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Old 9th April 2008, 10:56 PM   #11
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Pins 2 and 3 are probably clamped because of the DC on the outputs.

What's the voltage from pin 5 to pin 6 of each of the opto-couplers?
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Old 9th April 2008, 11:05 PM   #12
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Opto-couplers? If your talking about the 3120's, the voltage across pin 5 and 6 is 0V.
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Old 9th April 2008, 11:17 PM   #13
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Yes, the 3120s.

That means the optos are not driving any of the outputs on. The outputs may have failed. It doesn't appear that they're shorted because they aren't driving full rail voltage to the speaker terminals. If they're leaking, you may have to remove them from the board to determine which one is defective. After finding the defective one, reinstall the others to see if you still have DC on the speaker terminals.

There are 5 outputs in parallel on each side of the sink. See if you can find one that has more voltage from the gate to the source (1-3) than the others. If you find one, it's probably the culprit.
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Old 10th April 2008, 07:51 PM   #14
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I pulled the output mosfets which all 10 are IRFP240's, I tested them and im getting ~.5V across the Drain and Source on all of them, and nothing in any other combination. With this particular fet, should I be getting a connection between the Gate and Source since there is "virtually" a Zener diode between the two?

Here is the data sheet: http://www.vishay.com/docs/90274/90274irf.pdf

I tested them in the board powered up, but of course there was no voltage difference between each on the same side of the board because there in parallel.

On the negative side I was getting -rail voltage on the Gate and Source, and the drain had around ~8V if I can remember clearly, and on the positive side of the board I was getting +rail voltage on the Drain, the Gate and Source had ~8V.
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Old 10th April 2008, 07:59 PM   #15
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You shouldn't get any reading (ohms or diode-check) between the gate and either of the other legs. The intrinsic diode is between the drain and the source. That's the 0.5v you read. You should have only read 0.5v when the red lead was on the source and the black lead on the drain.

What happens if you power up the amp without the outputs (do you still have DC on the speaker temrinals)?
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Old 10th April 2008, 08:14 PM   #16
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Yeah, I still have ~8V DC across the outputs with the Mosfets removed.
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Old 10th April 2008, 09:34 PM   #17
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Connect a resistor (100-1000 ohms) across the speaker terminals. What's the voltage before (assuming 8v) and after connecting the resistor?

What's the value of the resistor that you used?
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Old 10th April 2008, 09:42 PM   #18
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7.92V DC before, then 0.89V DC after connecting a 220ohm resistor.
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Old 10th April 2008, 09:57 PM   #19
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I think the voltage may be coming from a defective op-amp.

Is the voltage on pin 6 of the op=amp closest to the 3120s ~8v with no resistor across the speaker terminals?

Is the voltage across R412 (near the 3120s) ~4v with the resistor in place and ~0v with no resistor?
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Old 10th April 2008, 10:12 PM   #20
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On the 8 pin op-amp closest to the 3120's it's 0.14V on pin 6.

And across R412 it has a 0.008V drop with the 220ohm resistor, and a 0.114V drop without the resistor. There is +- 15V reg voltage on both op-amps just to confirm.
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