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Speaker Terminal Arcing

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Yeah, that's not supposed to happen. Was looking for DC offset on my 2nd board- source connected but not playing. No load. I put the black probe on the black speaker terminal and the red probe on the red speaker terminal. Couldn't get a stable reading, which I thought was odd. I pulled the probes to change the scale on my DMM, and when I placed the black probe on the second time it arced. Now my DMM probe is black and pitted.

What went wrong and how can I fix it?
 
I think it's a ground (earth) issue, but I don't know what I should be looking for/fixing/replacing. Could the chip have a short? I know my transformer is (slightly- 28 vs 25) over the recommended range, but the first channel worked right away. I guess I have a different question now.

Is it worth connecting the tested power supply to the oscillating amp? That one step will tell me if it's the power supply causing the issue and it doesn't risk frying the good amp board. If the amp has the same issue with the good power supply, then it's definitely the amp. I think I just talked myself into it. Probably could have checked this last night.
 
Well, no sparks, but no measurements either. I think my DMM is toast. Tried testing the working one too- all I'm getting is "1" which is an overload, even at the 2000V scale. I no longer trust this tool and I'm going to go replace it. I'm going to return it and try again with one straight out of the box.
 
Alright, brand new DMM with brand new battery says 39.4V on the "bad" board between PG+ and V+, matching 39.4V between PG- and V-. The working board is 39.5V on both sides. It's close to the 42V limit, but definitely and obviously under it. I can safely say that the power supply is not the problem.
 
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