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Old 17th November 2010, 10:47 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Crossfire CFA202 wont power up

Just working this little amp through. Someone worked on it before, and theres a couple things going on with it that I'm trying to sort through atm. This amp is missing R151 near C67. Both of these pieces tie into the non-bridging speaker terminals.

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Old 17th November 2010, 10:54 PM   #2
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If the cap and missing resistor are in series and one end of the pair connects to the negative speaker terminal and the other end of the pair connects to the positive speaker terminal, it's a zobel network. The amp will work without it. There should be one for each channel.

If you can't find the fault, post the voltages on the TL494 (or whatever they're using).

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
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Old 17th November 2010, 11:07 PM   #3
Dr Zeus is offline Dr Zeus  United States
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It has a 494P.

IC#
Pin 1: 0.003
Pin 2: 4.85
Pin 3: 0.075
Pin 4: 0.010
Pin 5: 1.492
Pin 6: 3.662
Pin 7: 0.002
Pin 8: 12.30
Pin 9: 5.25
Pin 10: 4.86
Pin 11: 12.45
Pin 12: 12.61
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 2.744
Pin 16: 2.471

The power fets check out on the DVM, but one of them appears to have been replaced and has a crack in it. Both are IRFZ44. I removed the one with a crack in it (Q48) so only Q49 remains. Q48 on the bench confirms it is messed up; but not shrted. Transformer displays a "decent looking" sqaure wave on the scope but only +/- 2.5v.
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Old 17th November 2010, 11:18 PM   #4
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What's the DC voltage on leg 1 of the FETs?
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Old 17th November 2010, 11:21 PM   #5
Dr Zeus is offline Dr Zeus  United States
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11.92V on Q49 pin1. I took Q48 off the board so nothing.

Well, not nothing. ~3.5vDC where the Q48 should be
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Old 17th November 2010, 11:23 PM   #6
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The other FET(s) defective. 12v is leaking to the gate.

You should have had ~5v to the gate pad for the FET you removed. If you don't, the driver transistor has failed.
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Old 17th November 2010, 11:26 PM   #7
Dr Zeus is offline Dr Zeus  United States
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Two bad FETs. Got it. I have some IRZ44 spare from another rebuild and wil give them a try once I confirm 5v are on the gate w/o fets.
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Old 17th November 2010, 11:30 PM   #8
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Don't forget to check the gate resistors.
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Old 17th November 2010, 11:47 PM   #9
Dr Zeus is offline Dr Zeus  United States
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Both gate resistors measured OK, but one of them looked a little brown. I replaced them both.

Amp powered up - now the indicator LED is showing bright-normal. Time to check it through the rest of the way...

Thanks!
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