"Audio Gone" question for Audiogon experts

I have an original NAD 3020 integrated amp. I like it very much, but within the past several months sound levels have started to change. When I turn it on, the left and right channel sound volumes are roughly equal. Over the next 15 minutes or so the sound balance starts to change and one channel, the right in my case, starts to get louder, or the left channel starts to get quieter, I'm not sure which. As the 3020 has pre-out and power-in jacks I decided to try the power-in section to see if the problem was in the power amp section. It is not - I have "balanced" sound output when I only use the power amp section, regardless of how long I listen. I swapped out the old power amp sections large capacitors about a year ago, carefully replacing them with the same microfarad values so as not to foul the Q of the amp (still sounds good to me). So, the problem is in the pre-amp section. Could a faulty capacitor in the pre-amp section be the fault? A transistor? What is most likely the problem? Thank you.
I guess its possible that if there was a crack in one of the connections, it could move farther apart as the preamp warms up. I'm not sure how likely something like that can be, but I have heard of it happening before. Have you tried any of the other inputs? Doing so might help you locate the problem.
Have you tried using the preamp by itself? Running that section into a separate amp?

I'm just guessing, but maybe by combining the two sections with the jumpers, is overtaxing the power supply section somehow. Where one or the other section by itself is fine.

You might even try using regular interconnects to reconnect the two sections. Maybe the jumpers are not quite tight enough.
Does this occur on all functions?
Try pulling the jumpers out and clean them.
Play with controls and switches and see if volume is restored at all. Volume balance and input switches in particular. If that helps at all a good cleaning of switches and pots would be in order. I have heard similar with my trusty old nad 3020 .
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That's a better idea. Use any crisis, no matter how small, as an excuse to by new gear.
The problem occurs on all inputs. The pre- power- connects are firm, clean, and uncracked. There is some crackling and associated volume drop/gain when I wiggle the Mute switch, but I don't usually use it so this particular problem seems unlikely. Then again.... I will crack open the unit and take a peek.
As for "upgrading", I've listened to a lot of "high-priced spreads" and found all their claims wanting. This little beastie, at low to mid volume, sounds as nearly as good as the best of them to me. To use a famous line "They'll have to pry it out of my cold, dead fingers if they want it!"
Thanks all!
OK. Next is to find out if the one channel is fading, or if the other is getting louder.

I've seen feedback resistors fail before, so getting louder is something that should be checked out.
I will likely never part with my NAD 7020 receiver. Its an oldie but goodie. The pre-amp in particular, including phono still sounds great even in comparison to more modern and much more expensive gear. The 20 w/ch amp sets a high bar as well with the right speakers. The tuner is not the most sensitive but sounds great with what it gets. I keep it on the bench normally until something else breaks and I need a sub, and it never disappoints. Last time I put into active duty as a pre-amp, it stayed in my second system for over 2 years until I finally decided to get something newer and let it rest a bit once again.

The controls and switches do need occasional cleaning though otherwise the symptoms as you describe with your 3020 crop up.
Chances are that if it has a "Tape Monitor" switch but you don't own a tape device and never use those switches, it gives you that exact symptom. The audio has to go through the TM buttons all the time. Not using them typically causes one or both channels to fade.

If you were to put IC's on the tape out and run them to a different component that has line level inputs (pre amp, headphone amp, etc...), would that catch the signal before it got to the tape monitor circuit?