FLUKE 8920A multimeter automatic range maintenance help--Successfully repaired

[daisizhou]

Hello everyone, I have been using this 8920A multimeter.Recently, I made an 8920a interface board
Finally, I gave up using this interface board.(https://www.youtube.com/watch?v=-3n-NkzEpIc&pp=ygUFODkyMGE%3D)
But now when 8920a is turned on, it shows full range display.I don't know what's wrong with 8920A
If you know, please let me know. Thank you
https://www.elso.sk/media/download/pdf/fluke/man/Fluke_8920A_8921A_Manual.pdf

In addition, I first checked the interface J501 on the 8920A motherboard corresponding to the interface board,I found that the resistance between VIN and GND is around 103.8 ohms, is this normal? Is U210 already damaged?

I separate the ACPCB and the baseboard,Check the resistance of ACPCB TP5 and GND and find that it is 103.8 ohms
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The current failure is that the gear display LED is wrong

The normal operation is in the "mv" position, the "mv" display LED will automatically light up, when the voltage input is in the "V" position, then the "V" position LED will light up.And now the "V" LED is always on there, it doesn't change with the input signal.

The same is true for the "RELATIVE REFERENCE" LED. After pressing the REL button, the digital display returns to zero, but the corresponding LED does not light up.

I am planning to repair these gear display LED indicators, but I am not sure how to proceed with the repair. If you know how to inspect them, please let me know. Thank you

[wn1fju]

I have the Fluke 8921A which is identical to the 8920A around J501.  With the unit powered off, I get a resistance of 93 ohms across J501-3 and J501-6.  So I don't think you have a problem there with your 8920A.

[daisizhou]

Thank you for your reply, but please see the picture. After I turn on the power, the Nixie tube and pointer meter display the full range

I short circuited the input BNC interface, and the same phenomenon still exists

Please watch the video I shot
(https://youtu.be/DFAqgIGnU3A)

[wn1fju]

I was merely suggesting that my resistance measurement was similar to yours and so it would appear unlikely that you have a problem with the circuitry immediately surrounding the meter or with at least one-half of the RMS sensor. 

However, since both the analog meter and digital display indicate an overload, the problem is probably way earlier in the signal flow.  You'll have to march through the  troubleshooting steps in Table 4-9 of the 8920A/8921A service manual.  It could be something simple or it could even be a problem with the auto-ranging circuitry or controller.

[daisizhou]

Step 17, the voltage between TP201 and TP202 is 1.134V

Step 33、34, TP3 is 0.883V

Step 45、46  TP1 is 2.016V

Step 52 checking Q3 Q4 Q5 Q6
I checked the Q3 and Q4 transistors and found that the resistance values were 132.1 ohms and 121.3 ohms

[daisizhou]

Thank you everyone.The 8920A is now functioning properly, and it appears to be the fault caused by the "LO RANGE ENABLE" or "AC+DC DC" or "HOLD AUTO" buttons

[daisizhou]

Unfortunately, there is still a malfunction with this 8920a. I found that the mv gear cannot be displayed, only a slight flashing light persists

I will recheck. If you have encountered similar faults, please let me know. Thank you
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I re checked the fault according to the 9-4 table and found that the mv gear was constantly flashing faintly, and I input a 1v AC signal. Please see the test video
https://youtu.be/KQIlldUxdGM

https://youtu.be/HBErov7jMIU
The relative reference indicator light has not been lit, and the LED is normal

[daisizhou]

I disassembled the display panel section and found the schematic diagram, but did not find any faults

[daisizhou]

I found a strange component, which is R208. The appearance, packaging, and other resistors look different,On the schematic diagram, R208 has a resistance value of 10K, but in actual online testing, it was found to be a resistance value of 108K,U203 appears to be a gear selection function, while R208 is related to power supply.
Strange component, I don't know why

[picburner]

Based on the lettering and the photo it would appear to be a 1N4157, a tight tolerance stabistor, not a resistor.
Perhaps a circuit modification?

[coromonadalix]

could be,  if it is indeed at r208,  they must create an supply line for the ic, better stabilized than a simple resistor

1n4157 is a 2.66v diode, check if you have this voltage ...on its pin




if the meter was or could have worked fine before, you may simply have oxidized switches / selector,   you dont chane parts for fun

And when you do measurements on the pcb, you can be influeced by nearby parts ...  testing transistors in ohms is not always the best idea

[daisizhou]

This is what has been confusing me, it looks like a diode (IN4157).But I am online, using a multimeter to measure the resistance (forward and reverse) is 107.9 kohms

[daisizhou]

Do you mean I measure like the picture below?




