nad 304 annoying fault

[sollerbeen]

My NAD 304 has a problem with the right hand channel. This stays mute for about 10 minutes after switch-on, then bursts into life and works fine until switched off. There is no intermittent on and off, no break up or crackling. The pre-amp section is OK -headphones work fine. The speakers work ok when swapped over. I can live with this fault, but would prefer not to. Anyone got any ideas?

Cheers

Neil

 

[phase_accurate]

This could either be the output-relay itself or its control circuitry (i.e. DC protection).
This is what jumps to my mind and is not meant to be a terminal listing of possible causes.

Regards

Charles

 

[sollerbeen]

Thanks for that Charles. Any idea why it comes to life after a few minutes? Effect of heating possibly?

Cheers,

Neil

 

[ChocoHolic]

If this amp has two separate speaker relays with seperate control circuits for the speakers, then I would guess that the reason is more in the relay control circuit.
May be one of the delay caps (mostly E-caps) got old and
has a high leakage current. Leakage current of E-caps
is typically decreasing during the first minutes after applying
a voltage....
The most simple idea would be to change the E-caps in the relay delay circuit.

If the amp has only one output relay for both speaker, then I would
suspect more the speaker relay itself.

... or less probable the input selector/volume control etc...

Good luck
Markus

 

[anatech]

Hi sollerbeen,
Shut the unit off and remove the top cover. Turn the unit on and lightly tap the relays with a plastic (or similar) rod. The sound shout cut in once you are near the faulty part or connection. You may have to try this a few times.
Relay contacts can oxidize, or get burned a little bit. Or there may be a cracked solder joint. Any switch contact may be suspect so keep an open mind.

-Chris

 

[sollerbeen]

I think it must be something you guys said, but the fault has now cleared itself! I particularly like Chris' idea of smacking bits with a screwdriver and I think it must have been the sight of me advancing on the amp., soldering iron in one hand, giant screwdriver in the other that persuaded it to mend it's ways. I think I'll take the opportunity to study the schematic and get to know the innards a bit better. Trouble is the sound is so good I don't really want to mess with it.

I've got a nice old Trio KA 4002 with a faulty mains switch and a slight hum at low volume, so I think I'll give that a bit of a seeing-to instead.

Anyway, thanks for the advice.

Cheers

Neil

 

[anatech]

Hi Neil,
No, I said use a non-conductive rod. What you are talking about is called an "attitude adjuster". It is used on some customers and service department heads.

-Chris

 

[mastertech]

ChocoHolic -

" Post #4 If this amp has two separate speaker relays with seperate control circuits for the speakers, then I would guess that the reason is more in the relay control circuit.
May be one of the delay caps (mostly E-caps) got old and
has a high leakage current. Leakage current of E-caps
is typically decreasing during the first minutes after applying
a voltage....
The most simple idea would be to change the E-caps in the relay delay circuit.

If the amp has only one output relay for both speaker, then I would
suspect more the speaker relay itself.

... or less probable the input selector/volume control etc...

Good luck
Markus"

i agree with markus, check control circuitry thoroughly resistors, semiconductors etc...

cheers

 

[sollerbeen]

The fault came back. I took Chris' advice and tapped around a bit (with the blunt end of a pen -not a screwdriver!). There is only one relay - I assume this has one actuator and two sets of contacts. It clicks when the red LED turns green- there is no further mechanical sound when the right hand speaker cuts in. Tapping the relay makes no difference to when the channel cuts in, but prodding around the big electrolytics in the centre of the board does seem to get it going. I've yet to work out where they fit into the scheme of things, it's been many years since I've delved into the workings of amplifiers. Thanks for all the comments so far, any further advice or enlightenment would be very welcome.

Neil

 

[anatech]

Hi Neil,
Okay, you seem to have bad solder joints. The big caps only give you leverage on the PCB, the the actual bad joint is nearer to one side or edge. Tap around the channel that cuts out. The heavier components are more prone to bad connections, but it could be anywhere.
-Chris

 

[ChocoHolic]

ughhh, quite inconvinient such sort of fault
... probably Chris is right...
good luck in finding the bad solder joint.

In first step you could suspect all mechanically stressed solder joints.
I.e. traditional suspects could be a solder joint of the input terminals on the PCB..

Don't worry if you don't find it the reason for the fault immediately.
I had a similar fault in an amp of an friend.
After 2 hours searching, I decided to resolder all joints.
2 more hours. No success.
I had to follow every PCB track systematically with the scope
and finally found a cracked track. The crack was so small that you
could not see it with your eyes, even after I knew where to look....

But before you start to much trouble in searching, you should first make sure that the speaker relay is really OK.
Let the musik play and measure with a DMM if the Signal is missing
on both sides of the related relay contact, or if it gets lost across the contact...

 

[jaycee]

I had a fault in an amplifier where the music would suddenly cut out with a loud *THUD*, intermittently and unpredictably. After checking the usual faults, and deciding that the STK chips were fine, i was flummoxed. It was only by chance that I noticed a transistor configured as a zener follower was cold when it should have been hot. Replacing the transistors made no difference however.

Poking around the area with a multimeter probe to check voltages, I slipped and hit a resistor. *THUD* the music came back on. It turned out the resistor was cracked internally even though on the outside, it appeared fine... and the -ve voltage to the preamp and frontend of the STK chips was disappearing.

It seems sometimes when it comes to fault finding, a certain amount of karma is involved

 

[anatech]

And of course you are correct. Good techs don't charge to replace the part, they charge to find the bad part and replace it.

-Chris

 

[sollerbeen]

After a lot of prodding about I still can't pin down the fault. On the whole I'm inclined to put up with it. It occurs less frequently now than it did a week ago. (I should mention it started after I had moved the amp. a few miles in the back of the car - with a fair number of road humps involved). About three out of four times the channel comes on instantly, on the remaining occasions it's on within about five minutes. I have noticed that the channel sometimes comes on at a reduced volume, jumping to normal vol. after a few minutes. I've also found that momentarily twisting the volume control to a high level will sometimes start it up.

Anyway thanks for the advice and suggestions. If the problem worsens I'm going to have to get more involved, but as I said before, it sounds great and I would really like to avoid resoldering every joint.

Thanks

Neil

 

[kbook]

This thread was a year ago but I found out that my NAD304 acts the same way where my left channel mute and unmute itself. I have to increse my volume up and then the sound came back. It is weird. I did the simple current and voltage adjustment. All values are ok. Furthermore, my problem seems to be a balance pot. When I turned to to the right channel, I can still hear the left channel.