Author Topic: Fluke 87 1st gen. AC and DC Voltage not auto-ranging  (Read 429 times)

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Offline Gary INTopic starter

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Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« on: November 15, 2024, 08:06:44 pm »
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
« Last Edit: November 15, 2024, 08:13:20 pm by Gary IN »
 

Offline Gary INTopic starter

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Re: Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« Reply #1 on: November 16, 2024, 02:52:49 pm »
Any ideas?
Schematic pages from the Service Manual attached.
 

Offline dustooff

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Re: Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« Reply #2 on: November 18, 2024, 09:13:24 am »
Hi,
have you tried cleaning the rotary switches?
...
 
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Offline MathWizard

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Re: Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« Reply #3 on: November 18, 2024, 10:47:04 am »
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.
 
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Offline Gary INTopic starter

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Re: Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« Reply #4 on: November 18, 2024, 01:51:06 pm »
Hi, have you tried cleaning the rotary switches?
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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Offline Gary INTopic starter

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Re: Fluke 87 1st gen. AC and DC Voltage not auto-ranging
« Reply #5 on: November 18, 2024, 02:12:31 pm »
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.
Thank you for the suggestion!
I quickly tested those RC parts before but will check them again later today. The capacitance meter did show some nF without anything connected to it. So, I think you are on the right track. With testing 1 to 4.7 μF it seemed ok-ish, maybe 20% off compared to my good Fluke 87V. But I will double check that again. I know that the 1st gen. 87 has v. limited capacitance test range.

If the internal switches are fried, then I will just use this Fluke for μA/A, resistors and continuity buzzer.
 


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