Fluke 8920a malfunction occurs again, how can I repair it(already ruled out)

[daisizhou]

Hello everyone,A few days ago, my 8920A malfunctioned. I tried repeatedly pressing the "LO RANGE ENABLE", "HOLD/AUTO", and "STEP UP" buttons.Sometimes it returns to normal, but sometimes it has no effect.I disassembled these three switches and cleaned the inside of the switches,And I cleaned the connection between the lower part of the switch and the PCB,But the fault is still the same.
The current fault phenomenon is that after starting up, the digital display shows 700V full range,Pointer gauge right full range,Sometimes it swings repeatedly,I mean, when it reaches position 0, it immediately turns right.I don't know how to repair it. If you know, please let me know. Thank you

manual-->(https://www.elso.sk/media/download/pdf/fluke/man/Fluke_8920A_8921A_Manual.pdf)

[daisizhou]

I will conduct the inspection according to the inspection forms 4-9

Step 17, the voltage between TP201 and TP202 is 1.134V

Step 33、34, TP3 is 0.883V

Step 45、46  TP1 is 2.016V

Step 52 checking Q3 Q4 Q5 Q6
Continue to check according to inspection forms 4-10. I am not sure what these represent and cannot proceed to the next step

[m k]

Are both meters acting equally?
Is it always positive?

Since both meters are acting up you can expect that the problem is the measuring and not indicating that measurement.
One candidate is a kneeling supply voltage.

If meter change is instant you can expect a contact problem.
If it takes more time then a faulty component is more possible.

[daisizhou]

Thanks.I removed some traces of corrosion under one resistor.Currently testing and observing, I hope the fault has been eliminated