Interesting problem and solution

[Randall]

A Dr Z MAZ Jr came in with no output, not a peep. One EL34 wasn't lighting up, and the HT fuse was blown. I replaced the dead tube with a good used, and put in a new 1/2A fuse. Now it was making scary noises and not passing signal. No schematic of course, so I went about tracing B+. Good at the rectifier side of the standby switch and also the side going to the fuse. But then on the other side of the fuse it would start off at about 150v, slowly drop down to around 40 - 50v, and then I could hear a very faint pop or click, and it would jump back up to around 150v and repeat. I didn't care for hearing that click much, so I didn't leave it on long.

Well, it turned out to be the HT fuse holder was faulty, of all things. It is the type with a screwdriver slot in the cap that must be turned a quarter turn to lock in. I have never cared for those. I replaced it with the better quality and stronger springed one that CED sells, and problem solved. I don't know the chain of events that caused the failures, but it's fixed now. Plus, when I tried to check the AC mains fuse, the holder was stripped and twisting inside the chassis, so I replaced it as well.

A fuse holder, of all darned things.

[The Dude]

I like that story as a lesson to logically troubleshoot. There are those who would have first installed a new power transformer.

[pdf64]

Is the standby hot switching the reservoir cap on to the rectifier?

If you've traced the schematic, I'd be grateful for a copy (with one repair after another I can see you're a busy guy so understand that you may not have time to do that, so don't go to any extra effort )

[Randall]

"Is the standby hot switching the reservoir cap on to the rectifier?"

I didn't draw anything, and the amp is being picked up soon, but I can tell you the B+ goes from rectifier tube to standby switch to HT fuse to first filter cap. That's as far as I needed to go.