My first guess is that it’s the fader…. I would offer one, but I’ve literally just run out of spares for the 828.
Another possibility is that the busrails connecting the channels have come adrift, but my best guess is that it’s the fader. You will have to check it with a multimeter.
I must advise against replacing components unless you are sure that they are faulty. The 828 has proved to be very reliable and if you are getting a good signal on PFL, then there is nothing wrong with the channel.
There are a number of places that sell 60mm faders, the problem is always the mounting of the fader knob; in manufacture we actually cut down the shaft of a standard fader to fit the knob that we used at the time.
For others reading this, I do offer a ‘service’ for 828 Mk1 and Mk 2 (The above one is most likely a Mk 2 as there’s an op amp on the board.)