Seventh Circle Audio

I've just gotten around to building a rev 1.6 C84. I applied power, and was trying to get pins 1 & 7 of U1 to 0V. I couldn't get past -124 mv or so. Eventually C44 popped and smoked a bit - it's now obviously burned with a bit of the foil poking out. Any ideas?

Cheers,
Bob

There's no way adjusting R19 can make C44 fail, so that's just a coincidence. Most likely, C44 was installed backwards or developed a short. Check the polarity of the rest of the tantalum and electrolytic caps just to make sure. Snip out C44 and replace it.

Tim, C44 was in right, and I've checked all the other caps. I've got a replacement on the way, but is there anything else I should check before I power it up again?

Cheers,
Bob

Double-check the orientation of the diodes and the solder joints at Q1 and Q2. Make sure you haven't swapped the black 1K and 1K58 resistors. You can power it up for testing without the capacitor.

Well, I replaced C44, Q5, Q6, Q9 and Q10. I powered it back up and all was as described in step 34. I adjusted R19 for 0V between pins 1 & 7 of U1, and just about had +/- 18v at U1 pins 1 and 4 when Q46 popped. I've replaced Q46 and now when I attach to the power supply and power up the power supply shuts down.

Q46 popped

I guess you mean C46? Are you sure you have the tantalum caps oriented correctly? The small stripe on the cap indicates the positive lead, which is the opposite of the marking on the aluminum electrolytics.

I am lining up the '+' on the side of the capacitor with the '+' on the PC board.

Well, whatever the cause you seem to have a number of shorted bypass capacitors, and I suspect you haven't found all of them yet. Try measuring resistance directly across the pins of C44 - C49 and let us know what you find.

C44 - 974r
C45 - 970r
C46 - 974r
C47 - 970r
C48 - 970r
C49 - 964r

So the PS03 indicators light with no load, but when you connect the module it shuts down?

Tim, just for the record, I've built several guitar amps from scratch, and 4 N72's. I've checked every component on the board at least twice.

I plugged the C84 into the PS to make sure the symptom I saw last night would still hold, and it actually powered up and stayed on. Either something was overheated last night or it was the combination of the C84 and the 4 N72's, which I've since disconnected. Sensing victory, I attached my probes to pins 1 and 7 of U1. Within 30 seconds or so, R43 and R44 started smoking and are now black.

The first thing to check is Q5 and Q6. Keep in mind they're oriented the same way as Q9 and Q10. Shorted Q3 or Q4 could also smoke those resistors. Check for shorts between C and E and C and B of all four parts.

Q6 was showing a short, but I think that's because R43 and R44 were shorted - I took them out and I don't see the short on Q6 any more. C25, which was mounted correctly, is also fried. I've removed that as well. No shorts on Q3 or Q4.

Sounds like you'll need to send it in.

Okay, send it to:

Seventh Circle Audio
1234 47th Ave, #9
Oakland, CA 94601

Yes?