Author Topic: Please help for strange problem with Fluke 867B DC measurement  (Read 1035 times)

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Offline marcaurell

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on my Fluke 867B, which otherwise works perfectly, DC shows a level of approx. 14v in the view window. Does anyone have a solution for this ?? See Photos. Thank you very much!
 

Offline shakalnokturn

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Re: Please help for strange problem with Fluke 867B DC measurement
« Reply #1 on: July 02, 2020, 08:57:04 pm »
Main display is handled by RIC (1727P) the graphing/scope comes through RIC too but the ADC is done separately by U10 (MP87L75).
My first suspects would be U14 (74HC4053) and U9 (MC34081D) check concerned power supplies first of course.
 

Offline CDaniel

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Re: Please help for strange problem with Fluke 867B DC measurement
« Reply #2 on: July 02, 2020, 10:48:16 pm »
For processing the waveform ( scope mode ) there is a DC bias to shift the input of the fast ADC U10 to half range ( so it can go up and down with the alternating signal  ) , maybe this is wrong . First you should check if with the probes shorted or not connected ,  VIN pin16 is at VREF/2 . The reference is 1,2V if i'm correct , so at 0,6V .

U10 is the 8bit fast ADC used only for scope , the precision ADC for meter is inside U30 RIC2 and both go in separate way to the display processor .
« Last Edit: July 03, 2020, 05:22:06 am by CDaniel »
 

Offline Analogdevice

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Re: Please help for strange problem with Fluke 867B DC measurement
« Reply #3 on: November 06, 2020, 06:17:04 pm »
Hi there,

I'm repairing this device for marcaurell.
I don't have so much time this weekend, so very quickly:

Measured between TP29 (non-inverting input of U9 before R31) and signal ground, DCV, auto-range.
Shorted input: 1,18V
Open input: 1,18V
300mVDC to input: 1,19V
It should be 1V at shorted or open input and 1,3V at 300mVDC at input.

It comes from RIC2, because this voltage is present also if I remove R31 or U14 and U9.
I checked Z6 (feedback resistors at COMP. AMP and feedback resistor at summary amp) - OK.

Can someone measure for me input to the comparation amplifier? Pin 1 of Z6 between signal ground (COM input)? I'm very suspicious that there will be problem, because comp. amp is working and it comes from RIC2 (it's a pity that I don't know what and where drives the switches inside RIC2).

Thank you very much!
Greetings from Slovakia.
 

Offline Analogdevice

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Re: Please help for strange problem with Fluke 867B DC measurement
« Reply #4 on: November 13, 2020, 08:16:15 am »
Hello,

nobody have 867? Please, take off 6 screws and some other 6, apply 300mVDC (e.g. from calibrator or power supply) and measure for me pin 1 of Z6 between COM input.
It will help me so much! Thank you!
 


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