Electronics > Repair
Please help, FLUKE 8920A Multimeter Repair
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daisizhou:

--- Quote from: Harry_22 on June 10, 2024, 01:43:19 pm ---Perfect!

The last experiment for today.
Please connect TP1 and #4 by clamps and switch on the device.

--- End quote ---
Harry_22:
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.
Harry_22:
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 :)
daisizhou:

--- Quote from: Harry_22 on June 10, 2024, 08:31:09 pm ---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 :)

--- End quote ---

Good 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"?






Harry_22:
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.
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