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Topic: Very strange behavior of NAD 7140 (Read 4355 times) previous topic - next topic
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Very strange behavior of NAD 7140

My NAD 7140 receiver has begun behaving very strangely.  Audio on one channel occasionally drops out.  As per another post in this forum concerning similar behavior of the NAD 7140, I've checked switches, pots, connections, physical hardware around output transistors, etc.  The strange and, in this case, different condition is that when the audio drops out and there is an electrical discharge in the room--light switch turned on, static body discharge, for example--the sound returns! ... for a time.  Then it will drop out again, another static discharge in the room brings it back.  I am utterly stumped what to look for and troubleshoot next based on this odd behavior.  Any help appreciated!  Thanks!

Very strange behavior of NAD 7140

Reply #1 –
I'd suggest calling the Ghostbusters.

Very strange behavior of NAD 7140

Reply #2 –
As you probably know, intermittant problems can be difficult to track down.  And with modern electronics, repair is often not economical.  It takes a small amount of cheap unskilled assembly-line labor to build this stuff, but it can take many hours of expensive skilled labor to troubleshoot & repair.
  And sometimes,  the only way to troubleshoot is to swap-out circuit boards 'till you find the board that fixes the problem. 

Have you tried swapping the left & right speaker connections to confirm that the problem is not the speaker?

It sounds like an intermittant connector or solder joint.  Try flexing the circuit board(s) (but not too much!) and try wiggling the internal wires & connectors.  Does the problem come-and-go when you tap (or bang) on the case? 

Sometimes, you can get a flaky (cracked, intermittant) trace or via on the PC board.  Sometimes these can be bypassed with a jumper-wire if you can find the bad trace/via, and if you can find a good (and large-enough) place to solder.

Quote
The strange and, in this case, different condition is that when the audio drops out and there is an electrical discharge in the room--light switch turned on, static body discharge, for example--the sound returns!
I don't think I've seen that before, but I have had bad connections that apparently "break through" the oxidation and start working when you increase the signal/volume.  You might be getting something similar, where you're getting a voltage (noise) spike on the signal, and that's breaking through.

Very strange behavior of NAD 7140

Reply #3 –
Many thanks!!  Yes, I've switched speakers, tried new speakers, etc., same issues remain.  I'll try the poke, bend, and pull (gently) method and report back if I have any success.  It may help someone else sometime.  It is also helpful knowing what could be going on (possible oxidation, etc.).  I appreciate your thoughtful response!

Very strange behavior of NAD 7140

Reply #4 –
I'd put my bets on the speaker protection relay. Symptoms would be quite typical. If an electrically and mechanically matching replacement can be found (sometimes some adaption on the mechanical side is necessary), it's easiest to just swap that in. Otherwise the relay has to be removed, opened (usually doesn't work well when installed, unless you're a safecracker), the contacts cleaned (no sandpaper, contact cleaner and paper strips only) and everything reassembled.