Author Topic: Please help, FLUKE 8920A Multimeter Repair  (Read 10255 times)

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Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #25 on: May 06, 2024, 02:22:53 am »
OK. this is a test video,The TP3 waveform looks a little different than before.
Looking forward to your analysis or further testing.



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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #26 on: May 06, 2024, 03:58:56 am »
Perfect! I this video you catches 700V overload.
I need some time to analyze the data.
 

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Re: Please help, how can I adjust this transistor?
« Reply #27 on: May 06, 2024, 04:00:30 am »
Ok thanks, I will wait for your results :-+

please 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/
« Last Edit: May 06, 2024, 04:14:09 am by daisizhou »
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #28 on: May 06, 2024, 05:15:14 am »
Yes your new video eplains my suggestion somewhat.
Now please measure voltage at two points:
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #29 on: May 06, 2024, 10:23:33 am »
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
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #30 on: May 06, 2024, 11:53:52 am »
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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #31 on: May 06, 2024, 11:59:15 am »
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?
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #32 on: May 06, 2024, 12:18:46 pm »
Yes you are right.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #33 on: May 06, 2024, 12:20:01 pm »
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.
« Last Edit: May 06, 2024, 12:57:04 pm by daisizhou »
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #34 on: May 06, 2024, 01:07:42 pm »
Did you measure at "LO RANGE ENABLE" ? Seems yes.
Please check that TP1 voltage still around 8.6V
Also 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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #35 on: May 06, 2024, 01:51:15 pm »
"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"?
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #36 on: May 06, 2024, 02:11:58 pm »
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?
« Last Edit: May 06, 2024, 03:21:08 pm by Harry_22 »
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #37 on: May 06, 2024, 10:57:13 pm »
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" flashing

About 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.
« Last Edit: May 06, 2024, 11:41:24 pm by daisizhou »
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #38 on: May 07, 2024, 02:13:00 am »
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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #39 on: May 07, 2024, 03:22:20 am »
Thanks

I 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.
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Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #40 on: May 07, 2024, 04:46:04 am »
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?
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #41 on: May 07, 2024, 05:52:50 am »
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:
« Last Edit: May 07, 2024, 06:00:31 am by Harry_22 »
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #42 on: May 07, 2024, 07:32:02 am »
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, C24

The 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........ :-//
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #43 on: May 07, 2024, 07:40:32 am »
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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #44 on: May 07, 2024, 07:44:31 am »
ok .i will wait for the error to appear again
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Re: Please help, how can I adjust this transistor?
« Reply #45 on: May 07, 2024, 10:21:19 pm »
Hi

I 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 it


Although I'm not sure if the data is meaningful, I recorded the data anyway
« Last Edit: May 08, 2024, 01:46:19 am by daisizhou »
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #46 on: May 08, 2024, 07:41:05 am »
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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #47 on: May 08, 2024, 08:01:36 am »
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?
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Online Harry_22

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Re: Please help, how can I adjust this transistor?
« Reply #48 on: May 08, 2024, 08:43:52 am »
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.
 

Online daisizhouTopic starter

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Re: Please help, how can I adjust this transistor?
« Reply #49 on: May 08, 2024, 09:09:52 am »
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
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