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Old 16th February 2012, 03:48 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Memphis 16-MCA3004

I've got this amp on the bench which isnt powering up totally well. I pulled it out of the sync, and found Q272 output, and IRF9540 shorted across legs 2&3, and 180 ohms across Legs 1-2.

I clipped the shorted IRF9540, but the amp still fails to power up. It does pulse power-up for about 1/4 second, but that is barely enough time for any troubleshooting. The PS is probably in good shape at this point.

In looking to find further faults, I'd like to be able to troublshoot this amp is a semi-powered on state. Is there anything I can do to let the PS run while connected?
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Old 16th February 2012, 06:07 PM   #2
timwebb is offline timwebb  United States
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U tried isolating the power supply?
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Old 16th February 2012, 10:35 PM   #3
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Power it up through a limiter and measure the DC voltage across the source resistors as it tries to power up. Do any read more than 0.000v DC?
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Old 20th February 2012, 01:04 PM   #4
Dr Zeus is offline Dr Zeus  United States
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Perry,

It barely has enough time to power-up. I basically only see signal on the gates of the PS fets for an extremely quick shot which is not enough time for my FLuke to stabilize any readings (not even rail voltage). If there was a way, like isolating and grounding pin 16 of the TL494, then I could proceed.
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Old 20th February 2012, 02:46 PM   #5
timwebb is offline timwebb  United States
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U can try pulling the rectifiers. To stop the rail voltage from reaching the output section. It should narrow the problem area down some.
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Old 21st February 2012, 05:59 AM   #6
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Lift Q704 on the driver board. Does that allow it to power up?
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Old 21st February 2012, 10:17 PM   #7
Dr Zeus is offline Dr Zeus  United States
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Q704 is a 3AX. Lifted, the amp will power up without a limiter. +-28.8 rail voltage.

I was able to replace just the IRF9540 output and bench/set test the amp out of the sync on all 4 channels.

When I put Q704 back in, I get the protection LED and no signal on the ps fet gates.
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Old 21st February 2012, 10:33 PM   #8
Dr Zeus is offline Dr Zeus  United States
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TL494

Pin 1: 0.003
Pin 2: 4.97
Pin 3: 0.062
Pin 4: 0
Pin 5: 0
Pin 6: 0
Pin 7: 0
Pin 8: 0
Pin 9: 0
Pin 10: 0
Pin 11: 13.4
Pin 12: 13.4
Pin 13/14/15: 4.97
Pin 16: 3.729
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Old 22nd February 2012, 05:07 PM   #9
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Are you sure that your meter was working properly on 4-10?
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Old 23rd February 2012, 01:38 AM   #10
Dr Zeus is offline Dr Zeus  United States
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Nope. Tight spot. better readings now

Pin 1: 0.003
Pin 2: 4.97
Pin 3: 0.062
Pin 4: 4.97
Pin 5: 1.464
Pin 6: 3.644
Pin 7: 0
Pin 8: 13.51
Pin 9: 0
Pin 10: 0
Pin 11: 13.51
Pin 12: 13.51
Pin 13/14/15: 4.97
Pin 16: 3.729

Another strange thing I notice it, if I leave the amplifier powered off for several minutes, the first time I power it back on it will go alright. As soon as I power cycle it wont turn back on. Its almost as if leaving it powered off clears it's 'memory' and allows one time power-on. I also confirm, that if I drain the rails then the amp will power once as well.
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Last edited by Dr Zeus; 23rd February 2012 at 01:44 AM.
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