FLUKE 8920A RMS AC meter failure needs help - Page 1
Forum section: electronics / repair

author: daisizhou
author_rank: Super Contributor
timestamp: April 02, 2024, 11:35:03 am
content: 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, thanksVideo of normal startup--->Service Documentatiewww.bramcam.nl/8920/Fluke_8920A_8921A_Manual.pdfMarketing Folderwww.bramcam.nl/8920/Fluke-8920-Folder.pdf

author: daisizhou
author_rank: Super Contributor
timestamp: April 03, 2024, 02:45:04 am
content: I uploaded the malfunction video, thanksIt 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 belowhttps://youtu.be/sO-RI9zRX3cI'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

author: daisizhou
author_rank: Super Contributor
timestamp: April 03, 2024, 01:04:51 pm
content: According to the inspection steps in the manual, it is found that the TP201 fault condition is 1.01V fluctuating voltage.

author: daisizhou
author_rank: Super Contributor
timestamp: April 20, 2024, 08:36:26 am
content: Can anyone help me? I need help

author: blackdog
author_rank: Frequent Contributor
timestamp: April 20, 2024, 10:47:22 am
content: 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

author: daisizhou
author_rank: Super Contributor
timestamp: April 20, 2024, 11:46:25 am
content: Great, I've been looking for you.So you are here toohttps://www.circuitsonline.net/forum/view/165140Considering 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

author: blackdog
author_rank: Frequent Contributor
timestamp: April 20, 2024, 12:14:38 pm
content: 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

author: daisizhou
author_rank: Super Contributor
timestamp: April 20, 2024, 12:19:13 pm
content: If you have any doubtful test points, please let me know and I will test and post the test diagrams.thanks

author: daisizhou
author_rank: Super Contributor
timestamp: April 20, 2024, 01:25:06 pm
content: 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

author: daisizhou
author_rank: Super Contributor
timestamp: April 21, 2024, 01:10:11 am
content: 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.

author: daisizhou
author_rank: Super Contributor
timestamp: April 24, 2024, 12:44:20 pm
content: I checked the fault according to the troubleshooting table on page 53 of the service manual.

author: daisizhou
author_rank: Super Contributor
timestamp: April 25, 2024, 11:53:53 pm
content: Can anyone help me? I'm still waiting, thank you

author: daisizhou
author_rank: Super Contributor
timestamp: April 27, 2024, 02:35:56 pm
content: I tested TP3, TP4, TP5 waveform video, please check

author: daisizhou
author_rank: Super Contributor
timestamp: April 28, 2024, 03:20:41 am
content: The "700" status TP3 waveform is displayed. Unfortunately, the TP4 and TP5 waveforms are not obtained.Normal state TP3 TP4 TP5 waveform diagram

author: daisizhou
author_rank: Super Contributor
timestamp: May 03, 2024, 09:06:07 am
content: 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 diagramI 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.

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 04, 2024, 02:27:44 pm
content: Just as a point of reference, I have the very similar Fluke 8921A meter. Mine will flash "700" upon startup as it is searching for the proper input range. This occurs whether or not I have a 50 ohm load connected to the input terminals. The meter takes 1 - 2 seconds to finally downrange correctly wherein a relay click can be heard.None of this happens if I start the meter up with the range held to a manual setting. So perhaps your problem is with the range detection circuitry.

author: daisizhou
author_rank: Super Contributor
timestamp: May 05, 2024, 12:32:32 am
content: Thanks.I tried replacing a 50 ohm resistor,Fluctuation seems to improve after startup.But as long as I press the "LO RANGE ENABLE" button, I see it still fluctuates，I don't know if this is normal or an error occurred.Please watch the video

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 05, 2024, 05:51:46 pm
content: I see no fluctuations at all on my 8921A in the lo-range-enable mode, so there is something definitely wrong with yours.It could be a problem with the FETs in the lo-range-enable switching circuitry. Also, since you said it often improves over time, perhaps there is a marginal solder joint. Try flexing the board vigorously with your thumb and see if anything changes.The autorange circuitry seems conceptually easy - it just steps the attenuators up or down until the A/D output is in a valid range. But you would probably need to instrument the A/D output and the attenuator control lines with a multi-channel scope to really see if anything seems wacky. Good luck.

author: daisizhou
author_rank: Super Contributor
timestamp: May 05, 2024, 11:02:29 pm
content: Thanks.As in the video I shot above，Sometimes as long as the 8920a power switch is turned on, the host starts to fluctuate,(To protect the analog pointer table, I disconnected),Sometimes the boot is normal, and as long as the lo-range-enable button is pressed, the fluctuation appears again.According to my observation,It usually takes 15-20 minutes and remains powered on, it will eventually stabilize and no longer fluctuate.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.At present, I only have conventional tools such as digital oscilloscopes, signal generators, and digital multimeters.I don't know how this fault should be repaired or maintained

author: daisizhou
author_rank: Super Contributor
timestamp: May 11, 2024, 10:34:31 pm
content: I found that my 8920a is missing the PCB circuit board shown in the picture below,is this normal?I don’t know whether this PCB is a standard default configuration or an optional configuration.Does it do anything?

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 12, 2024, 12:52:31 am
content: Not needed. That board is the optional Logarithmic Analog Output board.You do have the service manual, don't you?? That board is described there.

author: daisizhou
author_rank: Super Contributor
timestamp: May 12, 2024, 01:10:16 am
content: I understand, I mistakenly thought it was dismantled and lost by its previous owner.Thanks