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Old 28th July 2012, 05:01 PM   #11
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Hi Nrik,

Please post as much as you like. I need all the help I can get.

Yes, it is the base of the MOSFETs on bad channel that the negetive side that are only reading -308mv. The possitive side is 0.470V. The bases on the good channel are -0.424 and +0.424.

Also, on the good channel, I am getting about -022mv offset on the output and +029mv offset on the bad channel.

There are only main fuses. No fuses to pull on the PCB.

EDIT; I am plugged into a different outlet this morning and the Main rails are +/- 117V

Blessings, Terry

Last edited by still4given; 28th July 2012 at 05:13 PM.
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Old 28th July 2012, 09:49 PM   #12
srh is offline srh  New Zealand
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Hi Terry, have you got a 'scope? If you have, hook the bad channel up to a dummy load and feed it a 1kHz sinewave from an oscillator. Monitor across the dummy load with the scope and slowly increase the input signal. If one half of the sine starts to limit before the other, then you have one or more leaky or open cct fets. All this assumes that it doesn't blow fuses when powered up.
Good luck.
Steve.
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Old 28th July 2012, 10:07 PM   #13
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I should have stated earlier that the bad channel does make sound but it is distorted. I have to believe that it is not going work properly until I can discover why I have only -300mv where I should have -0.5V. I don't know where that voltage comes from. Is it generated by the MOSFET or Q4?

Thanks, Terry

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Old 28th July 2012, 10:26 PM   #14
srh is offline srh  New Zealand
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Hi Terry, leaky fets or faulty gate protection diodes will pull the voltage down.
Regards,
Steve.
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Old 28th July 2012, 10:38 PM   #15
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I do have a scope but don't remember how to hook it up. It was 6 years ago that I was playing around with this stuff and was only just getting the hang of it then. Also no sinewave generator.

When you say "leaky fets" are you refering to the MOSFETs? Those Hitachi's are getting hard to find.

Last edited by still4given; 28th July 2012 at 10:49 PM.
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Old 28th July 2012, 10:56 PM   #16
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Hi Terry, yes, I am referring to the mosfets. If you have a good quality dmm that can read millivolts, with the amp powered up measure the voltage across each gate stopper resister. Good fets should show a fraction of a millivolt or nothing. A faulty fet will draw gate current which will show as a voltage drop across the gate stopper. Check the resisters first to ensure that none are open or shorted. (This check with the amp powered down.) Otherwise you will have to pull the fets and test each one.
Regards,
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Old 28th July 2012, 11:20 PM   #17
Nrik is offline Nrik  Denmark
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Okay great.
So to narrow the fault options down, it could be:
1) Too low bias - adjustment needed.
2) defective MOSFETs ( one or more)

The voltage is comming from the current through the VR1 potentiometer, and its setting.
So to solve 1) carefully increase the bias voltage with by adjusting VR1 to a slightly higher ohmic value, and see if you can hit the magic -0.424 or close to it. If the bases at the P-channels stays at -0.3V and the adjustment only increases the N-channels bias, then as SRH states you probably have one or more defective MOSFETs in the P-channel range (2SJXX).

To solve 2), you could try to operate the MOSFETs 1 pair ( N+P) at a time, by pulling out the other ones (switch the amp off before pulling them of course). As long as you are not loading the amp it is no problem at all to operate it with only one pair, just make shure you have the same amount of N-channels as P-channels mounted at all times.
When you find the P-channel(s) that is/are defective and you can isolate it/them and operate without, you should experience perfect operation, both measurement-wize and sound-quality-wize.

If it is only one defective unit, the alternative to hunting down original spareparts is to remove it completely, and also pull out one N-channel even though they might all be working fine, to have the symmetry. Pull one pair form the other channel also.
Of course this is not the optimum solution, but four pairs is still decent and can still deliver a huge amount of current. So while you hunt for spares you can use the amp fully. (Just avoid throwing a party with 3 pairs of B&W or Magnepan speakers in parallel)


You can increase
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Old 28th July 2012, 11:32 PM   #18
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Quote:
Originally Posted by still4given View Post
First of all, I have +/-120v on the rails instead of 110. I'm not sure why.
Standard mains voltage in the US has gone up from 110-115Vac to 120-125Vac.

In the case of the Perreaux, that's not a fortunate event, the J56/K176 only handle 200Vdc between drain and source.

(good to see you're healthy and still kicking audio, Terry)
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Old 28th July 2012, 11:56 PM   #19
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Thanks,

Is there a way to test the MOSFETs out of circuit? I just remembered that I have a old Soundcraftsmen that I never could get going. It has a pile of those MOSFETs in it but I would need to be able to test them.

Steve,
When you say gate stopper are you speking of R17- R26? If so, they all measured 218r to 220r. They measured 0000.0mv with amp powered up.

Thanks, Terry
I did test the
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Old 29th July 2012, 12:49 AM   #20
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Hi Terry, could you confirm that you have measured the +/- rail voltages at the mosfet drains, not across the filter capacitors. Thanks.
Regards,
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