"N Cook" wrote in :

Greg wrote in message
...
Hello All,
I hope someone may help me with fixing this NAD317 integrated
amplifier made in 1997.
The owner reported strange distortion only at low levels of output on
both channels.
Hook up a 'scope & feeding in a 1kHz sine wave shows intermittent high
frequency oscillation distorting the waveform. You can see the traces
on the link below:

http://personales.ya.com/cannet/

My first suspicion is the PSU electrolytic caps (6 off 4700uF 63V & 4
off 220uF 160V), when I tap around them sometimes the fault goes away
if present. Similarly I can sometimes cause the fault by tapping them.
But strangely I can see no sign of the oscillations on the power
supply rails to either pre or power amp (the distortion is present at
the pre amp output). Also strange is that sometimes just passing my
hand over the PSU board can cause or cure the fault. Perhaps this
could be a dry joint also?

Any tips on trouble shooting this are welcome.
Best regards,
Greg.


The fact that it is a positive going oscillation on positive peaks and a
negative going oscillation on the negative peaks rather than going above
and below the 1 kHz signal should be some kind of a clue but I am not sure
exactly what it is telling us.
The oscillation looks like it is 15 or 20 kHz.

Could you have a bad ground connection somewhere?

Have you tried looking with your scope at the ground for the preamp and the
ground for the PA, using the PS ground as your reference?





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bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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