It seems that the test digital tube display is normal, but it shows that the mv gear indicator LED light has been weakly glowing.
No matter how small the voltage is, the V LED is lit, and the RELATIVE REFERENCE LED has never been lit.

[daisizhou]

Please go through these 2 videos, this is why I want it repaired.
https://youtu.be/KQIlldUxdGM
https://youtu.be/HBErov7jMIU

I cleaned the key switch, but the problem still persists

[picburner]

Do you mean I measure like the picture below?

Yes!
However, the polarity should be reversed from what you marked.

[daisizhou]

Thanks.
I measure the voltage between pin 1 of U203 and GND is 1.570V.

In order to see clearly, I have taken a clearer video of the malfunction. Welcome to browse, thank you
https://youtu.be/lRWcksniObQ

[coromonadalix]

First   it a led  not a nixie ...   and what i see is som flashing in the led,   it could be simply  the way things are displayed, in a matrix scheme, scanning  etc ...

Sometimes on old stuff you can see some artifact of this phenomenon, is it the led blinking your problem   when you are under 1 volt range ?


and i must say i dont understand your reasoning

on some videos  you show us  your generator,   since its in chineese i dont see the frequency,  but the meter was near enough,  maybe a simple calibration can or could be in order

your way of explaining things is not clear,  at least for me

the 1st video link  (bidoulleur)   he talk about some tantalum caps who play tricks .....       and finally maybe when you played with your added interface  you may have caused some problems ... interface, signal levels, or bus problems who may affect the main mcu

what was the interface  schematics ?   to see on what is it conected and what it control       i think i saw this inerface here not    just something to read this instrument if i recall ?

Is it this one ?  A 7 part series 


Have you touched  J501 connector in any ways ? 
since you have to connect something on it to read the ranges if i recall ??  for the mentioned project  ...     

i downloaded the project files and i see where it connect   ........

[daisizhou]

Yes, originally I made this breakout board circuit,However,
it is found that the USB interface circuit and the internal power supply of the 8920 are directly connected,I am concerned about current surges or static electricity when connecting to the J501 interface, so I gave up this interface circuit.
 (I have attempted to connect to the J501 port only once---After completion, the entire machine is normal,I didn't pay special attention to the gear position display of these LED lights. By normal,I mean, the 8-digit LED Nixie tube displays normal numbers and changes).

The frequency of the signal generator is 1KHZ, sorry I forgot to show

The normal operation is in the "mv" position, the "mv" display LED will automatically light up, when the voltage input is in the "V" position, then the "V" position LED will light up.And now the "V" LED is always on there, it doesn't change with the input signal.

The same is true for the "RELATIVE REFERENCE" LED. After pressing the REL button, the digital display returns to zero, but the corresponding LED does not light up.

I am planning to repair these gear display LED indicators, but I am not sure how to proceed with the repair. If you know how to inspect them, please let me know. Thank you
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Please take a look at the following image. I found an image on the internet where the LED displayed in the mv gear will light up when the mv voltage is input,But after my device malfunctioned, the V display LED remained on there

The normal startup display should be that the mv display LED lights up first (no-load state),After connecting the signal voltage, if the voltage does not exceed 200mv, the mv display LED will continue to light up. When the voltage exceeds 200mv, the V display LED will light up,the mv display LED is off.
When the REL button is pressed, the RELATIVE REFERENCE display LED will light up

[daisizhou]

I conducted a logic test on the range,In the state of open input, 0.94 is displayed on the screen.I measured the voltage of pins U204-P13, U203-P14, and U204-P8,The following chart has been created.

[daisizhou]

I conducted further testing by inputting voltages of 1.5mV, 15mV, 150mV, and 1V respectively,
Here are my test data

It seems that there is some error in the 2mv gear logic,I don't know where the reason for this mistake occurred

[daisizhou]

The problem has not been resolved yet,I'm lost

[daisizhou]

Thank you to all those who have helped me. I have found the fault point of this 8920a, which is caused by the damage of this LED.
I plan to replace all the LED indicator lights, they may all have defects