Polytone - blown power transistor

[kdawg]

Hello - I have a Polytone Mini-Brute III that has a blown power transistor. It has an MJ4502 and MJ820. Question is - I have some MJ15024 and MJ15022 power transistors available, can I substitute those? Thanks

-kdawg

Schematic: http://www.murchmusic.com/Polytone%20Info/schem2.JPG

[Enzo]

MJ802 maybe?

Your original pair are rated at 30A and 100v 200w.

I am thinking that is a bot of overkill for your application, however, they are using only one output device per polarity here. And I don't recall the last time I saw a 0.075 ohm 5w resistor.

I like to think of the MJ15025/25 as universal replacements, but they could be marginal here.

I believe MJ15022 and 15024 are both NPN.

[kdawg]

Hmm, I just pulled the MJ54502 because it was reading a short across emitter-collected, but actually the D6 diode 1N4004 reads shorted... I have to remove to check it's really the diode and not something else. Transistor reads OK. Any ideas on what would have caused this?

I know this amp was making a really loud pop when turning on everytime before it stopped working. I'll check out filter caps.

Thanks
-kdawg

[Enzo]

One can get lost into trying to pin the blame on some certain event that caused a failure. SOmetimes we know, more often we don;t. We might know that this caused this, which caused that, etc. But what started it?

A static discharge when the speaker cord was last pulled out could have weakened a part in the output stage, then next time you use the amp, BZZZT, the part collapses. And there we sit trying to figure out what we did wrong. Well the problem occured last week, not tonight, and we didn;t do anything wrong.

It would be great if there were some cosmic cause/effect chart we co8uld look up each part on, but there isn't.