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Rockford Fosgate 120a.2


Dumdum1975
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Go on Amazon and find something equivalent to the mosfett shown. I found the data sheet for you so make sure the specs are the same as the N-channel mosfett shown. 

https://datasheetspdf.com/pdf/1457699/INCHANGE/BUZ100/1

 

(Word of advice, get some anti static tweezers when messing with mosfetts, a lot of people debate if static electricity damages amplifier parts but it has been shown that mosfetts and ic's are more sensitive.. a little proactive caution never hurts and they're cheap.. if not at least discharge by grounding yourself prior to touching especially in the winter. Other than that a soldering iron,Flux and Flux core solder will get you taken care of.. watch a video on soldering if you don't know how.. you'll need a desoldering iron or soldering wick to remove the existing soldering then pre-tin the board and mosfett prior to soldering so you aren't over feeding the solder. Hope this helps, good luck!)

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Its very wise to replace all the mosfets that are the same model number even if they are good with new ones that have similar date codes and come from the same batch (ie made at the same time).

 

Problem is the new mosfet will have slightly different tolerances and even sometimes specs, so when you change out a bad one with a new one that is potentially weaker or stronger (even if its the same part number from the same manufacture) you can cause other mosfets to blow that were previously good.

 

Also mosfets don't just blow, there is a chain of command and something will fail on the driver board, the failed component on the driver board will then cause a gate resistor to blow, that gate resistor will cause the mosfet to blow. Without tracing the circuit back and testing everything on that trace to be sure that everything is fine will cause that new mosfet to blow again if you missed anything. Example if you replace that mosfet but don't fix that blown resistor then that new mosfet is going to blow again as soon as it turns on. This is why an oscilloscope is needed for amp repair.

Typically when repairing an amp, you are safe to fire the amp up on low voltage using a benchtop powersupply at 9ish volts and limiting the current to a couple amps once you have all the mosfets removed from the board. The amp should turn on and come out of protect. If the amp is still stuck in protect with all mosfets removed then you still have damaged components elsewhere on the pc board that need to be replaced. Again if you dont replace all the blown things that you think "look" fine to you by visual inspection but are actually bad, the amp will just blow the new parts instantly. 

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Whoops, almost forgot RF amps use that secondary pc board that they attach their mosfets too which then gets bolted to the heatsink.

The mosfets are held on with a very high temp thermal compound. You will need a heat gun that has variable heating's that gets really hot. Typically without out the proper tools you will end up heatsoaking the mosfets that are attached to that secondary pc board and get them way too hot and damage them. 
So another reason to replace all mosfets at once.
You will also need to find a similar thermal compound that will also hold the new mosfet to that pcb since they are not bolted in place or held in place with spring clips like other amps. Failure to do this will cause them to over heat and blow.
With current market prices you maybe cheaper off buying a new amp sadly.

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1 hour ago, audiofanaticz said:

Whoops, almost forgot RF amps use that secondary pc board that they attach their mosfets too which then gets bolted to the heatsink.

The mosfets are held on with a very high temp thermal compound. You will need a heat gun that has variable heating's that gets really hot. Typically without out the proper tools you will end up heatsoaking the mosfets that are attached to that secondary pc board and get them way too hot and damage them. 
So another reason to replace all mosfets at once.
You will also need to find a similar thermal compound that will also hold the new mosfet to that pcb since they are not bolted in place or held in place with spring clips like other amps. Failure to do this will cause them to over heat and blow.
With current market prices you maybe cheaper off buying a new amp sadly.

 

 

Completely, agree.. I was under the assumption it was diagnosed based off the first post which further reading and not drinking I should of paid attention to the second lol. In the past I've used the same amp (when I can find) from the same month +/- within a month if possible (you can lookup the serial number on their site but I've had bad luck on a few that I think were knockoffs because of the casting  and parts used but again speculation... could be a shotty repair) solely for my personal equipment plus I have a boneyard for future scavenging should I need something different... But yes it is wise to replace all the fets. Barevids has a funny video on tracing issues with the fets and drivers but you would need an oscilloscope to check the wave. Sad but true on repair/replace.. sucks to get rid of a classic. Hated the side panels but made minor installs easier to hide everything.  

 

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