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Old 22nd January 2012, 04:41 PM   #1
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Default Direct short in Fosgate P300

I have a P300 the new style that is killing my battery.
I have a direct tone from power and ground.
I am certain the amp is pulling a constant load.

I opened the amp up and one of the heat sinks is toasted.
IRFP064N The amp still works fine, just drains my battery.

Anyway... I know the blown heat sink can cause the direct
short. is there something else I should look at.

When the bad heat sink was still in the amp, I could test for
tone on the main power and ground leads of the amp. It would get
a pulsating tone. Seems strange to me. The tone would go on and off.

Now... I removed the blown heat sink, I have tone for a few seconds
then it quits. I guess the amp is doing something. I totally don't
understand why the tone would come and go...

I basically have no idea what I am doing. I know just enough to
know the amp was the issue. Seems like it will be easy to replace
the bad heat sink.

So.. the question is... Should I test anything to look for the short?
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Old 22nd January 2012, 05:30 PM   #2
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Join Date: Nov 2003
Location: Louisiana
For clarification, the heatsink is the large piece of aluminum that the semiconductors are clamped to.

The IRFP64 is an FET and is likely in the power supply.

IF one FET in the supply has failed, the amp is likely performing very poorly. It's possible that it's drawing current when the amp is off. It's not common but possible.

When the meter reading is below a certain threshold, it will produce a constant tone. After the meter charges the capacitors beyond that threshold, the tone will stop.

Is this a P300-1?

If you replace the FET, you need to replace both of them. You also need to confirm that the gate resistors are within tolerance.

Since you're new to amp repair, read the entire basic amp repair page (link in sig line).
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Old 22nd January 2012, 06:17 PM   #3
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Join Date: Jan 2012
Mistake on calling it a heatsink. It is the Mosfet or resistor.
Yes, it is a P300.1
I am 100% certain it pulls constant power when on or off. Pulls 4 amps
when car is turned off. It is not my wires it is the amp.
Changed the amp and the drain stopped

Surprisingly the amp seems to work very well.
I bought it used. I am only pushing a P2/12

"When the meter reading is below a certain threshold, it will produce a constant tone. After the meter charges the capacitors beyond that threshold, the tone will stop. "

Ok, that makes sense. And then when the blown resistor was shorting out it would discharge the capacitor, thus starting the tone again.

"If you replace the FET, you need to replace both of them. You also need to confirm that the gate resistors are within tolerance."

There are actually 6 of them, total.

I will read the guide. I think this is an easy fix, everything works. I just need
to stop the drain on the battery.
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Old 23rd January 2012, 08:49 AM   #4
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Join Date: Nov 2003
Location: Louisiana
I don't see anything on a 300.1. Please post a photo of the inside of the amp.
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