Do the following:
use the 1KHz square wave signal from the calibrator in either channels. If the trigger starts decently but with the sinusoidal wave at the same frequency not, one of tunnel diodes in the trigger circuit is gone.
Check with the same signal first in channel A, then in channel B. There are separate trigger generators for chan A and B.
It's difficult that both channel have tunnel diodes burned.
Don't try to check TDs with a standard VOM. A curve tracer is required.
Check for presence of very abrupt change of about 400 mV across the tunnel diode (CR786) when trigger starts.
Max
