FLUKE 8920A RMS AC meter failure needs help

[daisizhou]

Hello,everyone.My 8920a meter is malfunctioning.

Fault description:
8920a as long as it is not turned on for a long time(Half a month or 1 month),First time booting,The screen displays "700" flashing,Simulate pointer swinging back and forth.Sometimes the "mv" and "V" gears automatically switch back and forth.
Please see the malfunction video-->

,Video of booting in faulty state--->

I don't know how to repair it, if you know please tell me, thanks



Video of normal startup--->


Service Documentatie
www.bramcam.nl/8920/Fluke_8920A_8921A_Manual.pdf

Marketing Folder
www.bramcam.nl/8920/Fluke-8920-Folder.pdf

[daisizhou]

I uploaded the malfunction video, thanks

It seems that the fault seems to be related to HOLD/AUTO,(To protect the analog pointer table, I disconnected the pointer table),
See my test video below
https://youtu.be/sO-RI9zRX3c

I'm lost, this is the latest test video, the flashing frequency seems to slow down and eventually return to normal. I don't know where the fault is.
https://youtu.be/zhFRpJvMZdY

[daisizhou]

According to the inspection steps in the manual, it is found that the TP201 fault condition is 1.01V fluctuating voltage.

[daisizhou]

Can anyone help me? I need help

[blackdog]

Hi daisizhou,

Check the Powerrails, are the voltages correct and is er noise on the Powerrails?
There are a number of tantalums across different power rails.
I repaired my 8920 a month ago, it also had bad tantalum capacitors.

Bad power rails can put the processor in the wrong state.

Do you have a Scoop to look at different power supply voltages?

Kind regards,
Bram

[daisizhou]

Great, I've been looking for you.
So you are here too
https://www.circuitsonline.net/forum/view/165140

Considering the aging and damage of the capacitors, I disassembled and tested these three capacitors and found that one capacitor had a slight reduction in capacity.So I replaced it with a 35v 680uf,See the picture.And I tested the ESR of the capacitor respectively, and the ESR values were 0.2289-0.2498.

I tested TP206 TP207 TP208 waveform diagram, please see the attachment

[blackdog]

Hi daisizhou, :-)

I can regularly be found on the Dutch forum Circuitsonline with topics of my own and responding to others' topics.

Read the link (translate) the topic, there you can read that it is about the capacitors after the linear regulators.
This especially about the tantalum capacitors that are over the supply voltages.
In the topic you quoted you can read that and tantalum capacitor was shorted across the negative 15 power rail.

This may not be the case with you, but what I pointed out, the tantalum capacitors that are directly over the +15V, -15V and the +5V power rail are suspect.

That's why I said check with a scope or multimeter to see if these voltages are stable.

Tantalum capacitors can fail especially with low impedance supply rails.
If the Fluke meter has a 100 Ohm resistor in front of it, they do not fail very often.

If you have a digital scope, you can use a low time bass mode preferably DC coupled to look at three channels simultaneously when switching on.
And then look at the +-15V and the +5V power rail.

Do the power rails come on cleanly?

Sincerely,
Bram

[daisizhou]

If you have any doubtful test points, please let me know and I will test and post the test diagrams.thanks

[daisizhou]

When the screen flashes "700", I connect a 50 ohm load resistor to the input terminal.
Following the troubleshooting steps, I checked that TP201-TP202 measured 1.120v DC voltage in step 17.
I don't know if this is a useful clue

[daisizhou]

I tested the TP201-TP202 and TP3 test point waveforms.
Let me add,in parenthesis,Sometimes the three test points TP206, TP207, and TP208 seem to be able to induce failures，TP201-TP202 can eliminate faults,Because when my multimeter or oscilloscope probe triggers TP206, TP207, and TP208, I can clearly feel the fault reappearing.And when the test probe contacts TP201-TP202, the fault will disappear,I expressed my actual observation and feeling. I know it is magical, but this is the reality.
Sometimes the digital tube flashes "700",The voltage of TP3 test point is 0.893v, and the voltage of TP201-TP202 test point is 1.138v.At the input, I always connect a 50 ohm load.

[daisizhou]

I checked the fault according to the troubleshooting table on page 53 of the service manual.

[daisizhou]

Can anyone help me? I'm still waiting, thank you

[daisizhou]

I tested TP3, TP4, TP5 waveform video, please check

https://youtu.be/qDG2JKikg8s