I know that issue! It is a VZ sickness sadly
It is an issue in the function board with all the switches that pop out each other when getting pressed. My guess is the one for tape, just fiddle with it a little while playing the radio, it should fix it temporarily. The only reliable fix is contact cleaner though. I am guessing it is all dust stuck in there, mine has the same issue but works quite well for a while when the tape switch is fiddled into the right position.
It is a very strange issue though because the sound does not go away on the affected side but it is lower...I have no clue how that can come but I know how to fix it on mine

Need to bring her to the shop maybe soon..I was never too keen on opening those grails
As far as the Crown 5100, I am guessing it is something similar like the function switch but my guess is the EQ that maybe does not allow heights on one channel anymore. Work it with lots of contact cleaner or bypass it somehow on the inside
Mine played weird things on me..Like the VU lights would just stay on, I gave her a gentle knock on the top and it worked again
Then the bottom tape played very fast once. Problem got fixed the same way
Yeah, I guess I am the boombox-whisperer...brought the switch of a MasterBlaster back to live last year in Dessau..It would not go on radio...I pressed it and it worked
