Electronics > Repair
Please help, FLUKE 8920A Multimeter Repair
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daisizhou:
"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"?
Harry_22:
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?
daisizhou:
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.
Harry_22:
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.
daisizhou:
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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