NAD 7150 Sound Issues

[a_mozart]

After about 37 min. of listing my NAD 7150 makes a small pop sound in the speakers and then no sound. If I let the receiver sit for several hours, it will play the same for another 37 min before doing the same thing. I did some testing and trouble shooting and concluded that the relay was bad. I replaced the relay and while I was in there replaced the 4700uF caps. After replacing the components, I tested the unit and passed the 37 - 40 min mark and was about to crack a celebratory beer when the speakers popped and no sound. But this time after about a minute they popped back on, and then turned off again.
I realize there is the possibility that I got a bad relay, but is there anything else I should be looking at as a cause?
A side note the speakers do make a pop sound when the unit is powered on and off or speaker A B selector is changed. Also when sound stops coming through the speakers the ambient heat coming off the unit was 118.4 F, But when the unit is in use I typically have fans keeping it cool.
Any ideas are greatly appreciated.

 

[duncan]

The soft-clipping, bridging and impedance switches in the back should all be looked at - make sure they're all the way slid into the desired position, make sure they're clean. Tap on each of them lightly during play. Also if you normally play with soft clipping on, turn it off for a while and see that it has an effect.

I don't have a schematic for this in front of me but often when there's a relay replaced, there are two inexpensive parts that should be replaced with it at the same time or at least checked. One is a reverse-biased diode across its coil, and the other will be a driver transistor somewhere.

Those are the things that come to mind right away.

You might also try posting your question in the DIY area. Some highly experienced tech types hang out there. Good luck and welcome to AK.

 

[Goldie99]

I don't know the unit, but the apparently repeatable ca. 37 mins timing suggests a thermal issue to me. The schematic shows a thermal cut-out in the relay circuit that would also be worth checking, e.g., is it well mounted to the heat sink etc., new thermal grease maybe ?

Otherwise, I'd probably also focus on the relay & protection circuits more generally.

 

[a_mozart]

Thanks for the insight duncan and Goldie99. I will post how it all works out.

 

[slimecity]

That its affecting both channels makes me think to focus on the switches as referenced above, or there's something amiss in the power supply - ie: agree a thermal issue, possibly with a regulator or small signal transistor in there

 

[SoCal Sam]

I don't know the unit, but the apparently repeatable ca. 37 mins timing suggests a thermal issue to me. The schematic shows a thermal cut-out in the relay circuit that would also be worth checking, e.g., is it well mounted to the heat sink etc., new thermal grease maybe ?

Otherwise, I'd probably also focus on the relay & protection circuits more generally.

This. And replace all trimmers. NAD cheaped out and used exposed trimmers which corrode and become unstable.