blown channels on rockford 851x amp

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hello, I have a rockford fosgate 851x amp 4 channels. 3 of the channels are blown one of them works fine.

I am looking to see if someone can point me into the right direction as to fixing them. Ive never worked on amps before and would appreciate the help.

what im asking i guess is what would have to be replaced (which transistors) how do i detect them, and what will i need to do that.

Ive attached a pic of the board to help visualize it to you experts.

Thank you very much

rf851x board
Well first let me start out by saying this is one tough amp for your first attempt to repair amps. Fosgate in a lot of their amps utilizes the BUZ1 MOSFET output, which an stp70n06 MOSFET will work as a great replacement. The thing about FETs to us technicians, we refer to them as Fire Emitting Transistor, they any many cases explode and burn thus making it easy to determine the bad MOSFETs. You will definitly need a good multimeter to check the mosfets. In most cases you want to change out all Mosfets In a blown cascade. Example: one MOSFET exploded you would want to replace the other 3 in that order, it's a kind of rule of thumb. The second task is removing and installing the MOSFETs since they are silver soldered to the heat sink, which the heat sink robs your heat you will want to get a large iron. Intense heat and get away from the new MOSFET. Also be sure and check the gate resistors for open or out of tolerance.
So then how would i go about testing which mosfets are to be replaced. I know that testing them in circiuits are different then when they are not on a board due to other components.

I just guessed that if i tested each gate using the continuance on a DMM and read 1 or below the transistor was bad and needed to be changed.

so i guessed that if i just replaced all of the transistors that i found below that point then the channels would be repaired.

but now your telling me that there is a rule of thunb to replace the surround transistors of the blown transistors too?

If the ones that appear to be working with fluctiation resistance across the DMM, even if it was next to a blown transistor would it still need to be changed?

Thanks a lot for your help

One of the best ways to test the MOSFETS is under load,that can be accomplished in a couple of ways. One , in the unit under power and 2nd, with a FET tester. If you plan to troubleshoot amps, the tester is a great investment. You can use a good DMM and do an impedance check across the MOSFETS.(diode scale) Depending on the FET ,you are going to read somewhere in the area of .600 - .750 ohms. This is tricky because of the highly capacitive circuit. Of course resistance of 000 is shorted. There is alot of info left out in your problem amp. I am assuming and should not, that one channel is still working. If so why is it not in shut down?.Visually you can determine faults in alot of circuits. Burnt fets ,resistors,etc. Make sure you have a 16KHz signal,sqaure wave, to the MOSFETS of the power supply. Compare impedances of cross channels. You do not have to replace all the MOSFETS, but if a mosfet in a channel has shorted it is a good idea to replace all for that channel. I'm not telling you the amp will not work if you do not replace all of them it very well will work,bbbbbbbbuuuuuuttttttt in most cases not very long due to inrush. I hope this has been a little help for you.
Good Luck
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