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Old 29th November 2010, 03:33 PM   #1
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Default Sherwood XA-1240Q

I have this Sherwood amp I am trying to get going again. Problems are channels 3 and 4 work fine, channel 1 has no sound, and channel 2 has sound, but it has static and cuts out. This amp is also fan cooled, which if I remember right, the fan was always on. But one this amp the fan is off, no matter what the amp does.

So far, rail voltages are +29.9 volts and -27.7 volts. Nothing looks bad, may have found a bad Zener D560, which reads .757 one way and 1.87 the other way (on diode check), (this is for the cooling fan). I do have a schematic for the amp if needed.

Any ideas???
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Old 29th November 2010, 03:53 PM   #2
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The symptoms seem to indicate that there is some sort of electrical leakage. This is generally due to electrolyte conducting between traces. You can also have leakage through fixative that's broken down.

Do you have a scope?

Email me the schematic diagram if you can't find the fault.
babin_perry@yahoo.com
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Old 29th November 2010, 04:31 PM   #3
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schematic has been sent.

Yes on the scope.
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Old 29th November 2010, 10:02 PM   #4
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What's the voltage across the diode in the circuit when the amp is on?

What is the voltage on all 3 terminals of Q560?
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Old 30th November 2010, 01:47 AM   #5
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The diode is reading 10.96 volts.

The terminals on Q560 are as follows, with meter black to amp ground,
Terminal one: 10.35 volts
Terminal two: 12.86 volts
Terminal three: 11.00 volts
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Old 30th November 2010, 01:57 AM   #6
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Measure the voltage directly across the terminals of the fan.
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Old 30th November 2010, 04:41 AM   #7
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Fan voltage is 10.35 volts. If I give the fan a small push it will run. BBut, once you power it off, you have to help it start spinning again.
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Old 30th November 2010, 07:54 AM   #8
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If it spins freely with no power applied, the fan may have a defective drive component. If it doesn't spin freely, the lubrication in the bushing may have dried up. In most fans, you can remove the old lubricant and the fan will work normally. A drop of motor oil will generally be a good lubricant (after the old lubricant is removed completely) if the bushing isn't worn too badly.

When you begin on the audio problem, you'll need to follow the signal from the point where the RCAs solder into the board until you find the point where the audio stops or becomes noisy. I'll assume that you've already operated all of the switches and pots through their entire range to confirm that none are intermittent.
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Old 30th November 2010, 02:05 PM   #9
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Cool, thanks Perry, I will start checking things, and will let the forum know what I find.
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