Let me talk about my observations,It may be wrong, I hope it can provide you with a reference.
I think the oscillation is always there, but the amplitude is different:
(1、small amplitude oscillation)Sometimes it starts normally,The 8920a digital tube can display "700", and in about 3 seconds, it displays 0.01,I mistakenly thought it was normal,As long as you press the "Lo Range Enable" button, you can still see the fluctuation
(2、Large amplitude oscillation)As seen in the video, it shows "700" flashing or "mv,v" flashing
About measurement:
Since I removed the metal shield case, it is difficult to confirm the status through the 8920a display,As long as I get close to the metal shield module, numbers appear on the 8920a display and keep beating.So I can't be sure of the data authenticity of the measurements,What I am referring to is data with error information. It is possible that it is data measured in a normal state.
So, how to measure I'm waiting for your opinion
(I have some unfortunate news for you. The 8920a has been in a normal state the last few times it has been powered on. I am waiting for the malfunction to appear.)
So far, I have performed the power supply section test and calibration according to page 41 of the service manual.TP206, TP207, TP208 match the description.No other calibrations have been performed yet.I noticed that this 8920a was produced from 1983 to 1990.