The 7A26 has small plug-in fixed attenuator modules underneath the aluminium cover that sits immediately behind the BNC connectors. These are T-section resistor/capacitor networks and can give high or low errors depending if the poor contact is in the arms or leg of the tee. If cleaning the contacts on the cam operated switches fixes the problem then at this stage you don't need to investigate the plugin modules. If after cleaning the cam switches you still have trouble then carefully remove these modules and clean the pins.
If the Ch1/Ch2 error is the same for all positions of the attenuators then you need to look elsewhere as others have suggested.
Best of luck,
Roger
