[SOLVED]Please help, FLUKE 8920A Multimeter Repair - Page 1
Forum section: electronics / repair

author: daisizhou
author_rank: Super Contributor
timestamp: May 01, 2024, 04:49:39 am
content: It looks like this triode is self-oscillating.I don't know how to adjust it for correct operationIf anyone knows please tell me, thanksService Documentatiewww.bramcam.nl/8920/Fluke_8920A_8921A_Manual.pdf

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 01, 2024, 08:02:28 am
content: You marked operational amplifier A. Is it correct?

author: daisizhou
author_rank: Super Contributor
timestamp: May 01, 2024, 10:57:25 am
content: I saw this sentence in the troubleshooting list.Did I identify it wrong?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 01, 2024, 12:28:42 pm
content: What is the voltage at TP1?

author: daisizhou
author_rank: Super Contributor
timestamp: May 01, 2024, 12:38:28 pm
content: Please view the connection video herehttps://www.circuitsonline.net/forum/view/message/2506099#2506099I 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

author: daisizhou
author_rank: Super Contributor
timestamp: May 01, 2024, 12:42:54 pm
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 01, 2024, 01:29:59 pm
content: Please measure the voltage/signal on TP1 by oscilloscope.

author: daisizhou
author_rank: Super Contributor
timestamp: May 01, 2024, 01:47:04 pm
content: ok please give me some time。I will wait for the failure to occur again and take a video

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 01, 2024, 02:54:27 pm
content: How does the malfunction occur? Are there prerequisites? Warming up, cooling, vibration...Try to find any regularity.

author: daisizhou
author_rank: Super Contributor
timestamp: May 02, 2024, 04:11:29 am
content: 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 meThank you for your help

author: daisizhou
author_rank: Super Contributor
timestamp: May 03, 2024, 09:05:00 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: Harry_22
author_rank: Frequent Contributor
timestamp: May 03, 2024, 09:20:42 am
content: 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 followingAClevels carefully :

author: daisizhou
author_rank: Super Contributor
timestamp: May 03, 2024, 09:51:36 am
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 03, 2024, 09:54:55 am
content: Please don't wait the failure and make all steps that were asked.

author: daisizhou
author_rank: Super Contributor
timestamp: May 03, 2024, 10:41:57 am
content: HelloThe 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?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 03, 2024, 01:36:50 pm
content: Open the shield and make the photo from upper point.

author: daisizhou
author_rank: Super Contributor
timestamp: May 03, 2024, 11:28:33 pm
content: 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

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 04, 2024, 02:32:36 am
content: Connect input o-ring terminal to the shield by short wire. Switch on and check for self oscillation.

author: daisizhou
author_rank: Super Contributor
timestamp: May 04, 2024, 04:05:49 am
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 04, 2024, 05:03:08 am
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 05, 2024, 03:30:15 am
content: Dear Daisizhou, if you don't work hard I won’t be able to help you.

author: daisizhou
author_rank: Super Contributor
timestamp: May 05, 2024, 03:50:17 am
content: 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.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 05, 2024, 05:20:11 am
content: Why do you force 50 Ohm termination issue? You don't need it for major measurements.

author: daisizhou
author_rank: Super Contributor
timestamp: May 05, 2024, 01:09:49 pm
content: 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 rangeMeasuring 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 itThanks

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 05, 2024, 03:37:14 pm
content: Please measure the same on TP1

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 02:22:53 am
content: OK. this is a test video,The TP3 waveform looks a little different than before.Looking forward to your analysis or further testing.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 03:58:56 am
content: Perfect! I this video you catches 700V overload.I need some time to analyze the data.

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 04:00:30 am
content: Ok thanks, I will wait for your resultsplease allow me to say one more thing,The video below may have clues."LO RANGE ENABLE" button.https://www.eevblog.com/forum/repair/fluke-8920a-multimeter-repair-needs-some-help/

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 05:15:14 am
content: Yes your new video eplains my suggestion somewhat.Now please measure voltage at two points:

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 10:23:33 am
content: I measured the voltage between GND-R1 and GND-VR1,If you mean the voltage between R1-VR1 please tell me and I will re-measure.thanks

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 11:53:52 am
content: Could you measure the following points by multimeter not oscilloscope.Be careful and don't short the pins with the transistor case.We don't need the generator any more.

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 11:59:15 am
content: You mean I disconnected the 8920a input?In other words, the 8920a input is left floating,Then use a multimeter to measure the voltage value in the picture above, right?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 12:18:46 pm
content: Yes you are right.

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 12:20:01 pm
content: If you confirm that the 8920a input port status is open circuit,I use the DC range of the digital multimeter to measure the voltage values 1-6 as shown in the figure below.**Also please tell me the conditions of measurement, I hope the measurement value is valid,For example: the input interface is open circuit or connected to a 50 ohm resistor...,Use the DC or AC range of a digital multimeter...**In addition, I saw "its31129" mark on the outer shell of this tube.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 01:07:42 pm
content: Did you measure at "LO RANGE ENABLE" ? Seems yes.Please check that TP1 voltage still around 8.6VAlso measure DC voltage at R40, R41 R42 and C24. You will get two values for each component. It doesn't matter which side is where.

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 01:51:15 pm
content: "LO RANGE ENABLE" button always pops up,"2Mhz MAX" indicator light is off.All buttons are pop-up,The 8920a host is in normal state for measurement, and does not display "700" or "mv, v fluctuations"TP1:-0.024V(DC)、R40:13.48V,14.41V(DC)、R41:12.77V,-14.38V(DC)、R42:13.50V,14.40V(DC)、C24:0.685V,-0.676V(DC)Do you need to measure the values of TP1, R40, R41, R42, and C24 at "700" or "mv, v fluctuations"?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 06, 2024, 02:11:58 pm
content: Yes, I need the same measurements but with 700V overload. You can check overload by measuring 8.6V at TP1.May be if you press "Lo Range Enable" you will get 8.6 volt at TP1?

author: daisizhou
author_rank: Super Contributor
timestamp: May 06, 2024, 10:57:13 pm
content: Let me talk about my observations,It may be wrong, I hope it can provide you with a reference.I think the oscillation is always there, but the amplitude is different:(1、small amplitude oscillation)Sometimes it starts normally，The 8920a digital tube can display "700", and in about 3 seconds, it displays 0.01，I mistakenly thought it was normal,As long as you press the "Lo Range Enable" button, you can still see the fluctuation（2、Large amplitude oscillation）As seen in the video, it shows "700" flashing or "mv,v" flashingAbout measurement:Since I removed the metal shield case, it is difficult to confirm the status through the 8920a display,As long as I get close to the metal shield module, numbers appear on the 8920a display and keep beating.So I can't be sure of the data authenticity of the measurements,What I am referring to is data with error information. It is possible that it is data measured in a normal state.So, how to measure I'm waiting for your opinion(I have some unfortunate news for you. The 8920a has been in a normal state the last few times it has been powered on. I am waiting for the malfunction to appear.)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.I noticed that this 8920a was produced from 1983 to 1990.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 07, 2024, 02:13:00 am
content: Here I need to summarize our research.The malfunction usually appeared after long-term storage of the device. This is similar to the failure of an electrolytic capacitor in which the absence of external voltage the oxide film is destroyed and further under applied voltage it slowly restores gains capacity and reduces leakage. There was also a 366 Hz signal present on TP3. Therefore was an assumption that self-oscillations occur in the circuit. This was confirmed by video from output of operational amplifier B where the sine wave was stretched like a spring.Further when we measured voltage at the output of Operational Amplifier “A” it turned out that it was constant and equal to +8.6 volts. Where do “self-oscillations” come from?Amplifier “B” has two more inputs. Their purpose is to reduce the strong signal preventing RMS detector overload. Thus the signal of 370 Hz frequency is a protection result.The circuit after TP1 works.It remains to find a reason for the skew of operational amplifier “A”. It was assumed that the positive voltage appeared at the direct input. Very high probability for this is a breakdown of overvoltage protection circuit Q1 collector junction. Most measure equipment failures occur at the input.But measuring voltage at the positive input of the operational amplifier it turned out that it is zero. The output voltage divided by 50 (feedback circuit coefficient) comes to the inverse input in the value of +0.17 volts. In case of OPAmp balance the forward and inverse input voltages must be equal. But here we see the clear bias in opposite direction. A positive voltage at inverse input should cause a negative voltage at the output not vice versa.Thus the operational amplifier “A” is faulty.

