Help - Marshall 8008 Power Amp Blowing Output Transformers

[daryl]

I have a Marshal 8008 power amp that I have owned about 15 years after about 4 years ownership it stopped working. I opened it up saw blown fuse (can't remember which were gone) tried  replacing the fuse but I remember it just blew again so I put it in the loft and forgot about it as I knew nothing about electronics then.

I've been building my own effects pedals for about a year now and remembered the amp the other day, and now that I know a bit more I thought i'd have a go at repairing it. I've only worked on pedals so I am completely new to working on amps.

Image of internal layout:


Link to schematic:
http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/Marshall%20Amp%20Schematics/Valvestate/8008%2080Wx2%20Rack%20Mount%20Power%20Amp/rackmount_2x80w_8008.pdf


What I have done so far:

Pulled all 6 transistors on the heatsinks and tested them, found a short on the T64 /T65 pair on channel A. So I swapped them out for TIP 142 /147. (Although I did completely forget and bought them in a smaller package size than the original output transistors.)

Plugged a guitar into Input A and a 4ohm cab into Output A, turned guitar volume to 0 and amp volume to 0 and turned it on. The output transistors on channel A blew instantly and were smoking so I turned it off quickly. None of the fuses blew though.

So I lifted the base leg of all of the smaller transistors and tested those, all of them were fine.

So I removed the rectifier and tested that and it was fine.

While the rectifier was out I tested the voltage reading on the red and black wires on CON1 and got 57v reading.

Removed and tested the 2 big caps and both were fine so I put them back.


Does anyone have any Ideas what I should try next?

[Enzo]

I first saw this post over at DIY, so I will follow it there.  It is confusing to follow separate forum threads for the same repair.   To me, anyway.

[DrGonz78]

One thing to think about in a situation like this is other components that may have been damaged. Do you have a light bulb limiter? It is the best tool you can build for about $10 cash, so why not build one. Even though the fuse has not blown out this time it will be useful as you do repair work on shorted amplifier circuits. So, are you sure the replacement parts were good?? I mean they blew up and all... But maybe they were not specified for the voltage or amperage on this amp??? That is the first question that needs to be answered. Send a link to the datasheet that shows the output transistor that you purchased for the amp. I mean to say that the casing types may differ but maybe heat transfer to a heat sink caused the trouble. But it sounded as if these output transistors blew up rather quickly, which does not sound like a over heating of the output chips. I would be looking for bad resistors or other shorted components as I inspected the related circuits.