Twin Reverb Issue

[buildAndPlay]

Ok - I have a Twin Reverb that I recapped several months ago. It has worked well for me for several gigs but this past weekend it made a loud pop and let the smoke out. One of the 100uf filter caps had exploded. It turns out I had one of them in backwards. How I made it through a half dozen gigs is beyond me, but I did.

Anyway, I ordered two new caps. I replaced the one that blew up and fired her up. Immediately blew the fuse. I checked the rectifier diodes and they checked out. I also checked the 220k resistors that parallel the filter caps and they are fine. I replaced the other filter cap but that didn't help. This is all with the standby switch off btw. The transformer checks fine (no shorts between windings). Also, if I disconnect the two transformer leads from the rectifier section I can power it up, not blow a fuse, and read proper the proper AC voltage across the transformer leads. There's another 48 vac lead that I left connected and I had the proper voltage there. So it seems to me like the problem is in the power supply (since the standby switch is off). Oh, I also removed the two filter caps and fired it up and the fuse didn't blow (again, standby off).

So now I'm wondering if I've got a bad filter cap. I can charge them with a 9 volt battery and watch the voltage slowly drain off with my multimeter conntected. Other than that, I'm not sure how to test them.

So...any suggestions on what else to test? Any way to test the capacitors beyond what I tried? All I have is a DMM.

Thanks,

Jim

P.S. - I have an associates degree in Electronics Technology...but that was from 25 + years ago, so my theory is out the window, but I know how to not kill myself.

[The Dude]

If I read you right, the fuse holds until you take the amp out of standby. That leaves likely tubes or cap(s) at points B,C, and D in the schematic posted above. With the amp off, attach a meter reading continuity from ground to B+ with the amp out of standby and start removing those parts until the short is removed. Obviously start with tubes since that is the easiest.

Edit: Just re-read your post and I may have it wrong. The terminology "standby off" or "standby on" is confusing to me. "Standby off" means the amp is out of standby to some people and the amp is in working state. To others it means the switch contacts are not engaged and the amp is in non-working state. Believe me, I'm not nit picking- just trying to understand. I usually use "out of standby mode" or "in standby mode" so that there is less chance of misinterpretation.

[buildAndPlay]

I mean the amp was in standby mode. The standby switch was open and not allowing current through.

[The Dude]

Well then ignore my post above and thanks for clarifying. We don't need to worry about anything past the standby switch then. If you check continuity from B+ to ground what do you get?

[g1]

Agree about the standby terminology, I like to use the terms "standby mode" and "run mode" (or play mode) to avoid confusion.
You seem to have narrowed it down to the main filters. Note that they are in series, so pos. end of one cap will connect to neg. end of the other. Triple check that they are wired up right.

[buildAndPlay]

Agree about the standby terminology, I like to use the terms "standby mode" and "run mode" (or play mode) to avoid confusion.
You seem to have narrowed it down to the main filters. Note that they are in series, so pos. end of one cap will connect to neg. end of the other. Triple check that they are wired up right.

Well...triple checking was a good idea. Initially I had one backwards and one the right way. The backwards cap eventually blew up. When I replaced the caps I got them BOTH backwards. Such a derp! Anyway, I finally got them in correctly this morning and believe it or not, the fuse doesn't blow.

It's good to be humbled once in awhile. I don't know if that's actually true but I'm going with it.

Thanks for the help.

Jim