Greg wrote:
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.

Greg,

I've got a couple of other service notes which I don't think really
apply to your case, but what the heck, have a look at them:

Factory service hint: AC line clicks are audible. Grounding problem of
input jacks. "SOLUTION: mount a **FOIL** 0.33uf, 400V cap between the
housing and the ground of the RCA jacks. The best is to loosen the
screws holding the RCA jacks and clamp one leg of the foil capacitor
between the plastic of the RCA lag and the back panel: the other wire
can be soldered to the ground of the RCA jack.

I've got my own notes mentioning that R823 (150K) can go high in value
which will cause the unit not to function. Preamp lights may be out.

I've got another one where I noted that a bad 5V standby supply filter
capacitor (C502 or C505, C502 more likely) can cause the main power
relay to chatter.

I'd also try poking at the main power relay, in case its got bad
contacts and is arcing. Lastly, I don't think the power supply PC board
is grounded to the chassis through its mounting screws, but I'd check
and make sure that they are tight. I'll let you know if I can think of
anything else.

Regards,
Tim Schwartz
Bristol Electronics