
content: Quote from: diegosfb on January 16, 2015, 02:35:48 pmSo that takes us to the question of, is it something common or is it a co-incidence due common components failing on both modules (a noisy potentiometer would explain jumping voltages while not necessarily testing faulty, and I have had problems with Pots not being overly fond of DC).If it was on my bench I would be considering replacing the voltage set pots on spec and/or jiggling them, wobbling them pushing the shafts from side to side to see if mechanical abuse (aka 'percussion testing') caused the fault.QuoteYou should be getting at least 10% accuracy on a Zener, and often more like 5%, so beyond 13.2V rings alarm bells, and if the -12V rail is as low as -15.33V on both modules, then that is very suspicious (or, it could just be that some dim-wit at the factory loaded a cassette of 15V Zeners instead of 12V Zeners)Often in trouble-shooting it doesn't pay to over think things.I have a rule of thumb that; if you see a problem/anomaly, fix that before you go looking for problems you can't (yet) see - even if you can't immediately understand how that would cause the fault you are chasing.In my experience this approach works at least 8 times out of 10.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: January 18, 2015, 11:16:22 am
content: Quote from: RJFreeman on January 16, 2015, 11:20:45 pmI support RJFreeman's view. In fact, I have fixed yesterday a very similar PSU with similar symptoms. I had 9.35 V instead of 12V at the regulator. The culprits were the 220uF capacitors (2C1A, 2C2A, 2C3A in your case). They were not bulged and apparently OK, but when I desoldered them, I found that they were completely out of specs. After replacement, everything was fine.

author: diegosfb
author_rank: Contributor
timestamp: January 19, 2015, 01:51:28 pm
content: I will give that a try.I will have to wait until I receive the parts (zener, caps, etc.) and let you know how it wentThanks so much for the help guys