Nikko Alpha 220 - Please help!

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#42
BSST said:
This is only a wild guess, but I suggest checking the Zobel network: R790 and C742, 10 Ohms, 0.047 uF. An Ohmmeter will allow check of R790. The cap has reactance of about 3.1 ohms at 1.1MHz, so if you measure 200mV at the output, there should AC volts dropping across both R790 and C720 if they are both viable.
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Thanks for giving it a thought!
Perhaps just a production line change that was not documented:
R790, and its peer on the left channel, R789, are in fact 4.7 ohms @5% tolerance, not 10 as stated in the manual.
But both are measuring 5ohms, a tad out of spec, but not sure if that would cause the issue.
Mylar cap C742 measures 47nF, not that I expected a reading from the multimeter, but it is there. C741 is not displaying any reading (0L), but that is on the "good channel", so I wont bother about it.
 
#43
Turbowatch2 said:
As this is a very old amp, caps should always be considered "not new".

If you got ripple on one side only, this can be the result of a dry cap.

As long as one side of a stereo amp is working, fault finding should be possible, just by comparing the same points, left and right.

Did you check the voltages at C705 /C706? (707/708) Is there a lot of dirt on the DC? If this cap is bad it could induce noise (and make the FET look bad) only on one channel.
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Take a look at the beggining of the thread, you can catch up with most things I have already done with this amp.
IMO they are worth restoring.
 
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