Approach to finding intermittent treble loss in amp

jupitersspot

Active Member
I've been working on an NAD 701 receiver (some back story over here) that I received as a DOA. It's up and running now and I'm generally happy with it, but for one thing -- there's an intermittent loss of treble in the left channel of the amp. This receiver has the ever-handy pre-out & main-in jacks that allow you test the preamp and amp separately, so I know the issue is in the amplifier section. I can listen for 20 minutes and have great right channel, and in the left have only bass and mid-range, when suddenly the left treble comes alive and things are perfect. This can last 5, 10, or 15 minutes but eventually the left channel treble disappears for 10 to 15 minutes, only to randomly later return.

How do the experienced folks here approach resolving this sort of issue?

Several ideas come to mind, but I'm not sure if they're good ideas, or where I should start. Some ideas:
- With no signal applied, check DC values in amp circuit, comparing values between channels, and also to the few published values in the service manual.
- Generate a treble tone - say 3kHz - and insert it into the amplifier inputs and use an oscilloscope to trace progress through the left and right channels, comparing parallel channel values.
- With music playing, go tap, tap, tapping on components with a dielectric stick to see if that makes any difference (this strikes me like looking for water with dowsing rods...)
- Maybe the description makes the smart people say "oh, it must be a failing <insert part name>", and they go off to test candidate <insert part name>. If that's the case for you, please mention the sort of component that might cause the issue.
 
Cold solder connection... dirty relay contacts...dirty speaker select switch if applicable....input coupling cap if applicable.
 
+1 on the solder. Poke at it with a suitable insulated pokenstick, you'll probably find the general area that causes it to flake out. Guessing at it, its probably somewhere around the tone controls or the line stage amplifier area.
 
I've done this many times... A 400Hz~1KHz squarewave or triangle is much more suitable as the test signal. You want lots of harmonics instead of a single HF tone because sines make standing waves that trick your hearing.
 
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