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Old 15th December 2012, 06:23 PM   #11
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The caps are the silver metal cans. They are notorious for causing problems but they look clean on the preamp board. You can see it better so you'll have to determine if there is any corrosion.

The same caps can cause problems throughout the amp and to make the amp reliable, long term, you may have to replace all of them if any have begun to leak.

The resistor is part of the feedback circuit. Is the one on the other side (R133) similarly discolored?

If you touch the fluid on the board with your soldering iron and it smells like anti-freeze (or some say it smells like fish), it's electrolyte and the cap near it will need to be replaced. You'll need to clean the residue with solvent (acetone works best but alcohol will also work).

Dive a 100Hz signal into the amp and measure the AC voltage on pins 8, 12, 17 and 18 of the preamp board. Post the voltages. Have the gains set the same for both channels. Set the input level from the signal source so that you read at least 0.1v AC on pins 8 and 12. Place the black probe on the RCA shield when taking the readings.
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Old 15th December 2012, 06:40 PM   #12
dhamia is offline dhamia  United Kingdom
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Thanks Perry, can I swap the caps for regular ones or do the need to be exactly the same?
Also how do I determine the values?
I might just change them all
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Old 15th December 2012, 06:53 PM   #13
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They are generally all 10uf@16v. You can use standard caps but there's no point. The hardest part is getting the old SMD caps off of the board without damaging the pads. Replacing them is very easy (on the main board, it's more difficult on the preamp board because they're so close together). If there is no corrosion on the preamp board, I'd leave them alone for now.

Post the voltages.
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Old 20th December 2012, 07:36 PM   #14
dhamia is offline dhamia  United Kingdom
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Hi again, I removed the audio driver board, inspected and cleaned it an re installed,
it powered up and the gain on the bad channel actually worked but then Q1 and Q2 went up in smoke.

Any ideas?
Also what are the replacement part numbers please?Click the image to open in full size.
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Old 20th December 2012, 07:45 PM   #15
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Well those were your power supply fets. You might have shorted outputs.

You need to replace the 2 power supply fets, the power supply fet gate resistors, and the power supply drivers.

Test the output fets and make sure they are not shorted.

Your first priority now is to get the supply up and running, you can do no further testing until the amp will power up.
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Old 20th December 2012, 07:51 PM   #16
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R2 and R3 are the gate resistors.

I'd replace them with 47 ohm resistors and use IRF3205s as replacements. Power up the amp and measure the DC voltage on the ends of R2 and R3 farthest away from the FETs. What voltage do you read?
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Old 20th December 2012, 07:54 PM   #17
dhamia is offline dhamia  United Kingdom
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Thank you gents, I'll give it a go and message back,

one thing I think that could have caused this is, I sprayed some wd40 into 2 screw clamp holes as they were tight, maybe it caused a short?
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Old 20th December 2012, 07:56 PM   #18
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WD-40 won't cause any problems at the low voltage that's being used in car amps.
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Old 21st December 2012, 10:31 AM   #19
dhamia is offline dhamia  United Kingdom
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Ok I replaced the 3205's and the resistors, I also pulled q203 and q206 both are defective

I powered it back up and it works,

So any idea what q203 and q206 replacements are?
Thanks
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Old 21st December 2012, 11:12 AM   #20
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I generally use IRF540s for replacements. Check R118 and R126.
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