Power supply problem, maybe op amp related. - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 20, 2014, 10:58:26 pm
content: I have two Farnell linear power supplies, an H60-25 and an H60-50.One is working, the smaller one, the large one is missing a power supply board and has another issue.There's a control board PCB that's exactly the same part number and revision as the one in the working H60-25. I decided to swap it over into the working PSU in order to test it. Using this other control board I get near zero volts out from the known good H60-25 PSU. I visually compared the two boards for obvious issues, then went in on the component level. I swapped out all electrolytics as a matter of course, for new ones. I removed and checked all diodes and Zener diodes and found an immediate issue. D1 had been changed to a 3.3 V Zener diode and it should be a normal rectifying diode. The other diodes and Zeners checked out fine under test. I also removed all the semiconductors bar the op amps. these too checked out fine with a Peak semiconductor tester. Resistors and pots gave similar readings and were the same value across both boards. I changed the Zener wrongly fitted at D1 for an IN4007 and tested again, but still no control of the supply. I am now thinking something's happened to an op amp, maybe as a result of someone fitting a Zener at D1, maybe coincidence. I am not good enough to work out what fitting a low wattage 3.3V Zener at D1 might do.... I wouldn't know how to start testing an op amp, in or out of circuit, and there are 3 off CA741CT used. I swapped one over between boards. The "good" board kept on working. The "bad" board remained inoperative. It was IC1 that I swapped. I managed to lift a trace doing this, so if there's a none substitution method I'd like to try testing the other two. Is this possible please? The schematic for the board is at:http://www.chriswilson.tv/schematic.jpgI have taken voltage measurements where this PCB slots into an 18 pin socket using PCB track "fingers" to carry the signals from and to the board. Some are radically different between the working board and the none working board.I have altered the schematic to make the pin out numbering clearer. Please see the one attached below and the modded one still athttp://www.chriswilson.tv/schematic.jpgOn the working board I see relative to the 0V rail on pin 5:1: -15.5V2: -39.7V3: .017V4: 6.27V5: 0V (negative lead of meter here for all measurements)6: 1.55V7: 25.23V8: 15.22V9: 0.0V10: 8.64V11: Blank, not used12: 6.28V13: 0.0V14: -.348V15: 0.0V16: Blank, not used17: 8.63V18: -0.347VOn the none working board :1: -14.98V2: 0.417V3: 1.47V4: 6.22V5: 0V (Negative lead of meter here for all measurements).6: -9.23VAll the rest of the voltages were pretty much identical to the good board.Measurements done with the supply set to give 40V out in both sets of tests, which the good board held steady.Voltage out from the PSU on the bad board was under 0.5V, with the settings left the same (40V out set).So pin 2 was -39.7V on the good board, but 0.417V on the bad board, pin 3 was .017V on the good board, and 1.471V on the bad board. Pin 6 was 1.55V on the good board, but -9.23V on the bad board. Does this mean anything worthwhile?I also measured ACROSS C3 (4.7 uF 63V) and on the good board saw 39V and on the bad board 1.4V.I have put an interconnection schematic at:http://www.chriswilson.tv/schematic-general.jpgThanks for any help here

author: Fank1
author_rank: Regular Contributor
timestamp: April 21, 2014, 10:45:24 pm
content: First check the voltage on the base of VT2, if it's very low then check the voltages on pin 6 of all of the IC's.Something has this in shutdown and that will help us in where to look next.

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 21, 2014, 11:12:13 pm
content: I took some more measurements this afternoon, including at the base of VT2.On the working board, measuring voltages between the 0V middle rail and the cathode of D3 I see 1.3VOn the none working board I see -9.7VOn the anode of D3 I see 2.1V on the working board, and -8.6V on the none working board.On D1 the none working board shows 0.0V on anode and cathode relative to the middle 0V rail. But on the working board I see .003V on the cathode, and .64V on the anode.I have put some measurements from the NONE working board on this updated schematic athttp://www.chriwilson.tv/schematic3.jpgI have edited the jpg to show these voltage test points relative to the zero volt rail. Does this help at all?I can measure voltages at the outputs of the op amps tomorrow. I am now 99% sure the op amps themselves are OK, having swapped all three between working and none working boards. The working board kept working, the dead board stayed dead.... Thanks for the ongoing advise, I find op mps difficult to understand, especially as these seem AC coupled?

author: Fank1
author_rank: Regular Contributor
timestamp: April 21, 2014, 11:56:07 pm
content: Chris; IC1 is shutting you down by pulling the base if VT1 low.Check the voltages on all of the IC 1 pins and post.Jerry

author: Fank1
author_rank: Regular Contributor
timestamp: April 22, 2014, 01:13:22 am
content: Late over there;Also dog lover and hoarder;No race cars but airplanes;Anyway;Measure the voltages on both sides of P1 also.The purpose of D2 is to bias the op amp output positive in a no fault condition.With the board out check the resistance across P1 and P3, they should be close to the schematic.If they are ok, unhook the wire to pin 3 and adjust P3, the offset balance pot, to get a positive output on pin 6 of IC1.Could be a screwdriver mechanic had it before you and set the offset pot wrong.Jerry

author: Chris Wilson
author_rank: Super Contributor
timestamp: April 22, 2014, 01:21:43 pm
content: Thanks very much indeed Jerry. I checked the voltages on all ins, on all 3 IC's and there were differences for sure. Pin 6 on IC1 differed as did pin 3. This gave me enough of a clue to re examine the faulty board and I found a microscopic crack in the track to pin 3 on IC1. Enough to break continuity. I repaired it with a link wire and immediately the dead board sprang to life. I re did the IC pin voltage checks and now all pins on all three op amps were very very similar across both boards, with the pots left on the manufacturing marks left by Farnell. Now to walk ten dogs, some of which don't get on, so several walks with amicable min packsI also think I have slightly more of a grasp as to how op amps work thanks to this job.My wife keeps thinking of taking flying lessons. I think she's too old, and I know I am too poor!The bigger supply I have, which is missing a "supplies board" is now going on the bench (well, on a raised box, it's too damned heavy to lift single handedly onto the bench proper. I want to recreate the missing board from the identical one in the smaller supply. I am indecisive how to do this, so I will start a new topic. Thanks again Jerry, and all the very best!

author: Fank1
author_rank: Regular Contributor
timestamp: April 22, 2014, 09:26:51 pm
content: Glad you fixed it.Tell your wife to try ultra lights, I like mine better than the Cessna.