Rockford Fosgate P5002 Rail Voltage on Output FET Gates - diyAudio
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Old 5th September 2011, 01:21 AM   #1
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Default Rockford Fosgate P5002 Rail Voltage on Output FET Gates

Greetings,

I am trying to repair my RF P5002. I believe it stopped working after the speaker wires shorted. The amp powers on fine and only draws about 1.6amps at idle. Using the info in this forum and Perry's online guides, I was able to locate a shorted 28N15 (Q222). I removed and replaced all 4 FETs on that channel (Q218 Q222 Q217 Q221), which removed the constant negative DC voltage on the right negative speaker terminal. My issue now is that I am seeing what I believe to be rail voltage (+- 46.2v) on the gates of all the output FETs. I also see rail voltage on several of the driver transisters on multiple pins. With power disconnected, I don't see any shorts in the drivers.

I also checked both the LM337 and LM317 regulators and they appear to be fine.

I tried contacting RF about getting the schematics, but no response from them yet.

Any Ideas?
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Old 5th September 2011, 08:21 AM   #2
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It's normal to have rail voltage on the various transistors.

It's normal to have 'near' rail voltage on the gates.

Does the amp produce audio?
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Old 5th September 2011, 09:08 PM   #3
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I decided to hook the amp back up to see if it produced any audio. Low and behold, it works beautifully now. I'm not exactly sure what I did to get it working. After I replaced the Output FET's, it wasn't producing any audio. The only thing I can think of that I did differently was to use a 12V battery instead of just a high current charger. I am assuming that the charger produces half wave DC with almost zero filtering. Maybe the amp just needed a clean DC input?

I asked about the Rail voltages because I had read that the gate only needed between ~2 and 4 volts to allow the FET to cross over. And that any more voltage above ~20 volts could cause the internal connections to arc over. Am I misunderstanding something here?

Thank you so much for your help.
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Old 5th September 2011, 10:03 PM   #4
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The 3-4v is referenced to the source leg of the FET.If you measure the DC voltage from the source to the gate, you'll find your 3-4v.

The amp has over-voltage protection and it's possible that the charger was triggering the over-voltage protection.
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Old 5th September 2011, 10:10 PM   #5
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Fantastic! That makes complete sense. Thank you for the clarification.
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Old 15th January 2012, 03:55 PM   #6
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Hey guys this is my first time joining a forum so if I don't do something correctly or post incorrectly take it easy on Me! Anyways Im rebuilding a p5002 fosgate amp and I ordered and replaced all of the transistors on the mesha strips, my god... it kicked my a$$. I've destroyed 2 or 3 of the little and components on the boards. I need to order replacement components but im unable to locate them on the schematic ..can anyone tell me what they are? Thanks guys!
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Old 15th January 2012, 09:20 PM   #7
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Specifically, which little components?
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Old 16th January 2012, 01:17 AM   #8
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I believe them to be smd resistors and transistors. I believe them to be 10kohm un sure of the wattage.. and also a mmbt6521lt1.. since Ive destroyed one of the boards, would it be possible to mount the components on to something else and solder a wire inplace of the jumper? Or solder a wire directly to the components and like hard wire them to the board?
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Old 16th January 2012, 02:45 AM   #9
timwebb is offline timwebb  United States
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u have a picture?
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Old 16th January 2012, 03:06 AM   #10
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One is a thermistor. The other is a bias transistor. You can follow the traces from the pads to the 3-pin connector and solder the replacement transistor to the board. It needs to be in contact with the heatsink so that it can monitor the temperature of the heatsink/output transistors.

You can do the same thing with a thermistor.

I didn't notice that you posted this in a thread you didn't start. If you have other questions, start a new thread. I won't answer any more questions about your amp in this thread.
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