Reviving this zombie.
The switches on the HK 300 and 400 series are really tricky boogers! Finding this to be true over the past week with a recent 330C I purchased off CL.
Upon intial power up it was playing really nicely. I noticed the left channel crackled a little without turning any knobs. I played it for a few hours with no issues then powered it down. Next day, no Left channel on power up. Going to cut to the chase here,... the darn monitor pushbutton switch is quite wonky, touchy, fickle and downright hard to get to to clean out. I ended up pulling the faceplate to get to the front of the switch so as to inject D5 in there with some F5 Fader lube to follow.
Also, if the faceplate is not perfectly aligned, it can put some undo tension and stress on the pushbuttons. Those buttons seem to have the tightest clearances through the faceplate. I ended up loosening the faceplate and aligning it ever so gently. Also used some white lithium grease in the brown thru-holes to avoid any faceplate rub. Seems to be playing just fine now. I'll probably end up doing a recap on it. These receivers really do sound great!