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Old 12th July 2010, 08:28 PM   #1
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Default Rockford Fostgate T400-2 dead

T400-2 dead no power. no LEDs or any sign of life.

various scope tests on the TL494 reviels power is present, chip is oscillating. however DTC pin 4 is held a logic high at 5v. thus preventing chip from outputting.

Checked all thermal sensors, everything appears ok. its hard to probe this board with power on as its a riser card, and bunch of crap in the way.

anyone with a schematic? thanks.
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Old 13th July 2010, 02:02 AM   #2
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More testing has revealed an accidental power up. I was probing voltages on the 2nd LM339 when the probe slipped and shorted out pins 2 and 3. effectively pulling the output of one of the op-amps to 12V VCC. this allowed the amplifier to kick on for a split second.

So being more bold, i put the meter probe back on shorting it again, and the amplifier worked fine. sounded fine, nothing got hot. soon as i let go, power LED went back off, and everything quit again.

So i bridged the connection temporarily, amplifier played fine. if i remove the remote wire connection, the audio will go out, but power supply and LED stays running until i remove the short. Or, if i cut the full power off, and back on. the LED will stay off until i reconnect remote wire.

so maybe there is an issue in the remote circuit, and making the short bypasses it?

any ideas? thanks.
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Old 13th July 2010, 04:09 AM   #3
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The 339 will likely need to be replaced of you shorted an output to 12v when it was internally trying to drive to ground.

Post the voltage on all pins of the 339 (jumper/short removed):

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:
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Old 13th July 2010, 02:13 PM   #4
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Quote:
Originally Posted by Perry Babin View Post
The 339 will likely need to be replaced of you shorted an output to 12v when it was internally trying to drive to ground.

Post the voltage on all pins of the 339 (jumper/short removed):

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:
I cant get safely to most of them.. its on a riser card. any ideas?
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Old 13th July 2010, 03:01 PM   #5
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Use a short length of stiff wire bent so that you can get to the terminals. Insulate the wire so that only the very tip is exposed so you can't short between terminals. A small dental mirror (preferably one with a plastic frame) can be helpful to see the IC pins in some instances.
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Old 13th July 2010, 06:15 PM   #6
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Well, i went ahead and removed the board anyway. I found a 412ohm precision resistor reading 32Kohm.

I replaced this resistor and replaced the LM339 and the amplifier sprung to life.
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