Author Topic: Please help, FLUKE 8920A Multimeter Repair (Read 18603 times)

Harry_22 · « **Reply #125 on:** June 09, 2024, 12:52:08 pm »

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.

Harry_22 · « **Reply #126 on:** June 09, 2024, 12:59:05 pm »

Please use 1k resistor. 10k is too much.

daisizhou · « **Reply #127 on:** June 09, 2024, 01:23:06 pm »

Ok, give me some time and I will test it

daisizhou · « **Reply #128 on:** June 10, 2024, 02:38:56 am »

Quote from: Harry_22 on June 09, 2024, 12:59:05 pm

Please use 1k resistor. 10k is too much.

It looks like a lot of interference, I don't know if the test is valid

Harry_22 · « **Reply #129 on:** June 10, 2024, 06:48:27 am »

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.

daisizhou · « **Reply #130 on:** June 10, 2024, 09:15:50 am »

I tested it again.

Harry_22 · « **Reply #131 on:** June 10, 2024, 09:43:53 am »

Well done!

Is it possible to reduce the level to see the clear sinus? If not please use 10k.

daisizhou · « **Reply #132 on:** June 10, 2024, 09:58:15 am »

Quote from: Harry_22 on June 10, 2024, 09:43:53 am

Well done!

Is it possible to reduce the level to see the clear sinus? If not please use 10k.

clear 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?

Harry_22 · « **Reply #133 on:** June 10, 2024, 10:12:12 am »

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.