NAD317 Strange Oscillating PSU?

About what frequency is the hf oscillation? Can you make it change frequency by bringing your finger near anything.? loose/broken ground connection somewhere between ps/prea/pa?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N Cook" wrote in news:frk6td$j7p$ snipped-for-privacy@inews.gazeta.pl:

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?

I would start by taking an ESR meter and going through every cap in the amplifier.

--

JANA _____

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.

Thanks for the tips guys, Nigel I will check the grounds as you suggest, Jana - I have ordered an ESR meter & will try that. Here in southern Spain I have seen many dried out caps & this meter will help. Best regards, Greg.

Greg,

There is a NAD service bulletin that might relate to your problem:

"PROBLEM: a distortion is audible, sometimes after a few hours use, or one channel will play not as loud as the other."

"Cause: There is a bad contact between the power supply PCB and the power amplifier board caused by a bad contact inside the connector of the screened (shielded) cable."

"SOLUTION: Remove the two pin plug and solder the screened cable directly to the PCB. It is advisable to perform this modification when the unit comes in for any service."

"PRODUCTION: This modification will be implemented in production, there is no serial number information known at this moment."

This service note is from 1996. It might even be an audio cable that carries the ground that is the problem, so I'd suggest checking all of the shielded cables going to the amp boards. Sorry, I don't have connector number info.

Regards, Tim Schwartz Bristol Electronics

P.S. This also confirms what Nigel Cook and "BZ" thought. I doubt that the capacitors are the issue.

But make sure you still use the shielded cable to run power through. Great work, NAD.

Hi Tim, Thanks for the info. This amp has been modified I guess at the factory as it was made mid 1997. The wires were soldered to the board & covered with hot melt glue which I think is a bad thing. I have removed both input screened cables & replaced as a precaution. Sadly it has made no difference to the problem. I am checking the ground connections but I still suspect the design of the PSU is inherently unstable. I can induce the fault by switching on my heat gun & holding over the PSU board. I was trying to see if the problem is heat related but it seems not.

Clifto - I don't understand your comment. I have replaced the screened cables as described above but are you suggesting the power leads need screening too?

Best regards, Greg.

If the glue is at all browned, remove it.

Mark Z.

(piggybacking - somehow the original never got here)

I just have objections in principle to running power through a shield braid inside an electronic unit. It seems a cheap way to save a penny. I myself would add a 16-gauge wire for power (ground) connection.

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

Hi Tim, thanks for your continued interest! The amp doesn't seem to have any power or protection circuit problems as you have seen. Still seems to be a grounding or capacitor issue, I hope to have an ESR meter next week to try. I don't suppose you have the circuit diagram? That would help some! The amp is a pig to work on as the PSU board is attached by fairly sort cables to everything & I even had to unsolder the speaker binding posts to lift the board to look underneath! You are right that the PSU board is not grounded to the chassis directly. There is a single black wire from "chassis" on the silk screen that goes to a solder tag under one mounting screw, I have cleaned & tightened it. Similarly on the left hand side of the board one of the mounting screws is used to provide ground to the speaker output. All in all the standard of wiring & soldering is poor. I have checked & re solderered anything suspicious but still no luck.

I found a posting in another group that seems to be the same fault:

I emailed the guy but had no response.

I will persevere as this is really bugging me!

Best regards, Greg.