Author Topic: Please help, FLUKE 8920A Multimeter Repair  (Read 10254 times)

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Online daisizhouTopic starter

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Please help, FLUKE 8920A Multimeter Repair
« on: May 01, 2024, 04:49:39 am »
It looks like this triode is self-oscillating.I don't know how to adjust it for correct operation
If anyone knows please tell me, thanks

Service Documentatie
www.bramcam.nl/8920/Fluke_8920A_8921A_Manual.pdf
« Last Edit: June 12, 2024, 07:55:13 am by daisizhou »
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #1 on: May 01, 2024, 08:02:28 am »
You marked operational amplifier A. Is it correct?
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #2 on: May 01, 2024, 10:57:25 am »
I saw this sentence in the troubleshooting list.Did I identify it wrong?
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Offline wasedadoc

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Re: Please help, how can I adjust this transistor?
« Reply #3 on: May 01, 2024, 12:28:42 pm »
What is the voltage at TP1?
 

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Re: Please help, how can I adjust this transistor?
« Reply #4 on: May 01, 2024, 12:38:28 pm »
Please view the connection video here
https://www.circuitsonline.net/forum/view/message/2506099#2506099

I tested the TP3 test point,TP1 and TP2 test points require removal of the shielding casing,If the shield case is disassembled, the 50 ohm load will be disconnected from the input.There will be interference in the input signal, so I was not able to accurately test the TP1 and TP2 test points
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Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #5 on: May 01, 2024, 12:42:54 pm »
https://www.eevblog.com/forum/repair/fluke-8920a-multimeter-repair-needs-some-help/
There are also test videos and fault symptoms here.I checked according to the troubleshooting sheet and I don’t know if it is correct.
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #6 on: May 01, 2024, 01:29:59 pm »
Please measure the voltage/signal on TP1 by oscilloscope.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #7 on: May 01, 2024, 01:47:04 pm »
ok please give me some time。I will wait for the failure to occur again and take a video
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #8 on: May 01, 2024, 02:54:27 pm »
How does the malfunction occur? Are there prerequisites? Warming up, cooling, vibration...
Try to find any regularity.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #9 on: May 02, 2024, 04:11:29 am »
The fault will definitely occur when the machine is turned on for the first time after it has not been turned on for a long time.
After a failure occurs, the host will return to normal after a period of time.After normal testing, all functions are accurate.


 I did the full troubleshooting checklist test and below is the video of the test.I tested TP201 and TP3 waveforms,Please watch carefully, the second half of the video is when 8920a returns to normal state from fault state.If you need TP1 and TP2 waveforms please let me know as it is inside the metal shield and requires lead output to test.
I think the fault seems to have nothing to do with the signal input,If you have any better suggestions please tell me
Thank you for your help


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Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #10 on: May 03, 2024, 09:05:00 am »
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.
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #11 on: May 03, 2024, 09:20:42 am »
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 :
« Last Edit: May 03, 2024, 02:02:58 pm by Harry_22 »
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #12 on: May 03, 2024, 09:51:36 am »
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.
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #13 on: May 03, 2024, 09:54:55 am »
Please don't wait the failure and make all steps that were asked.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #14 on: May 03, 2024, 10:41:57 am »
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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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #15 on: May 03, 2024, 01:36:50 pm »
Open the shield and make the photo from upper point.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #16 on: May 03, 2024, 11:28:33 pm »
Please review the image.
The electronic components inside the metal shielding cover are very sensitive,If the metal shielding cover is removed, as long as the human body approaches, the 8920a host display will show a digital change.
So I don't know how to operate it correctly,to ensure the accuracy of measurement
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #17 on: May 04, 2024, 02:32:36 am »
Connect input o-ring terminal to the shield by short wire. Switch on and check for self oscillation.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #18 on: May 04, 2024, 04:05:49 am »
Do you mean the green connection like the picture below?

If it is connected like this, I tried.There is still a lot of interference, I mean human body interference.
If the test is forced, I am not sure whether the test results are accurate.
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #19 on: May 04, 2024, 05:03:08 am »
Please do the green connections.
Don't take care about human interferience. Check for lack of self oscillation and start testing:
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).

Check self oscillations by switching generator signal on/off. If you will see big interferience or oscillations please increase the measurement limit as well as the signal level.
 

Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #20 on: May 05, 2024, 03:30:15 am »
Dear Daisizhou, if you don't work hard I won’t be able to help you.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #21 on: May 05, 2024, 03:50:17 am »
I replaced it with a 50 ohm load,Found that things are developing in a good direction.(https://www.eevblog.com/forum/projects/i-made-a-50-ohm-load-with-bnc-connector-is-this-correct/)

I'm still continuing to test,it takes time,give me some time,I need to confirm if the problem is finally resolved.
Thanks.
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #22 on: May 05, 2024, 05:20:11 am »
Why do you force 50 Ohm termination issue? You don't need it for major measurements.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #23 on: May 05, 2024, 01:09:49 pm »
Replacing the 50 ohm resistor was a suggestion from a friend,He said it was the easiest thing to try.

I followed your suggestion and shot C16C18---C45C48 And TP3 test video.TP3 waveform looks the same as before,Not a sine wave.(You can compare with my previous video records).fluke 8920a is still the same as before。
Set 2mV manual range on RMS--After 8920a is turned on, "700" keeps flashing.Unable to manually select 2mv range

Measuring and shooting are all done by myself. There may be wrong connections. If you find any, please tell me.
If you have further testing please let me know and I will follow up on it
Thanks


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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #24 on: May 05, 2024, 03:37:14 pm »
Please measure the same on TP1
 


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