author: daisizhou
author_rank: Super Contributor
timestamp: May 07, 2024, 03:22:20 am
content: ThanksI checked Q3, Q4, Q5, Q6, and I found that their pin resistance is very small, only about 130 ohms。I measured it online, so it may not be very accurate.but very strange.The resistance values of Q28 and Q29 are also very small. I am still looking for the pin location. The resistance value is only about 40 ohms.Give me some time and I will test them all and mark the pin locations and suspected resistance values.

author: daisizhou
author_rank: Super Contributor
timestamp: May 07, 2024, 04:46:04 am
content: I measured all the transistors in the area,And the resistance value is less than 800 ohms.I checked all areas pointed by the green arrow,Does the area with the red arrow need to be inspected?I have marked the location points and resistance values in the green area. Are they abnormal?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 07, 2024, 05:52:50 am
content: You measured JFET transistors they are used as relays here. The low resistance is Ok at normal (disconnected) conditions. Q28 and Q29 are in low resitant section and also in parallel it is possible to have about 40 Ohm.You malfunction is impermanent. It is very hard to find because all parts are good for a long time.Please check if falure persists if you press "LO RANGE ENABLE" button. If yes plase measure the same voltages:TP1:R40:R41:R42:C24:

author: daisizhou
author_rank: Super Contributor
timestamp: May 07, 2024, 07:32:02 am
content: My original plan was to disassemble these 120 ohm JFET transistors,Detach the PCB circuit board and measure to confirm whether it is normal.After reading your message, I suspended this plan.I don’t know why, but today the 8920a always turns on normally.What I mean is that when I press the "LO RANGE ENABLE" button, there is no fluctuation.I have been waiting for the failure to appear, preparing to test TP1, R40, R41, R42, C24The fault appears to be the same as what you described,Much like the capacitor aging, the leakage current,All I can say is it looks like,But for the power supply, I tested the electrolytic capacitor offline and confirmed it.In the signal part, many are red tantalum capacitors and ceramic capacitors.(https://www.circuitsonline.net/forum/view/165140)I noticed this author "blackdog" who had trouble with tantalum capacitors.It looks different from mine. What I mean is the malfunction.Now I can only wait, I'm confused........

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 07, 2024, 07:40:32 am
content: It is a normal situation. Currently I am waiting for RF amplifier malfunction more than a week. it is very important not to do any manipulations with the hardware while you detect and localize malfunction.Please don't unsolder any parts including capacitors.

author: daisizhou
author_rank: Super Contributor
timestamp: May 07, 2024, 07:44:31 am
content: ok .i will wait for the error to appear again

author: daisizhou
author_rank: Super Contributor
timestamp: May 07, 2024, 10:21:19 pm
content: HiI tested again today,I found that if you open the upper cover (upper shell) of the 8920a host,Pressing the "LO RANGE ENABLE" button will have no effect.In other words,When I closed the lid, I pressed the "LO RANGE ENABLE" button, the "2Mhz" indicator light came on, and I saw fluctuating numbers jumping on the display.When I remove the upper cover (upper shell) the "2Mhz MAZ" light will go out,Nothing happens when the "LO RANGE ENABLE" button is pressed.It seems that the "LO RANGE ENABLE" function is related to interference.To describe exactly what I mean, I shot a video, please watch itAlthough I'm not sure if the data is meaningful, I recorded the data anyway

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 07:41:05 am
content: Probably I found your problem. This is leakage in C18 tantalum.If I connect 10 kOhm resistor in parallel with C18 the same voltage imbalance is observed.I attached .CIR file for Microcap below.

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 08:01:36 am
content: Very good, you have thought of using software simulation analysis.Should I continue to wait for the fault to recur and then measure the resistance value of C18 online?Or any other better suggestions?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 08:43:52 am
content: The voltmeter now shows positive values ​​of 0.83 (or 1.35mV at Low Range). Try applying a hot soldering iron to C18 and check the change in readings.

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 09:09:52 am
content: Do you mean, let me remove the C18 capacitor and observe the data on the 8920a display? (No components are installed at position c18 of the PCB circuit board)I noticed that C18 is a 35V 1UF tantalum capacitor

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 09:50:59 am
content: No just touch it by hot iron through termal paste.

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 10:08:53 am
content: Does it operate as shown in the picture below?To put it simply, the high-temperature heat is transferred through the soldering iron head to the C18 shell.Then observe the changes in the 8920a display?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 10:16:55 am
content: This is not C18. C18 is a smaller one. Don't overheat it.

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 10:40:32 am
content: OK.I will test the capacitor with the arrow pointing to it using your recommended test method.I am doing this test while waiting for the 8920a to display "700" flashing, or "mv, v" flashing alternately.Because only in this way the changes will be more intuitiveThanks

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 11:13:11 am
content: You can do this test now by monitoring the readings.Could you film this.

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 01:25:45 pm
content: I tested it, and it seems to have no effect.It's night here, the light may not be very good.Do I need to wait for the fault to recur before repeating this test?By the way, I found that my 8920a host is missing this PCB. Is this normal?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 08, 2024, 04:50:38 pm
content: Seems you have no ground connection of 8920.Please do the following:1. Connect metal case to the ground.2. Connect oscilloscope to TP13. Push Low Range button4. Switch on 89205. Check 8.6 volt at TP16. Touch C18 by hot soldering iron.7. Observe oscilloscope

author: daisizhou
author_rank: Super Contributor
timestamp: May 08, 2024, 10:55:07 pm
content: I took apart the electrical outlet panel from the wall,Found that it only has a "blue" neutral wire and a "red" live wire, and no yellow ground wire.So I don't know about your suggested metal shield case grounding, how should I connect itAlso, as suggested by "blackdog", replace these red tantalum capacitors,Because these tantalum capacitors will fail after long-term use.According to my experience, most tantalum capacitor failures are caused by breakdown and short circuit, and this is the first time I have encountered leakage current.Do you think I need to replace them all?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 09, 2024, 02:14:46 am
content: There is a whole topic regarding how to do grounding correctly. All equipment requires grounding. You can use a nearby metal base or pipe.I do not recommend replacing all capacitors. It is not for nothing that tantalum is used in medical equipment. Change only the critical 1uF in both amplifiers.

