rockford fosgate t40001bd

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Gate to drain?

The gate and source could have been at the same voltage and the drain at the other end of the rail.

How many sets of rail switchers failed?

When you go back to using full rail voltage, insert an incandescent lamp in series with the red power supply wire. If there is a surge in current, the lamp should limit the current.
 
when I initially got the amp only 2 gets were found bad. I replaced all 5 of that bank. checked the driver circuit and it bbn looked okay. it seemed that the original FETS were no properly tinned, got hot came loose and shorted. I powered the amp up and about 20 seconds the powered up after I had all 4 drive signals they blew 2 banks. this time it took out the driver circuit. I rebuilt the driver circuits, replaced FETS, replaced 5110 ic's and uc301 and u302.
 
The FETs generally fail when the internal silicone die overheats. If the FETs are tight to the heatsink and current is limited, they should not fail. Since much of the storage capacitance is on the other side of the limiter (lamp), the FETs will not have as much current to have to pass if something is wrong and that stored energy won't be re-charged due to the limiter.

On some amps I recommend using a low-voltage supply made from added windings on the transformers but the transformers used in these amps have epoxy blocking adding the windings. You could get around this but it would be a bit more work. An alternative would be to use an isolated power supply to feed the output section but the power supply has to be 100% isolated from the mains power. A battery can be used for testing but that too would need current limiting.

As a side note, the FETs 'can' be damages if the drive voltage is too great but that's not likely here.
 
I just swapped in another audio board and have the same issue. Although, the drive signals to the fets look a lot cleaner whe the rail voltage is applied the high voltage sharing does not take place. could there be fault from the powersupply that inhibits the rail switching. I will have to rebuild anothernpowersupply board to verify the powersupply is not the issue.
 
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