Fixing a 20MHz old scope is time wasting.
Buying a 30MHz with readout is 360RMB = 435HKD.
BTW, having a service manual does not imply you
have the ability to fix it. All techs can be layoff or
anyone can be a skillful tech, otherwise.
See photo. Even at this cheap price, with good appearance, noone wants to buy.
For repair purpose, at least 60MHz for crystal oscillation check to avoid -3dB distortion.