author: daisizhou
author_rank: Super Contributor
timestamp: May 09, 2024, 03:09:19 am
content: Ok, I'm going to spend some time looking for a real ground test point.My original plan was to replace tantalum capacitors with 35V or 50V withstand voltage specifications, not electrolytic capacitors.I understand that tantalum capacitors have special functions.Because these red tantalum capacitors have been in operation for 20-30 years,Isn't replacing them in advance the best option?Do you mean to just replace the C18 35V 1UF tantalum capacitor with a 35V1UF or 50V1UF tantalum capacitor?Other tantalum capacitors remain in their original state

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 09, 2024, 03:28:42 am
content: Let you repair it first.I would ask you to successfully complete C18 heat test to prove my Multicap simulation.

author: daisizhou
author_rank: Super Contributor
timestamp: May 09, 2024, 03:37:34 am
content: Ok, I understand.I will wait for the fault to reappear and connect a suitable ground test point

author: daisizhou
author_rank: Super Contributor
timestamp: May 09, 2024, 09:18:08 am
content: Hi Harry_22.I would like to ask if a real ground test point is absolutely required?I searched several locations,Due to the use of new materials, many pipes are currently made of non-metallic materials such as PVC, PPR.... There are also some old pipelines that have been severely corroded. I am not sure whether these pipelines are really connected to the earth.I plan to test the 8920a again in 12-16 hours.Do you need the TP1, R40, R41, R42, and C24 test point voltages (measured by a digital multimeter)?Also, I still use a soldering iron to bake C18 capacitors and take videos, right?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 09, 2024, 10:47:57 am
content: Hi! We need only TP1 to test amplifier A offset. It should be around +8.6 volt. After you try to heat C18 and observe oscilloscope. It is expected that the line goes to zero. Please film this.Regarding grounding are you shure that there are no yellow-green connection anywhere at your location. Try to find ground bar outside your building.

author: daisizhou
author_rank: Super Contributor
timestamp: May 09, 2024, 11:49:05 pm
content: I re-tested, but unfortunately the fault did not recur.I found a ground test point though, I'm not sure if that works.If there is anything that needs improvement, please let me know. I'm still waiting for the fault to reappear and test it again.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 10, 2024, 09:01:37 am
content: Dear Daisizhou. With grounding the background noise has noticeably decreased.It seems that your well-heated soldering iron scared off our malfunction and now it may not appear soon.You can go another way. You remove C18 and measure its resistance with a multimeter. It is up to you.

author: daisizhou
author_rank: Super Contributor
timestamp: May 10, 2024, 09:38:50 am
content: OK。I plan to do it in 2 steps.The first step is to buy a brand new 50V 1UF tantalum capacitor.The second step is waiting for transportation,Since shipping takes a few days until new capacitors are received, I perform one last test to see if the fault can be reproduced and measure the resistance of the old capacitors.Because it won't be a waste of time

author: daisizhou
author_rank: Super Contributor
timestamp: May 12, 2024, 12:16:14 am
content: Tested again today, the good news is that the fault reappeared, the bad news is that I haven't tested the heated C18 yet, the fault disappeared again, I took part of the video。I seem to have discovered a pattern，That is, the longer the power outage lasts, the longer the failure will last.It seems that it is not that the capacitance of C18 is reduced, but that there is a possibility of C18 leakage current.Sometimes the fault of closing the shielding shell can be maintained forever,When you open the shielding cover or connect the TP1 test point, the fault will disappear automatically. It’s more serious on rainy days.Watch the video below.

author: daisizhou
author_rank: Super Contributor
timestamp: May 13, 2024, 05:04:36 am
content: I received the 50V 1UF capacitor and tested it for LCR.Next I am going to disassemble C18 and replace it with a new 50v 1UF one

author: daisizhou
author_rank: Super Contributor
timestamp: May 13, 2024, 08:27:34 am
content: I replaced the C18 capacitor.and measured the old C18 and it looked like it was normal

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 13, 2024, 08:39:13 am
content: Please measure the leakage by multimeter.

author: daisizhou
author_rank: Super Contributor
timestamp: May 13, 2024, 08:58:47 am
content: Is this how it is measured?I tested the multimeter wire resistance

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 13, 2024, 09:26:19 am
content: Seems it's Ok.Now you need to wait and check the further behavior of 8920a... wait and check. It's such a malfunction.

author: daisizhou
author_rank: Super Contributor
timestamp: May 13, 2024, 09:40:50 am
content: Ok, I'm going to keep testing this for a while.Frankly,The old red tantalum capacitor has a better series equivalent resistance than the new yellow tantalum capacitor.6.197 ohm(YELLOW), 2.532 ohm(RED)

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 13, 2024, 10:11:29 am
content: Don't be guided by this resistance. Not everything is so simple.Please see the measurement of 1.5uF capacitor and 9.8k resistor in parallel.PSYou had no bias on tantalum during measurements. May be this is a reason of high Rs.

author: daisizhou
author_rank: Super Contributor
timestamp: May 22, 2024, 05:49:55 am
content: Dear Harry_22.When I press the "LOW RANGE ENABLE" button, the flashing occurs again.Please see the video I took

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 22, 2024, 12:49:25 pm
content: Hello again!These are the unpleasant consequences of repairs when you give equipment to the customer and he returns it again. Therefore you should always find the actual cause of the malfunction and not clean the connectors or replace all the capacitors first.Let's continue.Push "LO RANGE ENABLE" button.Check that TP1 voltage is around 8.6VMeasure DC voltage at R18, R27, R30, R35, R40, R41 R42 and C24. You will get two values for each component. It doesn't matter which side is where.

author: daisizhou
author_rank: Super Contributor
timestamp: May 22, 2024, 09:39:59 pm
content: Hi Harry_22.Normally, the upper shield cover is closed,Press the "LOW RANGE ENABLE" button, the 2Mhz MAX indicator will light up.If I test TP1, only the upper shield cover can be opened,Then when I press the "LOW RANGE ENABLE" button, the 2Mhz MAX indicator does not light up,Looks like the "LOW RANGE ENABLE" feature is gone,It may be caused by too much interference,The GND connection line is also connected normally.In other words, if the upper shielding shell is opened to measure "LOW RANGE ENABLE", it will not be possible, so how should I measure it?

author: daisizhou
author_rank: Super Contributor
timestamp: May 24, 2024, 04:08:27 am
content: Quote from: Harry_22 on May 22, 2024, 12:49:25 pmOnce again I tested the above test points as you requested, but unfortunately the fault disappeared as soon as I opened the upper shield cover.To be precise, when I opened the upper shield cover, the "LOW RANGE ENABLE" function disappeared and the "2Mhz MAX" indicator light went out.How do I test next?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 24, 2024, 08:25:27 pm
content: The most sensitive range up to 2mV is turned on only when the Low Range Enable button is pressed and the Auto Range button is not pressed. 2MHz LED is lit when this range up to 2mV is active. If the signal is more than 2mV then the range automatically switches to a higher one and 2MHz LED goes out.When the cover is opened the noise increases and the lower range no longer turns on.Use an oscilloscope to see what is happening at TP1.

