NAD 7240PE - No sound to speakers, headphones very faint.

[MB2021]

Hello all, I have been reading up on this thread https://audiokarma.org/forums/index.php?threads/no-sound-from-nad-7240pe.756209

I have made some progress from when I first started out. The issue is that the amp went silent suddenly, no magic smoke, no popping sounds, nothing. It just crapped out. Tested the headphone jack on the front, no audio. Checked the output with my oscilloscope, nothing is coming through.

Deoxed all the knobs... No change.

I noticed that the board for TH1 looked rough, and after removing the board it had major corrosion, so for the time being I replaced it with a 100-ohm resistor, to continue testing.

I went through and replaced all the large caps (C505, C506, C509, C510). No luck, I began reading the aforementioned thread and tested the voltages for R311, R312, and R505, all voltages were within 75mv of what I had expected to see.

While I was working on this section of the board, I also decided to replace C514, C515, C516, C517 and C518. Since I had some on hand.

After replacing the caps, I was able to get an output on the headphone jack, although faint at full volume.


I am at a loss at what to look for next, any suggestions?

Edit: I missed one of the caps I replaced, which was C517

 

[audiomaniac2]

Check power supply for correct voltages and correct voltages in different stages of the amp . Use your scope to signal trace through different stages. Track down the problem before changing a bunch of parts. Use a dim bulb tester.

 

[mbz]

Use your scope to test for audio at the pre-out/main in links at the rear. Check that all switches are in their normal positions, ie, tape monitor off, dirty switches are an issue in vintage, work through each switch in turn, ie, press a few times, note any change in audio. Since both channels affect it could be a power supply issue or a problem with the FET mute circuit. Start with pre-outs...

 

[northpaw]

I second mbz's comments about the switches. My experience with switches and pots (including the concentric volume and balance pots) in this era NADs is that they tend to foul up to the point of no sound, and are recalcitrant to corrective measures. They ultimately yield to persistent efforts, but only if those are applied to actual contact surfaces. And don't forget the rear panel switches (power off only).

So if you don't mind, please describe exactly the procedure you used when you "Deoxed all the knobs".

 

[MB2021]

Use your scope to test for audio at the pre-out/main in links at the rear. Check that all switches are in their normal positions, ie, tape monitor off, dirty switches are an issue in vintage, work through each switch in turn, ie, press a few times, note any change in audio. Since both channels affect it could be a power supply issue or a problem with the FET mute circuit. Start with pre-outs...

Thank you! There was a bad solder joint for the main-in jacks at the rear. A quick reflow later and it worked! The sound is still not quite as expected, a bit crackly as compared to what I expected, it seems to be related to the volume knob, so I will give that just a bit more of a treatment.

So if you don't mind, please describe exactly the procedure you used when you "Deoxed all the knobs".

Of course, I just sprayed inside the potentiometer with some contact cleaner exercised the knob for about 45 seconds or until the overspray started to evaporate off the board, and repeated the process.

 

[northpaw]

Glad you found the bad solder. On the cleaning regimen, cleaning first with simple contact cleaner is what I like to do as well, and many cases will respond well to that, and not leave an oily mess. But some pots and switches are helped by following up with a light touch with product like Deoxit, which has some additives that get at corrosion better. Also, you probably noticed that the balance pot shares the same shaft with the volume pot. Those two, and the speaker selector switch, are the ones I've had the most issues with on my 7240, and they did require repeated treatments before all sounded well.