I've replaced the channel switch with no luck.
I am keen to check the channel 1 enable signal NOT(VS1) but as this is a hybrid to hybrid signal it is not easy to access.
It would also be good to confirm the actual channel output signal from the preamps to the channel switch but again this is difficult to do as this is a hybrid to hybrid signal.
The scope has lost calibration (a row of dots is shown on the bottom readout line). Not sure if this could be part of the problem as the preamp gain is controlled by the microprocessor.
Any comments or suggestions would be appreciated.