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Fluke 8000A Multimeter Reading 1/2 of Actual Values

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TMA-1:
First time poster here.  My self-taught electronics knowledge is moderate but sporadic.

I have recently been given a Fluke 8000A multimeter, a 3.5 digit unit which is both very old (70's) and very cool.  Being as it's a Fluke, I would like to restore it to accuracy and actually use it for my hacks.

But here's the scoop:  On DCV and on Ohms, the meter reads a suspiciously consistent 50% of what's expected and what my comparison meter reads.  On DC mA, it reads a suspiciously consistent 200%.  (On ACV the error is gross but less predictable.)  Range switching does not seem to affect the error proportion.

The unit looks good inside with no apparent evidence of leakage, corrosion, bomb craters, or meatball surgery.  (Yet.)  :)   My poking suggests the power supply section is operating normally.

I can't help thinking these unlikely exact multiples are a red flag for the failure of some individual component, but my digestion of the manual found on-line is less than perfect.

Can someone who better understands how these circuits actually work point me in the right direction?  Or is my "observation" a random thing and this meter is simply "wildly inaccurate"?

Thanks in advance for your time.

bdunham7:

--- Quote from: TMA-1 on January 17, 2024, 04:46:17 am ---My poking suggests the power supply section is operating normally.

--- End quote ---

Start by verifying that completely as in the manual and posting the exact results including how you measured them. 

That whole set of issues sounds pretty interesting and I don't have any ideas offhand--but I'll have a look.  Can you verify which exact model you have--does it have batteries, LO_OHMS or any other options?  Maybe a photo of the front of it?  There were some pretty weird variations of these made and they have significant internal differences.

Do you have a way of providing reasonably accurate sources for a range of DC and AC voltages on at least the lower ranges?  Like a function generator or something.

I've attached a full-page schematic for the standard line-operated unit if anyone is playing along.  It looks to me that the first place to look would be TP3 to see how the ADC input compares to various front-panel inputs.  If everything was half-off I'd say it was just another defective U3 or something, but the current being double seems interesting.

TMA-1:
Thank-you for the swift reply.

I started to follow the debugging flowchart in the manual, and got readings close to, but not within, the initial ranges specified, ie. across the big capacitor the value I measured (with my cheap and possibly stingy meter) was 4.3VDC, rather than 4.5 to 5.5 "expected".

I'm afraid I lost a bit of confidence in the flowchart at that point, (though I love that it started with a diplomatic way of asking me if the unit is plugged in!), for the following reasons:

* The component numbers didn't match my best guess at the right schematic to use;
* The board itself does not indicate component numbers; and,
* It occurred to me that if that 5V voltage was used as a logic level, even a minor variance could be fooling some logic and creating discrepancies.However, measuring the spread at the emitters of Q19 and Q24, I got a solid 30VDC, which is what the schematic (same one you posted) says it should be.  So I just don't know.

At that point I decided to post, because the suspicious nature of the discrepancy led me to hope my situation might ring a bell with someone.

I am less confident about the 200% mA reading than I am about the 50% experiences.

I do have a signal generator, and a way of providing variable DC voltages.  I'll endeavor to take and record a variety of DCV, ACV, and Ohm readings.

AVGresponding:
The fact you're only getting 4.3V on the 5V rail would have me checking if either CR13 or 14 are o/c, or C19 might be a bit dead. Or both.

It has to be said, the 8000A is pretty notorious for being a bit crap, due to poor regulation of the 5V rail. There is a modification somewhere on this forum, posted by member MED6753, basically using a 7805 to make it a bit more stable.

bdunham7:

--- Quote from: TMA-1 on January 17, 2024, 04:46:17 am ---Can someone who better understands how these circuits actually work point me in the right direction?  Or is my "observation" a random thing and this meter is simply "wildly inaccurate"?

--- End quote ---

To answer this question in simple terms, the meter switches and input circuits process the input through the input divider network, the current shunts and/or the AC converter circuit with the intended result that a DC voltage from either 0 to +/-2.000 volts or 0 to +/- 200.0mV shows up at TP3 near U3, all depending on what range is selected.  So if you are on the 2V range, you would see the exact input voltage appear at TP3.  IIRC, the 20V and 200V range use the same input divider but for the 20V range it uses the 200.0mV scale at TP3 and for the 200V range the 2.000V.  You'll see the adjustment circuits for those two ADC scales just below U3 on the schematic.

So the first thing you want to do if you can is to use DC inputs of 100mV, 1V and 10V, select the correct range for each and then see what appears at TP3 and what the display reads. 

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