Oscilloscope Channel 1 problem on BK Precision model 2120

[albertho]

This 2nd handed oscilloscope's channel 2 is in function but the channel 1 found no reaction to signal input.
The channel 1 can shows the beam line and can be positioned up and down.
Is there any link I can download the services manual or schematic?
Or could anyone give me some guidance to repair the fault channel.
Thanks very much in advance.

[unclejed613]

most common problem with scope inputs is either an open resistor in the attenuator switch, or a blown diffamp transistor. the input amp of an o-scope is very much like in an audio amp, a diff pair with a current source. the diff amp feeds symmetrical VAS stages all the way to the deflection plates. between the input and diff amp, you have a stepped attenuator. if your trace can't be centered on the screen, suspect the amp section. if you can center the trace, suspect the attenuator.