author: daisizhou
author_rank: Super Contributor
timestamp: May 24, 2024, 10:56:39 pm
content: Normally, I turn on the 8920a power supply and observe the digital tube or pointer to see if there is a flashing fault,If it is normal, I will press the "LOW RANGE ENABLE" button and observe again whether there is flickering.Do you mean to let me directly open the upper shielding cover and directly use the oscilloscope to measure the TP1 test point?If this is the case, how can I tell if the 8920a is working properly or malfunctioning?Because sometimes when the upper shielding shell is opened, the flashing fault disappears immediately.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 25, 2024, 06:10:46 pm
content: Hi! Let you do two additional connections on the PCB. Please see the scheme. Thus we add divider by 10 to reduce the signal from first OPA. I expect that "Low Range" mode will more stable and you will see 2MHz LED now.After modification please press "Low Range Enable" and measure the signal on TP1 by oscilloscope.

author: daisizhou
author_rank: Super Contributor
timestamp: May 26, 2024, 04:07:41 am
content: Hi.Let me tell you good news. I tested today and the fault lasted for a long time.The fault phenomenon is that "700" continues to flash.I tested the voltage at the test point you need.Your latest connection, I haven't connected yet, if you need I will connect to test laterIn order to prevent mistakes, I took a video. The video quality is not very good. I hope you can forgive me.Thanks

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 26, 2024, 06:10:49 pm
content: HI!I need your help. Below is a diagram from Microcap. In the ovals there is the correct voltage in rectangles under the voltages measured by you.It is necessary to supplement missed ones and check those already measured.Pay attention to very different values ​​like at R27 and C24.Also measure the voltage at In+ and In- inputs.Please use a multimeter with mandatory monitoring by an oscilloscope 8.61V at the Out.

author: daisizhou
author_rank: Super Contributor
timestamp: May 26, 2024, 09:44:53 pm
content: ok .give me some time i will measure them.Before the measurement, I compared the voltage error values of my commonly used fluke 27 and TEK TDS2012C oscilloscope.If you need to get high precision voltage value, I would use fluke 8060a or 179C for measurement

author: daisizhou
author_rank: Super Contributor
timestamp: May 26, 2024, 10:29:45 pm
content: I have a question,For example R29，The voltage on the left is -7.328，The voltage on the right side is -7.326，Do I need to measure one to one according to the schematic?Or is it still effective if left and right are reversed?Can I measure the input interface BNC in+ and in-? Because I don’t know the pin definition of Q9A、Q9BWhen measuring, I still choose to disconnect the BNC input interface as before, right? Do I need to connect a 50 ohm load?Because there will be interference when disconnected

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 27, 2024, 04:58:28 am
content: Fluke 27 is Ok. I don't need high precision. Concerning drawing with R29 measure collector Q13 and VR3.Please round the values 7.326 to 7.3, 7.967 to 8.0, 12.732 to 12.7 and so on.Measure around Q17, Q19; Q11, Q16; Q8 both, Q10, Q12. Also R33, R39, In+, In-.Connect 50 Ohm to the input and monitor the malfunction - +8.6V at the ouput.

author: daisizhou
author_rank: Super Contributor
timestamp: May 27, 2024, 06:25:29 am
content: Hello, I found that the voltage changes and is not a stable value.I don't know which point in time test is accurate.The voltage increased from 1.46 to 1.651 and then increased to 1.806.The same goes for other voltages.When the final screen displays 0.01, the voltage displays the highest value.(I connected a 50 ohm load)I don't know how to measureDoes measuring these voltage values need to be measured under fault conditions? Or is it measured under normal conditions?

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 27, 2024, 07:17:35 am
content: Please measure all the voltages under fault conditions.

author: daisizhou
author_rank: Super Contributor
timestamp: May 27, 2024, 07:30:56 am
content: for example,For the test point on the right side of R17,Also in fault condition,The voltage changes from 1.46v to 1.651v,Should I log 1.46 or 1.651?Usually I test one resistance and then test the next one. I cannot complete multiple resistance measurements at the same time.When I measure the test point on the right side of R17, the voltage is 1.46,Then I measured the voltage on the right side of R13 to be 1.851,The actual voltage on the right side of R13 is 1.72.All voltages are changing slowly and I can't guarantee whether these measurements make sense.so i'm lost

author: daisizhou
author_rank: Super Contributor
timestamp: May 27, 2024, 11:00:44 am
content: I tried to measure,Please note that I cannot confirm exactly the left or right side of the resistor，In other words, the voltage value on the left side of the resistor may be on the right side.If you can please tell me and I will continue, if it loses its meaning I will wait for your adviceSay one more thing,During the measurement, I found that these voltage values can sometimes be stable,If a fluctuation occurs, the 8920a is about to return to normal

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 27, 2024, 07:28:07 pm
content: Very good.Now please measure resistance between points 1 and 2 as well as between 1 and 3. Connect the positive probe to point 1.Measure very gently without touching PCB parts!

author: daisizhou
author_rank: Super Contributor
timestamp: May 27, 2024, 09:55:28 pm
content: The forward resistance is infinite. I tested the reverse resistance. Please see the picture.

author: daisizhou
author_rank: Super Contributor
timestamp: May 28, 2024, 12:26:17 am
content: Hallo.During my test yesterday, I found that these two chips were heating up very violently.I'm referring to the fault status that shows "700" flashing,I used my finger to touch and it was so hot that I couldn't even keep touching that kind of heat.Today the 8920a is in normal condition, I also touch it with my fingers and they only have a little bit of heat.I don't know if this is a useful clue, I hope it will be helpful to your analysis.Do I need to use infrared vision to shoot video? Let me know if you need to check it out, the infrared temperature shot is at a friend of mine's and I need to borrow it.Or I have a K-type temperature probe. If you need to check the temperature, I will shoot a video for you.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 28, 2024, 08:03:07 am
content: Well done for finding the overheating.These are voltage stabilizers. This is another indicator that the operating mode of OPAmp_A is changing. It can be seen in Multicap too.Measure the voltage at points 1, 2 and 3 at the time of the malfunction.

author: daisizhou
author_rank: Super Contributor
timestamp: May 28, 2024, 11:20:52 am
content: I once again confirmed that the resistance values ​​of 1, 2, and 3 are the same as my last measurement.If you need the voltage values ​​of 1, 2, and 3, you need to wait for a few days, because the 8920a is in a normal state when it is turned on today. I need to wait for the fault to occur before measuring the voltage value.If you need other test points, please let me know and I will pre-mark them because the failure time is very short.

