Electronics > Repair

Please help, FLUKE 8920A Multimeter Repair

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daisizhou:
I found that it seems to be caused by the load resistor.
The 8920a always fails when I turn it on after connecting it with a 50 ohm load resistor,When I disconnect the 50 ohm load resistor, the fault no longer occurs.Here are my recent findings and I will continue to test.
Magical malfunction phenomenon.I took pictures of the actual load and drew a simple schematic diagram

I read the 8920a user manual, and it does not prohibit starting with a load resistor.At least the manual doesn't specifically emphasize that this is wrong.

Harry_22:
Hello! Your video from here explains the problem (pls. see from 25 s)

You have self-oscillations that occur probably in Amplifier A.
Please do the following:
1. Set up generator frequency 370 Hz and 2mV level.
2. Set up 2mV manual range on RMS.
3. Connect your oscilloscope to TP3 and check the signal. It should be around 1V.
It must be clear sine with symmetry around zero line (if not please stop here).
4. Please measure the following AC levels carefully :

daisizhou:
Thank you for your reply.
I will wait for the next failure to occur and implement your suggestions.

I found that when I connected a 50 ohm load resistor and turned on the 8920a, it was more likely to cause faults.
When I disconnect the 50 ohm load resistor, the above fault phenomenon will no longer occur.
Do you know why?
I checked the 8920a user manual, and it does not prohibit connecting a load resistor for startup.

Harry_22:
Please don't wait the failure and make all steps that were asked.

daisizhou:
Hello

The test point you gave has a metal shield on it.
If I separate the metal shield, the negative terminal of the input 50 ohm resistor will be disconnected.
I won't be able to measure the test points you suggested without disassembling the metal shield.
How do I do this?

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