Ampeg BT15 blowing fuse

[pontiacpete]

THis amp came to me blowing the main fuse and taking out the bridge rectifier. First off I installed new filter caps, 3 2200uf/80v caps. The third one is not shown on the schematic but changed it anyway. Put in new diodes and it played for a little while, and thought I was done. Turned it on the next day the fuse blows again and takes out the rectifier again. This time I thought that I put that third big cap in backwards because I was getting a weird almost shorted reading on the ohm meter. Pulled it out and measured it and it seemed fine so I reversed it and replaced the diodes and it fired up and played for a while turned off, thought I was done. Came back later and it blew the fuse, diodes and smoked a bit. The 2n3741 seemed to be getting hot. I disconnected and checked with a transistor meter and it measured OK no short. I’m thinking the whole output should be replaced. All the transistors on the output I think are original. Does this sound like a good plan? If so what do I use for replacements?Ampeg BT-15-18.pdf

[g1]

If a cap has been run with reverse voltage it will be damaged and you need to replace it. It could be causing your problem.

edit: you say it's not shown on schematic, but is it the one at the output, above the ext.amp jack on the schematic? (1000uF 50V)

[Helmholtz]

It's not a filter cap but the output coupling cap required to block DCV from the speakers with amps using a unipolar supply.

[pontiacpete]

It probably is the one on the output to block DC. I was fooled by its size and also being exactly the same as the filters. They most likely had a lot of these big ones on hand.
I need to trace out the area to make sure I get the polarity right. Thanks

[Helmholtz]

The larger the cap value the better the bass response.
Cap value depends on load impedance and desired lower corner frequency.

[pontiacpete]

Now it’s getting worse. I replaced the diodes again and the cap on the output and I traced out which way it goes. I found that the 33R resistor on the neg end of the cap is what was smoking. There is a short upon turn on.

[Helmholtz]

The 33R is in series with the 0.15µ cap (constituting the Zobel network). Check that cap for a short.
Apart from that I suspect the amp is oscillating.

[pontiacpete]

Checked the .15uf cap for a short and no it’s still good. I replaced anyway.
When you say it’s oscillating, does this mean the output transistors need to be replaced?

[pontiacpete]

I removed all the output transistors and found one that is shorted. It’s an RCA 10003. I’d like to replace all of these. There are 2 of the RCAs and 2 of 2n3442. Can I install some modern transistors like the MJ15024 or 25, can’t remember which is the NPN at the moment

[Helmholtz]

Checked the .15uf cap for a short and no it’s still good. I replaced anyway.
When you say it’s oscillating, does this mean the output transistors need to be replaced?

Oscillation is hardly caused by a bad transistor.
More likely a bad cap, e.g. an ecap which has lost its capacitance.
Start with checking the 1000µ/3V cap.

[pontiacpete]

i pulled the 1000uf cap and tested it. It measure 1175uf and .14 ESR. I changed it out.
What about the shorted transistor I mentioned? What can I replace that with?

[Helmholtz]

i pulled the 1000uf cap and tested it. It measure 1175uf and .14 ESR. I changed it out.
What about the shorted transistor I mentioned? What can I replace that with?

Cap values look good.
No benefit from replacing good parts.

First verify oscillation with a scope.

[pontiacpete]

How do I verify oscillation if I can’t power the amp up? One of the transistors is shorted

[pontiacpete]

G1 there are 4 altogether. Refer back to the picture I posted in my first post

[g1]

The schematic from post #1 is wrong. This unit has 4 output transistors on the heatsink in the picture from first post.
Pg.1 or 2 of schematic attached should be the right one.