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.jpg I 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 at
http://www.chriswilson.tv/schematic.jpgOn the working board I see relative to the 0V rail on pin 5:
1: -15.5V
2: -39.7V
3: .017V
4: 6.27V
5: 0V (negative lead of meter here for all measurements)
6: 1.55V
7: 25.23V
8: 15.22V
9: 0.0V
10: 8.64V
11: Blank, not used
12: 6.28V
13: 0.0V
14: -.348V
15: 0.0V
16: Blank, not used
17: 8.63V
18: -0.347V
On the none working board :
1: -14.98V
2: 0.417V
3: 1.47V
4: 6.22V
5: 0V (Negative lead of meter here for all measurements).
6: -9.23V
All 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