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 28, 2024, 05:55:20 pm
content: The next time a fault occurs measure the resistance and voltage at point 3.

author: daisizhou
author_rank: Super Contributor
timestamp: May 29, 2024, 11:47:12 am
content: Hallo.I had some free time today, so I tested the voltages at 1, 2, and 3 in normal conditions.Please note that I am referring to the normal state, which is the state where the 8920a display shows a reading of 0.011:10.94v 2:-1.405v 3:8.21vEspecially when I tested the P3 test points, I saw that the 8920a display could easily display 700 and flicker.It seems that the P3 test point is a location that induces the "700" flicker,I don't know if it's useful, I hope to tell you and give you a clueI mean the normal state, when the multimeter red probe tests the P3 test point, 8920a shows 700 flashes

author: daisizhou
author_rank: Super Contributor
timestamp: May 30, 2024, 01:01:20 pm
content: I'll add my test observations.Under normal conditions, if you use the multimeter's red positive pole to contact test point P1,8920a displays "700" flashing,As long as the multimeter positive pole and test point 1 are separated, the flashing of "700" will immediately resume and return to the normal display state.If you put the red positive pole of the multimeter in contact with the P3 test points,Then "700" will flash. If you separate the red positive pole of the multimeter, the 8920a "700" flash will still exist and will not automatically recover.At this time, I measured TP1 and the voltage was 0.114v

author: Harry_22
author_rank: Frequent Contributor
timestamp: May 30, 2024, 06:25:25 pm
content: In this case look at the signal with an oscilloscope at TP1.

author: daisizhou
author_rank: Super Contributor
timestamp: May 31, 2024, 03:08:38 am
content: I don't know if these 2 videos are helpful,Please check out the "TEST2" video.It seems that "700" will only flash when no probe is connected at TP1.thanks

author: daisizhou
author_rank: Super Contributor
timestamp: June 01, 2024, 08:08:10 am
content: Hi Harry_22.Did you find any new clues?Do you have any constructive suggestions?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 01, 2024, 05:01:06 pm
content: Hi!You have created a resonant self-support (pls see attached photo). This may be due to the lack of a shielding cover. This is not normal operation.Please give me a very good photo of the C13. To do this use any camera stand.

author: daisizhou
author_rank: Super Contributor
timestamp: June 01, 2024, 10:21:22 pm
content: I took several pictures，I don't know which one will suit your needs.https://i.postimg.cc/153x9Q4q/33.jpghttps://i.postimg.cc/HsZqnh7P/44.jpghttps://i.postimg.cc/x1XWz3vj/P40528-185739.jpg

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 02, 2024, 09:41:22 am
content: The photos are good of course but I asked for a C13.Please be more careful.

author: daisizhou
author_rank: Super Contributor
timestamp: June 02, 2024, 10:00:47 am
content: Sorry, I re-shot it.https://i.postimg.cc/SNwvnT4y/P40602-175153.jpghttps://i.postimg.cc/j55XgPTg/P40602-175358.jpghttps://i.postimg.cc/52Bn6fp3/P40602-175405.jpg

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 02, 2024, 02:49:23 pm
content: You need to wait for a malfunction to appear. While the scheme is working nothing can be said.I also cannot fully simulate such a phenomenon.You said that during a malfunction the power supplies get very hot.Try using a thermal imager to find elements of increased heating in the circuit of amplifier A.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 02, 2024, 06:43:01 pm
content: The next time a malfunction occurs please measure the underlined points again.

author: daisizhou
author_rank: Super Contributor
timestamp: June 05, 2024, 11:15:17 am
content: Quote from: Harry_22 on June 02, 2024, 06:43:01 pmHiPosition 1 seems to be very sensitive. As long as the positive lead of the multimeter is touched to it, the 8920a will easily display a 700 fault status.I am still waiting for the 8920a fault to reappear

author: daisizhou
author_rank: Super Contributor
timestamp: June 08, 2024, 01:40:05 am
content: Quote from: Harry_22 on June 02, 2024, 06:43:01 pmPlease watch the video, I took pictures of the voltages of the test points 1-6 that you need

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 08, 2024, 10:50:07 am
content: Hi! Thanks!I haven't seen TP1 signal. Please use oscilloscope.Also measure voltage by DVM at TP1 and at pin 4 again.Every time check for "700" on the screen.

author: daisizhou
author_rank: Super Contributor
timestamp: June 08, 2024, 01:07:52 pm
content: OK, please watch the video

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 08, 2024, 02:05:08 pm
content: Please connect oscilloscope to TP1 and don't disconnect it.Use digital voltmeter to measure voltage at pin 1 and 4.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 12:28:32 am
content: Quote from: Harry_22 on June 08, 2024, 02:05:08 pm

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 08:17:56 am
content: Could it be that the ceramic capacitors failed and caused the failure? Or is there leakage current?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 10:59:58 am
content: Good job!Look what happened this time.Previously we localized a fault in operational amplifier A assembled here using discrete elements.What it looks like. At the direct input we have 0V at the inverse 0.17V at the output 8.3V. (Gain factor was 50)There is a clear imbalance here.Now it has become clear that when you connect voltmeter probe to the inverse input oscillations occur.This despite the fact that the input is shunted with a feedback resistor 110 Ohms.At this rating there should be no interference!PSYou can change all the capacitor of cause but are you sure that this will 100% solve the problem?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 11:12:08 am
content: Could you connect generator to the device input and check the signal on TP1.Adjust the amplitude to see a more сlean signal.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 11:15:21 am
content: Since there has been no progress for a long time, I don't know where to start to solve the problem,Replacing the capacitor is just my guess.If you have any other places you need tested, please let me know.thanks

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 11:20:46 am
content: Quote from: Harry_22 on June 09, 2024, 11:12:08 amI can connect a signal generator to the input.Could you please tell me the type of input signal or the specific voltage value, etc.....The output is TP1. Use an oscilloscope to view the waveform of TP1?Please tell me the specific test details

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 11:24:37 am
content: Sinus 1kHz as usual. Gradually increase the amplitude until a good signal appears on the oscilloscope screen.Yes connect oscilloscope to TP1.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 11:51:45 am
content: Quote from: Harry_22 on June 09, 2024, 11:24:37 amHiFollowing your suggestion, I set the input signal.But I increased the amplitude from 2mv to 20mv, TP1 was always 8.4v DC.Finally, I set 1v and 2v to be the same.Is there something wrong?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 12:08:45 pm
content: That is good.We observe the circuit's response to external disturbance.Please connect the generator via 10k to pin 4 (the negative OPAmp input) and do the same test.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 12:22:35 pm
content: Do you mean for me to connect as shown below?Connect a 10K resistor to the positive terminal of the signal generator and then connect it to the 4th pin.The negative pole of the signal generator is connected to the metal shell.Then observe the TP1 waveform, right?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 12:26:08 pm
content: Yes, exactly!

