Okay... I was successful at checking the 10k presence pot @ CON104. At first, the reading was 4k3 with the presence turned to 100% CCW. After rotating the dial a few times, the impedance increased to 9k5/100% CCW. Close enough for rock. I had treated the pot with Deoxit, so maybe it took some time to clear the debris?
I also tested the Effects pot, which testes good @ 10k/100% CW. The only thing I don't dig, is the way the dry signal volume increases (in parallel mode) along with the effects. Maybe I need to use kill-dry on the effects? It seems I recall the Effects volume acting as a MV in parallel... Of all things, I suppose I should take a look at the owners manual.
I found that most of the 14AWG slip-fit connectors were loose... I cleaned w/D5 and tightened the connectors. Some of the insulation wasn't applied properly, so I had to do some minor trimming for 100% coverage onto the terminals. I'm going to let it sit for a few hours. Until then, I'm going to enjoy a bowl of some homemade Skyline Chili w/angel hair pasta + shredded cheese.
Story...
When I purchased my '87 SJ 2550 stack, I found that Marshall had neglected to solder one of the wires onto the standby switch. WTH? The former owner had complained about "popping at power-up, and volume loss..." I used the old power-it-up in the dark trick, which pointed me right to the arch point. Just a bare wire placed through, and then wrapped around the standby switch terminal. Zing!