Ripped into the 535A to try to find the source of the arc and bang. As usual it turned into a marathon.
First, looked over the Trigger A section and the only thing I found suspicious was two flying leads that were real close to each other. But no marks indicating an arc. I separated them anyway. Sorry for the poor focus. Camera doesn't like going that close. They are the 2 bare leads going across the center of the pix.
Next I powered up and checked the ref V and all the B+ voltages. They are all OK. This PSU is damn stout to put up with those arc bangs. They were LOUD. And so far there have been no further snaps and bangs.
Next determine why no free running trace in “Auto” mode on Time Base A. Turns out the “Trigger Preset” adjust is off. This is where things went down hill. The pot is hopelessly jammed. I took it out and apart. Tried to free it up and the resistive element fell apart. It's a 200K linear miniature pot. No gots so I have to order one. It's the pot on the stand off. BTW, this Trigger Preset pot does NOT appear on my 535A schematics. Nothing new, see this all the time with Tek equipment. So many revisions and the manuals can't keep up.
In the meantime Time Base B does work fine. I'll order a new pot and at the same time I might as well start ordering replacement capacitors for the PSU. And in other news.....I found a “parts unit” 485 on Ebay for a reasonable price. Coming next week. I may FINALLY fix the trigger issue with the one sitting in the closet.