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Old 11th February 2012, 02:11 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default MA Audio 4000

I've got this amp which some repair dude didnt know how to fix very well. It appears all the power supply fets were replaced. Nothings shorted, but the amp wont power on correctly. It sorta appears that the drivers were replaced, but then again I'm not 100%.

Pin 5, 9 and 10 of the TL494 look abnormal when the amp is powered through a resistor.

Through the resistor, the amp only gets 6.14v across power terminals. If I supply 13v to rem, the amp tries to power up;

TL494
Pin 1: 0
Pin 2: 3.609
Pin 3: 0.063
Pin 4: 0.055
Pin 5: 1.270
Pin 6: 3.354
Pin 7: 0
Pin 8: 5.93
Pin 9: 2.065
Pin 10: 2.183
Pin 11: 5.92
Pin 12: 5.96
Pin 13: 4.87
Pin 14: 4.86
Pin 15: 4.84
Pin 16: 0.302

I *think* this amp may need new drivers in the ps but not 100% sure
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Old 11th February 2012, 03:50 PM   #2
Dr Zeus is offline Dr Zeus  United States
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This amp:

Click the image to open in full size.


I removed the PS drivers and the amp can accept full 13v power. Upon inspection of the TL494:

Pin 1: 0.015
Pin 2: 3.700
Pin 3: 0.078
Pin 4: 0.070
Pin 5: 1.397
Pin 6: 3.507
Pin 7: 0.014
Pin 8: 12.70
Pin 9: 3.210
Pin 10: 3.197
Pin 11: 12.70
Pin 12: 12.75
Pin 13: 5.02
Pin 14: 5.02
Pin 15: 4.84
Pin 16: 0.014

Click the image to open in full size.

Pins 5, 9, and 10 appear on the scope way way faster than they should, as I can only measure the signals a 1uS which seems odd.

Pin 5 at 2v/div and 1uS:
Click the image to open in full size.

Pin 9 & 10 at 5v/div and 1uS:
Click the image to open in full size.
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Old 12th February 2012, 12:44 PM   #3
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Check or replace the timing resistor and capacitor.
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Old 12th February 2012, 01:02 PM   #4
Dr Zeus is offline Dr Zeus  United States
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Just checking Film caps: Is 472J100 the same as 4n7J100?

The timing resistor is OK measuring at 3.9k
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Last edited by Dr Zeus; 12th February 2012 at 01:10 PM.
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Old 12th February 2012, 01:03 PM   #5
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Yes
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Old 12th February 2012, 01:13 PM   #6
Dr Zeus is offline Dr Zeus  United States
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Cool. I just found this myself too Thanks perry

Capacitor Chart
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Old 12th February 2012, 01:19 PM   #7
Dr Zeus is offline Dr Zeus  United States
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OK now I'm cooking. The original 472J100 cap was out of spec and even dropped a legg while pulling it. The new 4n7J100 cap makes the TL494 output correctly.
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Old 12th February 2012, 01:46 PM   #8
Dr Zeus is offline Dr Zeus  United States
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I think this amp may be OK. It has a tough time powering on my 7A power supply, but on my 12A it comes up without issue. It will not power up with my current limiter.

Regulated voltage is +-12v, rail voltage is +-67v

Think it may be fair game to put this amp back in the sync for some load tests?
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