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Old 2nd May 2003, 11:14 PM   #1
manzoni is offline manzoni  United Kingdom
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Join Date: May 2003
Location: Scotland
Default Fried JLH MOSFET

Hello all
I have a JL Hood power amp built from kit in late 80s. It has Hitachi K135/J50 MOSFET pairs

The second speaker protection relay failed for the second time (the kit came with two relays and I managed to find the spare one) but looks like when it failed it shorted the output of one of the channels, as there is now -72V on the LS output.

1. Can I assume that the MOSFETS are fried good and proper, and
2. Is there anything I can use to replace them.. ? There is no other visible signs of damage and power supply fuses are intact.

thanks for any quick advice

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Old 3rd May 2003, 07:51 AM   #2
ashok is offline ashok
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Join Date: Jun 2002
Location: 3RS
Default MOSFET's

If your MOSFETs are shorted out, the fuse HAS to blow. On the other hand you could have a breakdown of a smaller transistor in the driver or predriver stage which is causing the output to sit at -72 volts.
Your relay must be opening when there is a fault. So maybe it has opened just because of the -72 volts and there is nothing wrong with it.

You will have to check out collector / emitter or drain/source resistance with a meter with the power down on the amp. Check each transistor from output to input and compare it with the similar one on the other channel. I guess you are not very experienced at trouble shooting.
In fact do a base/emitter , base collector , and collector/emitter check . Even if you do not know which lead is which you have only 3 leads on each device and you can pick them in pairs ( for a start). Compare with the same device on the other channel. and let us know.
By the way - if you have -72V on the LS terminals, the speaker (woofer) must be fried. Enjoy your dinner ! Can you smell burnt wire at the woofer dust cap ? It stays a long time ! You should have also "heard" it when it went "Blam".
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