Micromoog; troubleshooting 15v short

[WhinyLittleRunt]

Long story short, I had left my Micro on for a while (like a month) only to come home one day and realize the LED had gone out and there was no longer any audio. It pops when I turn it on, and if I adjust the volume you can audibly hear it getting 'hissier' and the scratchy pot, just no audio output.

I'm trying to figure out the best way to troubleshoot this without a scope because I don't have one, and maybe someone can tell me if it's even possible with just a dmm before I keep going...

Power supply, with the main connector disconnected that runs to the mainboard, gets a solid 15v +/-. When I reattach the connector, I measure 1-2v on the - side and 6-7v on the + side. So I know there is a short somewhere, it's just figuring out how to find it. From my troubleshooting I can say that it begins with the first connector on the mainboard (P1) where I'm getting the 6-7v right at the pins. If none of the connectors are attached to the board, but just the connector is plugged in at the power supply, I measure 15v. So I know as soon as I connect P1 (and only P1) I'm already identifying the drop in voltage. I guess the part that is confusing to me is how do I trace out where the issue is if I'm already seeing the short at the first point of entry? I know it's on the mainboard somewhere. Any tips would be appreciated. Thanks

[Dr. Phibes]

When I had my Polymoog (which was built around the same time) it had a number of failing op-amps. I wouldn't mind betting that one of those MC1458 dual op-amps has failed and is shorting out the +15V/-15V supply. There's probably a cleverer way, but without a scope I would resort to checking each output to see if it's really high or really low since that would indicate that normal feedback was no longer functioning. You could also check the inputs to see if there is an unusual large difference in voltage (more than a few millivolts).

[sam]

Look out for odd voltages....Feel if the op amps are hotter than usual...Resort to the service manual for checking voltages..

[WhinyLittleRunt]

I tried some freeze spray on the chips today and think I found the culprit; a super-hot-to-the-touch CA1458G op amp. I removed it from circuit and the voltages all measure out normal now, so hopefully that's it and all it is.

[sam]

Nice work....I do love freeze spray..

[WhinyLittleRunt]

Thanks. And that was all it was; soldered a new chip in and tested good.

Wish everything was this easy...

[Dr. Phibes]

Good call on the freeze spray trick, much easier than fiddling around forever. Didn't I say it would be one of those smelly little op-amps? At least there's only 20ish in the micromoog, seemed like there were 20 billion in the polymoog.