Precision Power 2150AM Repair

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J

Justind

Member
Joined 2025
#1
So after 30+ years of sitting in the basement, I pulled my old equipment out to resurrect for my son's car. In a bonehead move while hooking up the 2150AM (Board Rev K) I somehow reversed the input voltage and released some of that magic smoke from the amp. I pulled the amp apart and found the Q50 MOSFET was smoked. I removed that Mosfet and applied power back to the amp. The power light turned on and all components were cool.

In the next bonehead move I decided to see if the amp would still function. I hooked up the input and a 12" to the left channel. Surprisingly the amp was working as it should even without this FET. As I was bragging to my son how they "just dont make em like they use to" something happened and I pulled the power lead. I again touched the power lead to the battery and the power light began blinking then just a bit more smoke came from the FET side of the amp. I removed all 10 FETs from the amp and most are shorted and the others dont seem to test correctly. I had the cover off the back so I am guessing they overheated without being pressed to the heatsink, and the fact it probably is also not intended to work with the missing Mosfet. 🙁

The original ones are labeled as PPI25N06 I cannot seem to find these anywhere. I ordered substitutes from Digikey that seem to be a replacement? FQP30N06FS-ND. Does anyone have any idea if these will work properly with this amp?

https://www.digikey.com/en/products...?s=N4IgTCBcDaIGIEUAKBmADAOTQNjgZQFoMAREAXQF8g

What are the chances anything else was blown during this show? I checked the output transistors, and they all seem to test good.

Thanks for your time! Justin


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#2
Those may work. In general, people use something like the IRFZ44 and they generally require a change of gate resistors. Using something like the IRF3205 (another common sub) the gate resistors definitely need to be changed.

The high gate resistor value typically protects the PS drive components.

The supply would run at low power or at idle with the missing FET. You actually only need 1 FET per bank for it to function.

Not having the semiconductors clamped tightly to the heatsink can allow them to overheat and fail. This can happen within just a few seconds. When repairing this type of amp, you generally need to make some sort of clamp to take the place of the cover.
 
#5
I have an electronics background, but been awhile since I have done component level troubleshooting.

I was thinking with the low Input Capacitance, these would work with the original gate resistor values.

Any idea if there is anything else I should check other than the output transistors for failure since it sounds like this was likely a self-inflicted overheat issue?

Any experience if it's common for just 1 FET to pop when the input voltage polarity is reversed?

Thanks
Justin
 
#6
Is it common for only one link in a chain to break? If you would have left it longer, they would all have failed as the intrinsic diodes would have been passing enough current to destroy them. The others could have been damaged and took a bit longer to fail.


When you power it up, have the semis clamped tightly and insert a 10 amp ATC/ATO fuse in the B+ line. If the fuse holds, all else may have survives.

Are you left to use the car supply for 12v or do you have a small 12v bench supply?

A scope?

Multimeter?
 
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