NAD 314 problem (fading channels)

[Juko]

Hi all,

I picked up a NAD 314 for little money. The right channel was dead. The first quick examination learned that the pushing (not turning) the bass-control had an effect. It was not tightly connected to the metal case, so I screwed the nut on. Both channels worked now, but after a while it sometimes still fades and becomes then louder again. Strange, because the outside of the bass-control potentiometer should not have influence on the circuit (I suppose). But when I move the metal 'turn-pin' of the potmeter against the metal case you can hear it on the speakers.
So I thought it was a grounding problem and I made sure the outside of the tone-control potontiometer was connected with the grounded case by soldering. This helped away the fading, but now I hear a very loud click when I activate the tone circuit by pressing the tone defeat button.
When I click the button in (bypass tone circuit) I hear nothing, but when I click it again (turn on tone-circuit) I hear a click. The longer I wait, the louder. This makes me think there somewhere is a DC-current which shouldn't be there. But where?

This all makes me think the problem is bigger as I thought. The amp works fine now, great sound also. The fading is almost over, but not at all.

Can somebody help me? What could this problem be? I include the schematic of the line amp part (and tone-control circuit), which I got from Etsang (thanks again).

I have basic experience an knowledge with electronics and amplifiers and I like to learn, so any help would be greatly appreciated.

[unclejed613]

looking at the circuit, i can see 2 places where DC leakage could cause the popping you mention. C148 near the line amp output, and the 47uf electrolytic near the tone defeat switch. if either of those caps were leaky, you could get the popping you mention. put a voltmeter at the wiper of the switch and see if you have dc there (switch the tone control in and watch for dc, then switch the controls out and watch for dc) , then check at the junction of r 160 and r170 for dc. it most likely is at the switch. if you find dc at either place replace the cap causing it.

also it sounds like you should rebuild that noisy pot. how to do this is shown here:
http://www.geofex.com/Article_Folder...s/potscret.htm

[Juko]

Thanks for the advice. I think i traced the 'pop'-problem now. It was in the right channel of the pre/line amp. My multimeter couldn't find any DC at the points you told me, but I think that's because it's a digital meter (maybe to slow to catch the pop).

Unfortunately my fade-problem is not solved. Both channels are still fading/cracking/dead sometimes. I disconnected the pre-amplifier from the main-amp and connected a cd-player (with adjustable volume) to the main-amp. Still cracking and fading. Not sure if this is a right method, but I think this isolates the fading-problem to the main-amp. Am I right?

And again my question is, what could be the problem?

[unclejed613]

try checking the collectors on the diff amp (LTP) and see if the voltages are drifting there...... also check the emitters (could be the current source) if you have any zeners in your input stage, replace them. any problems with the diff amp will cause changes in gain. zeners get noisy (so do some diff amp transistors)

[EchoWars]

Does it use relays in the speaker protect circuit? Dirty relays can do what you describe too.

[unclejed613]

if it does use relays, tap on the relay with a screwdriver handle.
if the relay is bad, replace it, or if you feel you can do it, burnish the contacts with a piece of brown paper bag.

[Juko]

I tapped the relay, which indeed seems to be the problem. I will try to burnish the contacts. Thanks guys!
I will let you know in couple of days if it worked.

[Juko]

Problem solved! I have a very nice sounding amp now, for very little money.
Thank you guys!

[aes]

I just discovered this thread.

I have had a NAD 314 since new (Nov 97) and in past few years it has been exhibiting symptoms similar to what the OP describes. In my case, the unit would operate fine for about an hour or so till it warmed up, then the left/right channels would fade in and out randomly. If I adjust the volume up or down from the source input (eg TV), not using the volume pot on the amp, the problem temporarily goes away (few seconds). I pretty sure its not a dirty pot problem since I don't get the characterisic crackly sound when I turn any of the pots.

I suspect a bad solder joint/ground or something similar that loosens up when things get warm.

Interesting to read that the OP found that the speaker relays were a major contributing factor. A while back I did a similar test using a source direct into the power amp and found that things were working properly. I suspect my problem is in the preamp.

Anyway, all this to say thanks for sharing your experience, it may help me.

Alan

[Elvalys]

Hello,

I discover this threat with a lot of pleasure because I also have a NAD 314 and start to have some similar problem.

Therefor there is a little different, I only have the prob on the right channel. So it shouldn't be coming from the pot...

May someone have an idear to help me ?

I precise that I am not really use to electronic testing and just created a preamp in my life... I am full of good will so with appropriate explainations I will manege the best I can

Thks

Fabrice (from France)