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A Marshall JMD:1 came in with "lights on but no sound". I quickly determined the +/- 15V supplies were toast, including the L7815/L7915 regulators. After pulling the regulators and checking to see what might have fried them, I found that both rails were nearly shorted to ground, especially the negative rail (115 ohms to ground). But there were 5 dual op amps and 1 quad opamp, and some assorted caps that were possible suspects, all well and sturdily soldered in plated-through holes. I have good de-soldering equipment but I didnt relish removing them at random. No schematic either. Looking at the board, I noticed one NE5532 opamp near an XLR jack. Figuring that anything involving the possibility of human error, such as plugging a shorted or otherwise compromised XLR cable into the direct out seemed to be the most likely cause of opamp destruction, I removed the opamp after confirming it was connected to the direct out. Resistance to ground on both rails is now 1 meg.
. As well as the bad cable/user error possibility, some outputs don't like being connected to an input that has phantom power on it.
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