I only just noticed that there is a 'Cal' at the right end of the Var V/Div knob which is presumably for standard calibration signal to achieve the highest possible accuracy (corresponding to 5mV/Div).
The Cal indicates the position the control is locked where amplitude can be accurately measured against the graticules. If not locked in the Cal position graticule measurement don't mean a thing therefore any adjustment to amplitude accuracy must be made with the control in the Cal position.
In my circumstance, do you think is there anything I can do to make it correct?
First both channels must be in the Cal setting before channel amplitude can be compared and a quick check can be made with just a clean common DC source while inputs are set for DC coupling.
If there's a disparity then the fun begins, first which channel is right and then where is the error in the bad channel signal path ?
Much can be done just measuring points along the input signal path to where the channel mismatch appears then carefully check that part of the circuitry. Some manuals have DC measurement points to check while the scope is configured in a certain way.
Some go about this another way by starting at the CRT plates and working backwards along the signal path and both methods have their merits and also depends on the quality of guidance in the service manual.
Check of the Repair docs thread in the Repair board and grab the Troubleshooting Tek scopes pdf as it will offer more tips and understanding. Read, study, rinse and repeat !