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 12:40:36 pm
content: Quote from: Harry_22 on June 09, 2024, 12:26:08 pmExcuse me, can you please confirm once again that there will be no other failures in this connection?Because this is my first time to connect and test,I am worried about burning out components due to incorrect connections or testing.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 12:52:08 pm
content: There won't be any problems. Just need to be careful at the connection point of pin 4 and 10k. Do not short to other components.So you connect one leg of 10k resistor to pin 4. Other leg you connect to generator central pin.GND of generator to GND of DVM.Start from small signal level and monitor TP1 with an oscilloscope while increasing the level.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 09, 2024, 12:59:05 pm
content: Please use 1k resistor. 10k is too much.

author: daisizhou
author_rank: Super Contributor
timestamp: June 09, 2024, 01:23:06 pm
content: Ok, give me some time and I will test it

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 02:38:56 am
content: Quote from: Harry_22 on June 09, 2024, 12:59:05 pmIt looks like a lot of interference, I don't know if the test is valid

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 06:48:27 am
content: I haven't seen the resistor. It should be as close to pin 4 as possible.The suqare wave signal you see at power on is 50Hz induced interference.You have to see straight line on the screen before test.Could you also try 10k resistor.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 09:15:50 am
content: I tested it again.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 09:43:53 am
content: Well done!Is it possible to reduce the level to see the clear sinus? If not please use 10k.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 09:58:15 am
content: Quote from: Harry_22 on June 10, 2024, 09:43:53 amclear sinus?I don't understand what you're saying.What do you need to observe carefully?Can I change the resistor to 10K and test it again?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 10:12:12 am
content: Yes.Now we see clipped signal and I can't calculate amplification factor.Could you try to set the level to see sinus on your screen without any clipping.Seems I found out the malfunction reason. The operational amplifier has very high gain.My Microcap shows the same surprisingly.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 10:26:01 am
content: Measure the voltage at following points A and B.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 11:22:06 am
content: 10K resistor test video is being uploaded,It may take some time because YouTube is slow.A:-5.22VB:-12.57V

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 11:28:58 am
content: Please connect TP1 and point 4.The indication "700" should disappear.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 11:44:09 am
content: Quote from: Harry_22 on June 10, 2024, 11:28:58 amIs this what you mean?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 11:52:04 am
content: Yes.Could you also measure the following U203 pins.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 12:02:12 pm
content: U203?U206??

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:05:25 pm
content: Sorry for my mistake (in green).U207

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 12:26:51 pm
content: Quote from: Harry_22 on June 10, 2024, 11:52:04 amU203 seems to have been replaced. Please see the serial number.The voltage is shown in the figure below

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:32:48 pm
content: Good, and also in green. Please see my last picture.What about TP1 and #4 connection?

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 12:37:41 pm
content: Quote from: Harry_22 on June 10, 2024, 12:05:25 pmU203 and U207 seem to have been replaced manually.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 12:39:54 pm
content: Quote from: Harry_22 on June 10, 2024, 12:32:48 pmDo you mean for me to try connecting TP1and #4?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 12:41:22 pm
content: Yes, as on your picture

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 12:56:02 pm
content: Quote from: Harry_22 on June 10, 2024, 12:41:22 pmThere is no change after the connection, please watch the video

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 01:04:40 pm
content: Something wrong here (see the picture)Please remeasure.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 01:20:46 pm
content: Quote from: Harry_22 on June 10, 2024, 01:04:40 pmTo avoid errors, I measured the voltage before and after the R240 and R241 resistors.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 01:43:19 pm
content: Perfect!The last experiment for today.Please connect TP1 and #4 by clamps and switch on the device.

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 02:09:00 pm
content: Quote from: Harry_22 on June 10, 2024, 01:43:19 pm

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 03:08:00 pm
content: I can’t figure out where -4.48V voltage on the left side R241 came from.And it seems that it is not permanent.I'll think about it later on.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 10, 2024, 08:31:09 pm
content: After your measurements the situation has become somewhat clearer.The malfunction reason is undervoltage at Gate Q29. In this case JFET is closed and there is no negative feedback in amplifier A. The gain from your generator taking into account 10k resistor is about 75dB almost maximum.What you need to do is understand where -5 volt at Gate of Q29 comes from. Please look at the JFET name and measure voltages on the other pins Q29.It seems to me that it comes from the ether

author: daisizhou
author_rank: Super Contributor
timestamp: June 10, 2024, 10:35:38 pm
content: Quote from: Harry_22 on June 10, 2024, 08:31:09 pmGood morning.Thank you for your analysis.I took a picture of the Q29 and its voltage value.I don't understand what you mean by "ether"?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 06:56:25 am
content: Ether is something that does not exist. This is a medium in which radio waves, gravitational forces and other phenomena incomprehensible to modern science propagate.So, we have -5 volts from nothing.Check the resistor R241 -1MOhm.Look for flux residue around the trace from R241 to JFET Q29.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 07:59:31 am
content: Quote from: Harry_22 on June 11, 2024, 06:56:25 amI checked the connection lines, please see the picture.Most of it is clean, except for the two yellow ceramic capacitors.I used PCB cleaning water to clean the bottom of the two ceramic capacitors.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 08:19:45 am
content: What about -5 volt? Still exist?If yes please connect your DVM through the clip to the trace R241-JFET Q29.Take the cocktail straw.Blow through the tube to different places on PCB where the trace runs and observe changes in the voltmeter readings.There are 3MOhm leakage resistance between this trace and -15v rail. You can see that the voltage is variable every time you measure it: 4,48v 5,2v 5,32v 5,69vMy guess is that this resistance depends on the ambient humidity.You will try to increase the humidity through a cocktail straw.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 08:40:48 am
content: Quote from: Harry_22 on June 11, 2024, 08:19:45 amQ29 -5.14v voltage is always presentI do not quite understand what you mean."cocktail"--Do you mean alcohol?What concentration of alcohol solution?

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 08:55:03 am
content: Quote from: Harry_22 on June 11, 2024, 08:19:45 amIs C204 C205 possible?Is it necessary for me to remove these two capacitors?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 09:03:23 am
content: No, please don't do any hardware manipulations until we find the malfunction.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 09:19:44 am
content: Double check J102, P102 around pin 5 and C204I mean blow through a cocktail straw

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 09:56:31 am
content: Quote from: Harry_22 on June 11, 2024, 09:19:44 amPlease see the attached picture.R241-JFET Q29 connection wire passes through C204 and passes through the PCB at the bottom of C205.If we are looking for -5.XV voltage, I suspect there is a strong suspicion here.That's why I suggest trying to remove C204 and C205.If there is no change, put it back as it was.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:04:20 am
content: Quote from: Harry_22 on June 11, 2024, 09:19:44 amLook, it's right next to the PCB

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 10:05:31 am
content: Quote from: daisizhou on June 11, 2024, 09:56:31 amYes, you are absolutely right. It's a pretty tricky place.Вut why don't you want to blow? I'm not asking to make a film about this, although it will be very interesting if the tension changes.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:11:15 am
content: Quote from: Harry_22 on June 11, 2024, 10:05:31 am"blow through a cocktail straw" I don't understand what you mean by "cocktail".Is it a hose connected to compressed air?Are there any pictures?Do you mean any tiny plastic hose?

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:17:49 am
content: Can this be used instead?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 10:24:05 am
content: Quote from: daisizhou on June 11, 2024, 10:17:49 amSorry if I express myself incorrectly.I mean this. The breathing air has high humidity. It will wet the remaining flux and its conductivity will increase.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:30:50 am
content: You mean to let me measure the R241-JFET Q29 connection line voltage (-5.1xV) for a long time,Then use your mouth to connect the hose and blow air into various connection points on the R241-JFET Q29 link and observe the voltage changes?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 10:32:01 am
content: Yes, exactly!

