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Old 29th July 2012, 01:52 AM   #21
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Hi Steve,

Can you tell me which resistors those are on the Schematic I posted on the previous page?

Thanks
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Old 29th July 2012, 01:55 AM   #22
srh is offline srh  New Zealand
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Hi Terry, R17 - 26 are the gate stoppers.
Steve.
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Old 29th July 2012, 02:04 AM   #23
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Quote:
Originally Posted by srh View Post
Hi Terry, R17 - 26 are the gate stoppers.
Steve.
OK, those are the ones I measured. They each settled at 0000.0 mv.

Thanks, Terry
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Old 29th July 2012, 02:29 AM   #24
srh is offline srh  New Zealand
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Hi Terry, you have checked all the fuses on the back panel haven't you?
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Old 29th July 2012, 02:50 AM   #25
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Yes, fuses are all good
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Old 29th July 2012, 03:24 AM   #26
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Quote:
Originally Posted by Nrik View Post
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
Ok, I adjusted the bias until the P- side came up to 0.424V but now the P+ is 0.528. Also, the output offset is now +032mv

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Old 29th July 2012, 03:47 AM   #27
srh is offline srh  New Zealand
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Hi Terry, I'm not sure there is a problem with the fets. You are able to vary the bias voltage ok and both ends of the pot change voltage. Another thing that can be done is to reduce the load on the collector of Q4. On the p chan. side lift one end of the gate drive limit network D3-ZD2 and see if the voltage changes. Also lift one end of the gate stoppers one at a time to see if the voltage changes.(R22-26). Make sure the filter caps are discharged before lifting any leads.
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Old 29th July 2012, 03:48 AM   #28
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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.


Thanks, Terry
Just looked at the Soundcraftsmen and I was wrong. The MOSFETs are J50/k135. Not going to work. I did find some J52/K176 for sale but he wants $25 a pair with $28 shipping
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Old 29th July 2012, 04:06 AM   #29
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Quote:
Originally Posted by srh View Post
Hi Terry, I'm not sure there is a problem with the fets. You are able to vary the bias voltage ok and both ends of the pot change voltage. Another thing that can be done is to reduce the load on the collector of Q4. On the p chan. side lift one end of the gate drive limit network D3-ZD2 and see if the voltage changes. Also lift one end of the gate stoppers one at a time to see if the voltage changes.(R22-26). Make sure the filter caps are discharged before lifting any leads.
Steve.
OK, do I need to lift the gate resistors in pairs (one from each side)? Will this essentially take the MOSFET out of the circuit?
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Old 29th July 2012, 04:26 AM   #30
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Hi Terry, just the p chan (j52) one at a time. This will take the fet out of circuit. To be safe, temporarily ground the gate of the fet you take out of circuit to prevent accidentally turning the device on while testing.
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