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Old 30th July 2013, 06:47 PM   #341
Itsmee is offline Itsmee  England
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Is this on the + or - rail or both?

Your caps are installed correctly so it's most likely a problem with the output stage, with the power off, first thing to do is to measure the continuity between the heatsink and drain of the output FETs, anything less than open circuit is a leaky insulator.

If everything is ok so far, power the amp via a bulb tester, make one if you don't have one.

Next, short the base/emitter of Q13 & Q14 to ensure the output FETs are biased off, power up then measure to see if there is any voltage across the source resistors, if there is, you found your faulty FET(s).
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Old 30th July 2013, 07:02 PM   #342
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Hi itsmee

Thanks for the help.

I get symmetrical low voltages. I have checked that the heat sink is not shorted to any of the output stages - no problem there.

I did wonder if I destroyed some fets with static.

I'll get on to Q13/14 tomorrow
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Old 30th July 2013, 09:11 PM   #343
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With q13/14 shorted the light bulb throbs So does the voltage across all the source resistors except 2. Looks like I need a new batch.
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Old 31st July 2013, 08:06 AM   #344
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I must appologise, I'm an iddiot, it was a long hard day yesterday, I should put my brain in gear before typing.

Shorting the base emitter of Q13/14 will turn the transistors off and therefor will develop a voltage across them; the transistors need to be fully turned on to bias the FETs off.

Short the collector/emitter of Q13/14 then measure across the drain resistors.

Forgot to mention, fit the feedback links into the local positions to prevent any interaction with the front end.

Last edited by Itsmee; 31st July 2013 at 08:13 AM.
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Old 31st July 2013, 10:00 AM   #345
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No worries itsmee

With the emitters and collectors shorted the voltage is back up to 24v. There's also 0v across all the source resistors. This is with local feedback.
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Old 31st July 2013, 10:59 AM   #346
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D'oh, you said you where using the FQA16N25C FQA12P20, these require about half the bias voltage, put an 8K2 in parallel with the 5K6 in the bias chain (base/collector of Q13/14) that should fix the problem.

Last edited by Itsmee; 31st July 2013 at 11:26 AM.
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Old 31st July 2013, 02:47 PM   #347
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With 8k2 everything is quiet and I have the 24v again. However, I can't get the mosfets to turn on. Should I try a higher value?

I have been more than a little foolish thinking I could simply use the fqa's.


Last edited by chalkandtalk; 31st July 2013 at 02:54 PM.
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Old 31st July 2013, 03:18 PM   #348
Itsmee is offline Itsmee  England
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Yes, it looks like a little experimentation is needed.

A quick calculation shows that with the 8K2//5K6, adjusting VR3 (or VR4) you will get a bias voltage between 5 and 6 volts.

The original values (without the 8K2), the bias range is 8V to 9.5V

27K//5K6 should put you in the range of 6.7V to 8V which is probably where you need to be.

Last edited by Itsmee; 31st July 2013 at 03:46 PM.
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Old 31st July 2013, 04:07 PM   #349
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Yippee

I tried 15k first and was able to get some adjustment so went up to 22k. There's now correct settings all round.

Thanks so much itsmee.

Now for the other channel.
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Old 28th May 2014, 08:17 AM   #350
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You know if it is possible to use mosfet current mirror ?
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