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Old 9th May 2008, 01:47 AM   #1
frt96 is offline frt96  United States
diyAudio Member
 
Join Date: May 2008
Default memphis 16-ST500D

HI all,

I am new to this forum anda novice when it comes to car audio amps but I have worked for the navy for the past 13 years repairing and maintaining high power 10KW HF Radio transmitters and 50Kw Lf Transmitters. I have a tech shop with every piece of test gear that you could ever need.

My favorite nephew blew his anp up and asked if I could fix it.

I looked around the net and found you guys.

Here is where I am at.

Performed physical inspection of amp and found 3 blown fets on power supply side. removed and replaced with same part# from newark. removed and checked 3 large filter caps. They were fine. Pulled six fets on ouput side and found 2 shorted.
one IRF9640 and one IRF640 (same??)
checked 2 bipolar transisters they seem alright.

Powered with outputs removed and protect light came on.

Measured 13.5 gate voltage on all power suppy fets.

Have new ouput fets on order.

I have amp still in heatsink and hooked up to a 13.5V 22 amp output power supply with 7.5 amp fuse to protect amplifier.

any thoughts on what caused this??

Any thing I can check out while waiting for new fets??

Thanks
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Old 9th May 2008, 02:47 AM   #2
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Join Date: Nov 2003
Location: Louisiana
You should replace all of the power supply FETs so they'll match. The amp is likely to be more reliable. For testing, you can use the mismatched components.

With the power supply FETs out of the circuit, do you have a clean square wave on the gate pad for the PS FETs? The square wave should go to ground with a vertical falling edge on the waveform. If it doesn't, there may be open gate resistors and/or open PNP driver transistors. Check the signal on all pads after replacing the FETs to confirm that all have the same signal. The driver transistors are on the end of the vertical driver board near the PS FETs.

I'd also suggest replacing all of the outputs. There are probably two IRF640s and four IRF9640s. It's best to have matched parts when they are running in parallel.

If the other 640s and 9640s are OK, you can leave them in the circuit for testing. If the amp goes into protect and you're 100% sure that the 640s and 9640s are OK, pull the driver board. If it doesn't go onto protect with the driver board removed, there are likely blown drivers (A1023?), defective 12v zeners and possibly open 1 ohm resistors.
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Old 9th May 2008, 07:16 AM   #3
frt96 is offline frt96  United States
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Join Date: May 2008
All the power fets are the same I ordered 10. I also ordered all new ouput 9640's and 640s. I will check the drivers right now.

Thanks
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