Trace Elliott Commando killing output transistors

[Hadoque]

This is my first post here. I'm really happy to discover this forum with so many knowledgeable contributors, and I'm hoping to get some answers.

I'm trying to help my friend fix his Trace Elliott Commando. When I got it it was blowing fuses. I put it in series with a lightbulb, wich was, to begin with, really bright. When I pulled the output transistors it dimmed. I guess this doesn't really proove that the output transistors are the actual problem, but when I measured them they were both faulty. So I went to ELFA (I live in Sweden) and got new ones, and put them in. (I also replaced replaced both 4700uF 63v filter caps, since i had a couple lying around.)
I plugged in the speaker and turned the amp on. It sounded and worked fine for about a minute, then blew a fuse. Output transistors gone again.
I never attached the big heatsink to the chassis when I turned it on to test it. Could that be the problem? The heatsink was really hot after the fuse blew. If not, does anyone have an idea how I troubleshoot this one?

I'm a primarily a live sound tech but I've worked on plenty of tube amps, my own and amps belonging to the bands I've worked with. However, I have very little experience with SS.

[Jazz P Bass]

Heat equals current.
I do not think the amp repair was complete.
When you replace shorted output devices it is a good rule to also change out the drivers.
Also check the ballast resistors that are attached to the output devices.
The Last item to connect is the speaker.
All tests can be done without it.
ie: overall amplifier current draw, overly hot components...

[Steve Conner]

Agree with the above. You also have to check the bias when changing output transistors. Just like power tubes, they have production tolerances, the replacements might overheat with the same bias voltage that suited the old ones.

The details of how to check bias depend on the amp. Generally you will find some large resistors in the output stage, less than 1 ohm. These are what Jazz P called the ballast resistors. Clip your digital meter across one of these and adjust the bias pot to get the (very small) voltage specified in the service manual. If you don't have this then 5mV will do.

Also most SS amps have a thermal sensing transistor or diode. This is placed in contact with the output devices or heatsink. When changing the outputs, you must make sure this gets put back where it was, or the amp may overheat and blow up.

[Hadoque]

Thanks for the replies.

I did what you said and changed both drivers and power transistors (again). The thermal sensing diode never came off the heatsink, so I didn't have to worry about that. About the bias... The only pot that I could find on the pcb was one that had a terminal beside it that said "set to 21VDC". So i did that. Then I took a reading from the ballast resistors. The voltage kinda drifted up and down for a little while, but eventually settled to about 7 mV.

Up to this point everything was fine and nothing was overheating, so i decided to plug in the speaker and try the amp out. When i pluged a bass in and played it, it sounded fine, but the drivers became really hot and let off a little smoke(!). I turned it off and let it cool. Then I turned it on again and played it again. This time nothing overheats, and everything sounds fine.

Now, I've played music through it at medium level for about an hour and everything is fine and stable. So I guess I'm ok. Right? Would be nice to know why it became so hot the first time, though.