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FET Grouping or Bank replacement when?
FET Grouping or Bank replacement when?
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Old 14th March 2019, 03:01 PM   #1
Formas is offline Formas  Jamaica
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Join Date: May 2008
Location: Montego Bay
Default FET Grouping or Bank replacement when?

Hi guys, where does grouping fets for replacement is really necessary to use same batch, is it Power Supply fets or Audio Output fets? (or both?)
In my supplied image, if i was to group banks wouldnt the obvious "good" side be grouped with the damaged side (technically) because the gate buffer drivers is parallel with both sides? or the DRAIN and SOURCE is what really separates the sides?
In this typical design do i have to change all fets, or just replace the bad ones with new ones (different batch)? i think it was component failure on the power fets as the amp plays perfectly as it is now, so it wasnt any failure from the output stage.
Should i just fit in the new ones, or group the 4 used ones to one drive circuit and add 4 fresh ones to the other drive circuit?
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Old 14th March 2019, 05:04 PM   #2
jkrokenberger is offline jkrokenberger  United States
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That amp has dual supplies. Replace all fets on the side that went down. Make sure they are from the same batch. Remove rest of the fets on the side that failed then check all gate resistors and verify good drive, then replace all 8 fets.

What model are the mosfets that were in place? You may be able to use a substitute in the repair.
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Old 14th March 2019, 06:02 PM   #3
Perry Babin is offline Perry Babin  United States
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This appears to be a rare design where each supply has its own fuse. Most amps kill all FETs when any fail but this type of design can have half survive (rare in designs where all are fused together).

Here, you can replace only half of the FETs if only one supply failed.

You use parts that match (matching date codes/production batch is generally close enough) for parallel parts.

For the outputs, you would match parallel outputs. If the output had 3+3 outputs, each group of 3 in parallel would have to match.

For the power supply, this amp has 4 banks, each with 4 FETs. If one FET had to be replaced due to broken legs, you would have to replace the 3 good ones as well as the one with the broken legs to make it reliable. Here, you need to replace all 8 on one side.

If the other 2 banks are unharmed (no solder leakage out of the back or side of the FETs), you could leave them but the FETs are so cheap that you may want to replace them, in case they were stressed to their limits (like the other side).
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