Okay, I've examined R1375 (with the power off, and I'm not sure where to probe for anything other than that center tap on the pot, so I can't really answer whether it's seeing the full +50 V at one of the outer terminals) but the pot seems (to my uneducated eye) to be acting strangely. I was able to find a pad that is connected to the center tap of the pot, so I measured its resistance between center tap and ground, and between center tap and the +50 V rail, as I turn the pot through its range. It starts a effectively zero ohms (0.001 K ohm), goes up to about 6 K ohms, then drops down again to around 2 K ohms. The schematic says this is a 25 K ohm pot, so shouldn't I expect it to go from 0 to 25 K ohms?
If it turns out that the pot is dead, I guess that's good, but I don't relish the idea of removing the main interface board to unsolder and replace this trim pot.
