C84 working fine, then blew up, now shows problems

[monkeyxx]

with tim's email advice I got my N72 working just fine, but my C84 is still not right. I emailed Tim but haven't had a response in almost a week (his other emails were all next-day) so I thought I would post here also

I carefully assembled the C84, decided to go ahead and bias for + and - 23.5 volts for the extra headroom (is this a bad idea for next time?), I plugged it in, it tested fine, and it was working and sounding great. as I was patting myself on the back there was a loud pop, then a burning smell, and erratic noise on my C84 channel.

I took apart my rack and found that C27 was cracked and bulged on the top, and R28 and R29 were burnt to a crisp. I replaced these three parts, now the unit will power up without sending the psu into thermal shutdown (blinking lamp) I even got it biased to + and - 18V, but now my servo test readings are off.

the common mode servo reads only 12.2 volts or so, and the L3 and L4 voltages are way off, and separately read at -17.8 v (L3) and -8 v (L4) with my DMM black probe at pground. I have not tried connecting a microphone since these readings are off

I've double checked all the polar components and they are installed correctly. what's left to check now? I wouldn't be surprised if something else got toasted without being visibly burnt, but I don't know where to start. I'm totally fine with ordering more spare parts from mouser or wherever, and can follow directions.

I also have a lingering question of if the high initial bias is what caused the failure, or just a bad capacitor. just wondering why it was doing great and then "pop."

I have noticed that the bias tends to drift after the module has had a chance to warm up for a few minutes, so next time I will set the bias, then reset a bit later when it's warm and at working temperature

having fun with these just want to get them all working, thanks!

Dan

[tpryan]

We'll need to see the module. Please e-mail rma@seventhcircleaudio.com for return instructions.