About Channel 2 not working. A frequent problem in any scope is someone blowing up the input on one channel. Usually the input transistor or FET gets burned. These are usually dual FETS. You can check the FETS (and everything else) in one input and compare to the good input. With the usual dual FETs input, one of the matched FETs amplifies the signal while the other will control the horizontal position.
Sometimes the attenuator is burned. Some Tek scopes were prone to this failure.
Measure resistance on bad input compare to good input .
I have seen this many times.
About flickering. It is possible there is a portion of the power supply on another board. Probably the board that supplies the characters. Maybe a cap or volt regulator problem.
The information for the characters is applied to the trace separately from the signal trace.
I suppose you have cleaned all the potentiometers and connections,.
I am not familiar with this particular unit so I cannot be more specific, sorry about that.