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:39:33 am
content: OK, got it.PS: We rarely drink cocktails in China,We usually drink beer or liquor.And I am not used to using a straw.Only ladies or children occasionally use straws (Cultural differences)

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 11:12:58 am
content: Quote from: Harry_22 on June 11, 2024, 10:32:01 amVery good method,It's incredible.It is here C204 and C205

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 11:22:25 am
content: Dear Daisizhou, you are great!I was glad to solve such a non-trivial puzzle together with you. I haven't had such pleasure for a long time!PSNow you know what to do.It would be good to include Fluke 8920A in this topic name.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 11:26:25 am
content: Quote from: Harry_22 on June 11, 2024, 11:22:25 amI have some bad news for you.I removed C204 and C205 but the problem still exists-5.xV voltage still exists

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 11:40:32 am
content: What bad news could there be?Let's just continue.It is necessary to clean the entire area of ​​the sensitive path with alcohol. But this time without a straw

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 11:47:51 am
content: Can I use this?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 12:06:49 pm
content: It is non flammable.You need Contact Cleaner or Flux-Off or Isoclean spray.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 12:08:45 pm
content: Can I separate this module from the motherboard underneath?I mean use the resistance range of a digital multimeter to measure the resistance of the R241-JFET Q29 link and -15V.The fault is found in the module motherboard or the 8920a bottom motherboard below.If I confirm which motherboard it is, I will clean that motherboard

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 12:16:11 pm
content: Yes, you can measure here but be careful. This is static sensitive JFET. Connect your multimeter ground wire first.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 12:45:17 pm
content: Is this normal? Is this the problem?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 01:05:21 pm
content: It can be dust under P102 connector around pin 5. I'm not sure that there should be OL at this point.Unsolder R241 and check the trace resistance on other PCB as well as resistor itself.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 01:09:11 pm
content: Quote from: Harry_22 on June 11, 2024, 01:05:21 pmCompare it with P101 pin 2/4/6 . They are gates of other JFET.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 01:52:44 pm
content: Please watch the video. I don't know which choice is the right one.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 11, 2024, 02:14:19 pm
content: Quote from: daisizhou on June 11, 2024, 01:52:44 pmYou completely disconnected the frontend. The logic of the device’s operation is unpredictable.Check R241 on separated board, unsolder it and check the trace resistance. If exist clean it.Carefully check the gates at P101 pin 2/4/6 compare them with gate Q29 (P102 pin 5). First connect black wire of your multimeter to metal shield or ground next connect red wire to the gate.

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 10:40:13 pm
content: Quote from: Harry_22 on June 11, 2024, 02:14:19 pmR240-R241:https://i.postimg.cc/k5RyPwDf/r240.jpghttps://i.postimg.cc/B6xcX6TF/r241.jpghttps://i.postimg.cc/xCr3fHzY/r241-15v.jpghttps://i.postimg.cc/L52kfpDb/r241-1.jpghttps://i.postimg.cc/nr24GPwc/r241-2.jpgP101-P102https://i.postimg.cc/MTTqGM2P/P101-2.jpghttps://i.postimg.cc/T15xrzPn/P101-4.jpghttps://i.postimg.cc/3Rc7083K/P101-6.jpghttps://i.postimg.cc/xqr2g6kh/p102-5.jpg

author: daisizhou
author_rank: Super Contributor
timestamp: June 11, 2024, 11:37:10 pm
content: The fault still exists.Is it necessary to remove this P102 pin?Disassemble, clean and reinstall

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 05:31:22 am
content: I disassembled and cleaned the P102 pins,The fault of "700" flashing seems to have been eliminated.But when the "LOW RANGE ENABLE" button is pressed, the pointer still swings back and forth.Below are the pictures I took while cleaning the P102.Do the other two pins on this module's mainboard need to be disassembled and cleaned like P102?https://i.postimg.cc/pXmrx46Z/P40612-123927.jpghttps://i.postimg.cc/zBf57Rmj/P40612-123939.jpghttps://i.postimg.cc/05B5tZyJ/P40612-124009.jpghttps://i.postimg.cc/T3rR36qr/P40612-124021.jpg

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 07:52:28 am
content: After disassembling P102, I saw a dark red jelly substance in the part that contacts the PCB.Looks a bit like rust, but not solidIt is not a flux such as rosin.So I am thinking, is it necessary to clean the remaining 2 at once, and whether the bottom J102 also needs to be cleaned

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 08:22:24 am
content: Have you unsoldered R241 and measured trace resistance?Have you seen back side of the motherboard?

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 08:28:42 am
content: I measured the resistance of R241 online. I did not disassemble it. If you think it is necessary to disassemble R241, I will disassemble it.https://i.postimg.cc/k5RyPwDf/r240.jpghttps://i.postimg.cc/B6xcX6TF/r241.jpghttps://i.postimg.cc/xCr3fHzY/r241-15v.jpghttps://i.postimg.cc/L52kfpDb/r241-1.jpghttps://i.postimg.cc/nr24GPwc/r241-2.jpgDo you mean the biggest PCB at the bottom?If so, I checked it and it's very clean.Do you need a photo?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 08:38:10 am
content: Then don't take it apart again. We have a less difficult way to check.Assemble multimeter and measure the voltage from one side to the other of R241.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 10:08:31 am
content: Quote from: Harry_22 on June 12, 2024, 08:38:10 am**Add The video forgot to measure the right side of R241。The left side of R241Z is -1.631V, the right side is -1.565V

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 10:14:03 am
content: Perfect!Now you have 10 times more resistance than before.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 10:24:47 am
content: The video is uploading, it may take a while.Unfortunately, I pressed "LOW RANGE ENABLE" and the analog pointer still swung.In other words, I don't know if this 8920A host has other faults. I mean some faults that I don't know about yet.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 10:53:42 am
content: "Low Range" is quite sensitive.Close the shielding cover and check it again.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 11:03:30 am
content: Quote from: Harry_22 on June 12, 2024, 10:53:42 amYes.I am testing "LOW RANGE ENABLE" with the shield cover closed.Open the shielding cover and press "LOW RANGE ENABLE" 8920a has no change

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 11:34:16 am
content: Test voltage at R240 both sides.I need just digits no video.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 11:39:34 am
content: Quote from: Harry_22 on June 12, 2024, 11:34:16 amR240 left side -13.02, right side -14.96

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 11:43:23 am
content: At "Low Range" also

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 11:47:03 am
content: "Low Range" I need to put on the shielding cover to work,But I can't measure it with the shielding cover on.The shield is blocking my probe

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 11:51:57 am
content: But this resistor is aside. It is on processor board.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 12:03:18 pm
content: Quote from: Harry_22 on June 12, 2024, 11:51:57 amI mean the top cover of the 8920A host, not the metal shielding cover of the module board.Only when the upper cover of 8920A host is closed and "LOW RANGE ENABLE" is pressed, the effect will be displayed.If you need me to test it, I can't guarantee the results.Do you need it?

