Martin Logan ,klipsch bash amplifier keeps blowing fuse

Have Martin Logan dynamo 10" sub. Looks like it's a bash amp used on many other subwoofers.

I don't see anything blown except dsp 104 thermistor. With amp off the sub I put a new fuse and saw sparks coming from green dsp 104 thermistor

Fuse blows instantly

Thermistor reading off the amp is 17ohm.

This part is supposed to be 10ohm and 4A.

Could just a thermistor cause the amp to blow fuses or it's something else that caused this amp to blow?

Output transistors measured consistently around 1040ohm on outer terminals. Using diode test

Here is a thread of another gentleman with same amp on klipsh subwoofer

https://www.diyaudio.com/forums/power-supplies/192365-bash-amp-power-supply-repair-3.html

And pics of mine. I think it's power supply issue. Just curious if just s thermistor failure could be the culprit.
 

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I don't see anything blown except dsp 104 thermistor. With amp off the sub I put a new fuse and saw sparks coming from green dsp 104 thermistor

Fuse blows instantly

It's a pretty safe bet that something downstream is taking out the fuse and overloading the thermistor. If this is happening immediately, with no signal going through, you should probably be looking for a dead short across one or more of the supply lines to ground.
 

ehostin

Member
2020-05-03 8:42 am
Richard,

what was the final outcome of your repair? Hopefully a success.

I have a similar issue with my Martin Logan Dynamo subwoofer -- actually two of them.

The plate amp looks identical. I replaced the mosfet transistors (is that the same as the "chopper transistors" mentioned by JonSnell?) with IRF740 transistors to replace the existing ones. I got the amp to power up thereafter -- red light and all -- with no load. But then it blew again the following day when I tried to test it within the system. I never even hooked up any audio signal to it, but the fuse blew again.

All else looks OK. I didn't have any visible evidence of anything else going on. At least one of the removed MOSFETs was bad.

Emmanuel