Roland KC350

[lowell]

This amp blows the 2 500ma secondary fuses intermittently. It also causes the local bypass caps on the opamps to smoke. The caps seem to be the ones bypassing the negative 15v rail. Those smallish .1 ceramics. Right now it's working fine... again. I kept lifting one end of each smoking cap but this would cause a different one to smoke. Then it stopped doing it. I reassembled it and after powering on days later the fuses blew again. Have it apart again and it's working fine. So I'm pretty unsure of what could be the cause. I suspected that the voltage follower transistor in the low V supply was the issue... but alas it seems to be working fine. A check with the meter did not reveal anything either. Both rails look good at about +/- 15.4v.

Until it happens again I can't troubleshoot it. So figured I'd see if anyone has come across this particular issue. I've tapped all over every board and wiggled the Molex clips etc...

Says my pdf file is too large to upload

[g1]

Maybe those caps are all just bad. They can't smoke unless they are bad or have way to much voltage on them.
What is their voltage rating? Is the regulator supplying the correct voltage while they smoke? Any oscillation on that voltage rail?

[Enzo]

You could have an intermittent short on that +15 line causing it to go up to 25 or 30 or whatever.

[lowell]

I can't get this issue to happen again and been sporadically playing the amp and tapping and wiggling things. Putting it back together. Either it's fixed itself or it'll come back and I can troubleshoot if the issue is there.

[Jazz P Bass]

For future reference here is the file, cropped down to the schematics & layout.

KC-350-550.pdf