Electronics > Repair

Fluke 87 1st gen. AC and DC Voltage not auto-ranging

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Gary IN:
I got this old Fluke 87 first gen. with the following problems & could use some brain power to fix it.


* VDC always displays minus 39V (pic). When I measure 30VDC it will add or subtract only 5V - depending on polarity.


* VAC first jumps up and down in the mV range, which seems to look ok, but shows 24.5 VAC when plugged in to 115 VAC outlet.


* Diode test shows max (no zero or anything else). _______________________________

Strangely, everything else seems to work OK: mV DC. Ohms, continuity buzzer, Amps, mAmps and microAmps show accurate values.

Both fuses were blown when I got it

Gary IN:
Any ideas?
Schematic pages from the Service Manual attached.

dustooff:
Hi,
have you tried cleaning the rotary switches?

MathWizard:
So it looks like ACV, DCV, DT and Capacitance use mostly the same RC networks and internal switches in the IC. So does capacitance also not work right ?   You can check the RC parts around that IC. IDK about these meters, but I'm worried it's those switches inside the chip.

Gary IN:

--- Quote from: dustooff on November 18, 2024, 09:13:24 am ---Hi, have you tried cleaning the rotary switches?

--- End quote ---
I wiped that rotary switch and it is in perfect shape. The readings showing on the display are consistent.
Thanks for the suggestion.

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