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 12:24:22 pm
content: Press "LOW RANGE ENABLE", 2Mhz indicator light is not on, R240 left: -13.03 right: -14.96

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 12:44:20 pm
content: "Low Range" was not switched. You can control it via 2MHz indicator.Please measure the voltage on TP1 by DVM.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 12:52:02 pm
content: Quote from: Harry_22 on June 12, 2024, 12:44:20 pmNormally, I would think that it is effective only when I close the cover of 8920a, press the "LOW RANGE ENABLE" button and the 2Mhz indicator lights up.tp1:-0.088 ~-0.036Press LOW RANGE ENABLE TP1:-0.056~-0.037

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 12, 2024, 01:00:28 pm
content: Think it is Ok.You should not make any adjustments as this requires more precise measurement instruments than 8920A.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 01:06:50 pm
content: Quote from: Harry_22 on June 12, 2024, 01:00:28 pmGot it, I'm not going to make any adjustments, I mean those adjustable resistors, adjustable capacitors.

author: daisizhou
author_rank: Super Contributor
timestamp: June 12, 2024, 10:21:11 pm
content: Based on yesterday's maintenance experience of cleaning P102, do P101 and P103 also need to be disassembled and cleaned like yesterday?Because I saw P103 has a "LO SENSE" mark

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 13, 2024, 11:17:53 am
content: Please check it again with 50 Ohm load at the input.

author: daisizhou
author_rank: Super Contributor
timestamp: June 13, 2024, 11:43:30 am
content: Quote from: Harry_22 on June 13, 2024, 11:17:53 amThe input end is connected to a 50 ohm load, and the result is still the same.

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 13, 2024, 08:27:35 pm
content: Ok, make an ajustment according paragraph 4-48 on page 4-19 steps from 1 to 8.Adjust only R72 according step 5 and R26 - step 8.

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2024, 12:09:58 am
content: Quote from: Harry_22 on June 13, 2024, 08:27:35 pm**I haven't made any adjustments yet,I followed the steps to measure,I don't know if my measurements are correct,And whether some of the preparatory steps in the early stage are correct**It seems that R72 does not need to be adjusted because TP3 is less than 1mv。R26 may need to be adjusted since TP3 is greater than 1mv（2.3mV-2.2mV）(TP3 TEST NO2----- I modified AMPLIFIER A, TP3 voltage is about 2.3mV-2.2mV)Don't P101 and P103 need to be disassembled and thoroughly cleaned?https://i.postimg.cc/m2DfyqnC/55.jpg

author: daisizhou
author_rank: Super Contributor
timestamp: June 14, 2024, 12:20:47 pm
content: I need to use the hose again and blow air with my mouth to test the voltage at each test point of P101 and P103 and see if there is any change during the blowing process?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 14, 2024, 03:13:59 pm
content: Quote from: daisizhou on June 14, 2024, 12:20:47 pmNo, enough is enough.I will have a look soon.

author: daisizhou
author_rank: Super Contributor
timestamp: June 15, 2024, 01:52:14 am
content: I tried to capture this part of the waveform. I don't know if it can help your analysis. Please check it out.PS:I have a suggestion to disassemble and separate the module motherboard shown in the red box in the figure below.Then install and restore 8920A, turn on the power, and press "LOW RANGE ENABLE",If the fault disappears, then the fault is on the main board of the red-framed module.If the fault persists, then the fault is on the largest bottom plate.Do you think it is necessary to make such an attempt?I tried the above test,Without module mainboard installed, analog pointer keeps deflecting to the right (full scale)Return to normal state,"LO RANGE ENABLE" swing is always effective in "AUTO" gear,If I press the "HOLD" button and try to jump to 2mv, 200mv, 2v, 20v, 200v, 700v, there is no swing

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 19, 2024, 06:25:02 pm
content: First warm it up for about 15 minutes.Connect the load.Then switch it to "LO RANGE" and adjust the zero reading by R26.

author: daisizhou
author_rank: Super Contributor
timestamp: June 19, 2024, 10:23:23 pm
content: Do you mean to preheat the 8920a host for more than 15 minutes,Connect a 50 ohm load to the 8920A mainframe input port.Then follow the instructions on page 4-19 "DC Offset resistor selection",Select AC, press the "RANGE HOLD" button, and switch to the 2v position,Switch the digital multimeter to the mv position, connect the red probe to TP3, and the black probe to the metal housing (GND)，Observe the reading of the digital multimeter. If the reading is greater than 1mv, adjust R72 to make the reading less than 1mv.The actual measurement is (-0.59mv - -0.67mv)Then keep the above operation,Press the "AC+DC" button,Observe the digital multimeter reading,If the reading is greater than 1mv, adjust R26 to make the digital multimeter reading less than 1mv.The actual measurement is (1.39mv-1.46mv)Finally, I would like to ask, will this adjustment affect the overall accuracy of the 8920A host?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 20, 2024, 11:41:36 am
content: You are simply try to set zero volts on the low range so that the device stops jumping on the high range.

author: daisizhou
author_rank: Super Contributor
timestamp: June 20, 2024, 11:57:49 am
content: Quote from: Harry_22 on June 20, 2024, 11:41:36 amSorry, I still don't quite understand.For adjusting R72, the TP3 voltage is (-0.59mv - -0.67mv), and the manual requires it to be less than 1mv.For adjusting R26, the TP3 voltage is (1.39mv-1.46mv), and the manual still requires it to be less than 1mvAlso, when the picture shows "1.348", it means that the standby time is more than 15 minutes, and the pointer swing will automatically disappear. At this time, press "LO RANGE ENABLE" and the screen will display 1.348.I don't know which adjustment is the most correct one.In order to test the accuracy, I conducted the following test

author: daisizhou
author_rank: Super Contributor
timestamp: June 20, 2024, 10:57:29 pm
content: After recording data for a long time, I found that TP3 voltage is related to key selection.Follow steps 1 to 8 in paragraph 4-48 on page 4-19 of the Service Manual.Adjust R72, TP3 voltage is (-0.59mv--0.67mv). Adjust R26, TP3 voltage is (1.39mv-1.46mv)After the 8920a host is preheated for more than 15 minutes, the analog pointer will no longer swing back and forth.TP3 voltage is always (-0.59mv--0.67mv) or (1.39mv-1.46mv)I'm a little lost, I don't know how to adjust them,Is the most correct choice

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 21, 2024, 08:36:59 am
content: Try to set up zero voltage for both cases.Don't be afraid it won't get any worse.

author: daisizhou
author_rank: Super Contributor
timestamp: June 21, 2024, 08:53:26 am
content: Quote from: Harry_22 on June 21, 2024, 08:36:59 amDo you mean steps 1 to 8 in paragraph 4-48 on page 4-19 of the Service Manual?Adjust R72, TP3 voltage is (0mv), adjust R26, TP3 voltage is (0mv)？Finally adjust them to 0mv? Or 1mv?

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 21, 2024, 10:00:52 am
content: Yes

author: daisizhou
author_rank: Super Contributor
timestamp: June 21, 2024, 11:47:44 am
content: As you requested, I followed steps 1 to 8 in paragraph 4-48 on page 4-19 of the manual，Adjust R72 and R26 respectively, and finally stabilize TP3 at 0mv.If a 50 ohm load is connected to the 8920A input interface, the analog pointer will no longer fluctuate.If the 8920a input interface is not connected to a 50 ohm load, the analog pointer will still fluctuate when the "LO RANGE ENABLE" is pressed.is this normal?After adjustment, I conducted another accuracy test. Here is the test picture:

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 21, 2024, 03:21:57 pm
content: Quote from: daisizhou on June 21, 2024, 11:47:44 amThis is Ok.Quote from: daisizhou on June 21, 2024, 11:47:44 amYou need another more precise voltmeter to do this test.

author: daisizhou
author_rank: Super Contributor
timestamp: June 22, 2024, 12:11:48 am
content: Quote from: Harry_22 on June 21, 2024, 03:21:57 pm12 hours later, I conducted the experiment again. Please see the video below.