Author Topic: Fluke 88 VDC Inaccurate in Low Ranges  (Read 676 times)

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Offline Opie91Topic starter

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Fluke 88 VDC Inaccurate in Low Ranges
« on: September 24, 2023, 12:18:18 am »
Hi All,

Have been watching Dave for the last few years and thoroughly enjoy his videos. Recently, I picked up an old Fluke 88 for cheap and found it accurate with one exception: the lower DC voltages do not indicate correctly. Specifically the 4 and 40-volt DC range. Hopefully, someone here can point me in the right direction.

Details: This is an automotive meter. The AC Volts, amps AC/DC, ohms, diode check, and DC millivolts all work fine. The DC ranges 400 and 4000 also indicate just fine compared to a power supply. When DC voltage is selected in the 4-volt range with no voltage applied, the meter indicates -OL and -16.16 in the 40-volt range. The negative voltages do not change if the V and COM are connected. If you zero the meter and apply voltage, the meter indicates about 60% at 10VDC. At higher voltages, the gap between applied and indicated increases; at 32 VDC applied, the meter indicates approximately 55% of the applied value.

What I have checked:
Wafer switch - resistance checked and found to be good. Cleaned
Both fuses - good
1K fusible resistor - good
Resistor Net - checked resistance from the processor chip to the wafer switch. All resistances were found to be within specifications.

The board looks nice and clean with no burn marks, cracked solders, or apparent damage.

Any help is appreciated.

Thanks


 

Offline mwb1100

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Re: Fluke 88 VDC Inaccurate in Low Ranges
« Reply #1 on: September 24, 2023, 08:29:37 pm »
Is this a Fluke 88V or plain 88 (or some other variation)?  If it's an 88V it should still be under warranty.
 

Offline Opie91Topic starter

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Re: Fluke 88 VDC Inaccurate in Low Ranges
« Reply #2 on: September 24, 2023, 11:54:41 pm »
Hi, thanks for your reply. Straight Fluke88, old timer.
 

Online bdunham7

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Re: Fluke 88 VDC Inaccurate in Low Ranges
« Reply #3 on: September 25, 2023, 12:03:14 am »
I've no idea offhand what would cause that other than a problem with the main ASIC chip, but here's a manual with schematics.  Does your's say CAT II on the front?

https://res.cloudinary.com/iwh/image/upload/q_auto,g_center/assets/1/26/Documents/Fluke/88/88_doc_3.pdf
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline Opie91Topic starter

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Re: Fluke 88 VDC Inaccurate in Low Ranges
« Reply #4 on: September 25, 2023, 12:26:48 am »
Hi, thanks input. No, it does not say CAT II.

I was afraid that it might be the main chip.

Going to bring it to work tomorrow and take a look at some of the caps in the signal conditioning circuit, probably a long shot, but why not.

Thanks
 


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