27-01-2017, 10:02 AM
Hi Hayden,
QUICK UPDATE: My D-3800 is fixed, and I was wrong; IC2 (LM339) was not faulty.
I started removing IC2 by de-soldering pin 13, and I found that the solder pulled way from the pin completely and it was no loner connected. (As an aside, this PCB is really easy to work with. The holes are slightly oversize, not plated through, and slightly chamfered on the solder side. This means that you can easily disconnect any component by de-soldering with a good solder sucker.) Before going any further, I decided to check voltages on components normally connected to pin 13 of IC2, and I found that they were still too low - 7 V instead of 15 V. I then de-soldered pin 9 of IC2, and that made no difference. Finally, I de-soldered one end of C10, which connects between IC2 pin 13 and ground. Bingo, all the related voltages increased to 15 V! I have now removed C10, and reconnected everything else, and the supply is working normally.
C10 is an 0.1 uF ceramic, 25 V. That's the last thing I would have expected to be faulty! I currently have it connected to a 15 V power supply in series with 100 k. The voltage across the capacitor started at 5-6 V, and it is now 0.8 V. This explains all the symptoms that I was observing.
I have not replaced C10 yet, and I am contemplating replacement of the 5 small electrolytic capacitors on the PCB before putting it all together. I have just noticed that C26 is non-polarised.
This seems to be such a rare fault, that you might not dare hope that you have the same fault, but please let me know if you do.
Meanwhile, the second D-3800 has failed, and it seems to be a different fault, associated with IC1 (LM723). Stay tuned!
Best wishes,
Lew.
QUICK UPDATE: My D-3800 is fixed, and I was wrong; IC2 (LM339) was not faulty.
I started removing IC2 by de-soldering pin 13, and I found that the solder pulled way from the pin completely and it was no loner connected. (As an aside, this PCB is really easy to work with. The holes are slightly oversize, not plated through, and slightly chamfered on the solder side. This means that you can easily disconnect any component by de-soldering with a good solder sucker.) Before going any further, I decided to check voltages on components normally connected to pin 13 of IC2, and I found that they were still too low - 7 V instead of 15 V. I then de-soldered pin 9 of IC2, and that made no difference. Finally, I de-soldered one end of C10, which connects between IC2 pin 13 and ground. Bingo, all the related voltages increased to 15 V! I have now removed C10, and reconnected everything else, and the supply is working normally.
C10 is an 0.1 uF ceramic, 25 V. That's the last thing I would have expected to be faulty! I currently have it connected to a 15 V power supply in series with 100 k. The voltage across the capacitor started at 5-6 V, and it is now 0.8 V. This explains all the symptoms that I was observing.
I have not replaced C10 yet, and I am contemplating replacement of the 5 small electrolytic capacitors on the PCB before putting it all together. I have just noticed that C26 is non-polarised.
This seems to be such a rare fault, that you might not dare hope that you have the same fault, but please let me know if you do.
Meanwhile, the second D-3800 has failed, and it seems to be a different fault, associated with IC1 (LM723). Stay tuned!
Best wishes,
Lew.