[Sideband]

...but if it's fading out on both channels, surely this indicates another problem? I'd check power supply first. Clip your meter, set to DC volts (use the 50V or 100V range....whatever you have on your meter) to the main smoothing capacitors and see what happens to the voltage when it fades out (if anything). What I'm getting at is that a fault on the pre-amp board on one channel should not affect the good working channel. If both channels fade out it has to be something common to both, like the power supply.