Fluke 8000A Multimeter Reading 1/2 of Actual Values

[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]

My poking suggests the power supply section is operating normally.

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]

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"?

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. 

[TMA-1]

Very insightful, thank-you both.  I haven't made it back into the dungeon yet, but I will add those tests to my to-do list.

Regarding the 5V rail, yes, now that I take a closer look at the schematic--something I'm not terribly good at--I see that it is in fact TWO power supplies, and I AM capable of understanding how the 5V section works.  "Not very well", indeed, it seems.  I should also check those two diodes.  One of them may be limping.

I am excited by the idea of replacing the 5V rail wholesale, and I think it's within my skill set.  MED6753's modification is brilliant.  I have bridges, a 2200uF, and some 7805's.

Back downstairs, where my wife occasionally throws scraps of food and a picture of the children,...


[Later,...]  So.  I tested the two diodes of the rectifier, and found them both leaky.  After replacing them, the "5V output" was still 4.3V.  I suppose that leaves C19 as the most likely suspect.

So I did an experiment.  I "re-enforced" the 5V rail with an external 5V power source at C19.  The "noise" level (with the Fluke's probes unconnected) dropped from a very sporadic something-or-other, to 0.001 at most, and when connected to my little utility power supply now registered an entirely reasonable 13.67 Volts!

So it IS a 5V problem!  I suspect some logic circuitry is not being graced a healthy enough diet to function,... well, logically.

Now to fix.  Questions come to mind.

Could use be made of one of those darling little $1 buck converters, like this one?  https://www.ebay.ca/itm/225433817428

If I used the FULL winding of that part of the existing transformer, there's about 12VAC there.  I could bridge rectify, filter, then dial up about 7V on the buck converter, and add a 7805 for stability.  Or am I getting too brazen?

[bdunham7]

If I used the FULL winding of that part of the existing transformer, there's about 12VAC there.  I could bridge rectify, filter, then dial up about 7V on the buck converter, and add a 7805 for stability.  Or am I getting too brazen?

Why not just use a bridge, capacitor and LM7805 directly?  I'm assuming you can disconnect that center tap from the ground, the actual wiring up should be straightforward enough.

Were you able to verify that the meter was fully functional on a number of ranges with an external 5V supply?

[TMA-1]

I was thinking of lightening the load on the 7805 so I don't have to use a heat sink.  But yeah, here's where I get a little bit fuzzy.  The ganging of the two center-taps seems to happen inside the transformer, so if I disconnected one, I'd axe both.  Do I have to disconnect that?

I haven't tested a variety of ranges yet.  Having established that 5V is at least "a" significant problem, I'll defer that until that much at least is fixed.  I suspect it will rectify everything, but I will give the meter a thorough workout before putting it into use.

[bdunham7]

The ganging of the two center-taps seems to happen inside the transformer, so if I disconnected one, I'd axe both. 

That's exactly what I was worried about, I seem to remember that from somewhere.  Yes, that's a problem, just a simple bridge isn't going to work. 

[TMA-1]

Rats.  So close there,...

How about,.. replace the 5000MF 15WVDC capacitor (about the size of a tanker rail car), with a 4700uF 25VDC capacitor that I can salvage, and add a 7805?  Should I experiment with that?

[bdunham7]

How about,.. replace the 5000MF 15WVDC capacitor (about the size of a tanker rail car), with a 4700uF 25VDC capacitor that I can salvage, and add a 7805?  Should I experiment with that?

If the existing capacitor is signficantly degraded replacing it might help (and that part you are suggesting will be fine) but the voltage will become normal at most, no headroom for an LM7805.  You didn't mention what other meter you have, but assuming it's an ordinary digital DMM, try measuring the 5V rail using your AC range and see what you get for ripple.  Actually it would help to know the make/model of the meter as well.

[TMA-1]

Oh, right.  I got carried away there, maaaad with power.  I suppose I could use an up-converting buck converter, and then regulate it back down to 5V, but that's just too icky.

I'll do what you suggest momentarily.

I have a Vichy V97, which is not the best.  In fact the capacitance test is garbage, I just found out during this little escapade.  But the more expensive Mastech MAS345P CLONE that I have has a broken continuity check (NOT the buzzer element), so I tend to toggle a lot.

The Mastech clone let me down once though also.  It outright read the wrong voltage on one particular circuit, a conclusion eventually backed up by 3 other meters.  Scared the marrow out of me.

Hence I'd like a third opinion handy, in the form of this FLUKE, flawed though it may be.

They say never put to sea with two compasses.  If you can't take three, only take one. 


[Edit]  It works!  I'm getting 5.3VDC, and 0.06VAC ripple.  And my power supply once again reads 13.67VDC.  Yay!  I will of course check the other functions and ranges, but have no reason to suspect there will be other problems.

Thank-you bdunham7, and AVGresponding, for the excellent helmsmanship.  Your karma accounts runneth over. 

[tautech]

Eeeek, a TANT ! 



Just because it's lollipop red don't think it won't throw a curveball somewhere down the track.
Get that sucker outta there for a new and higher voltage rated one.

[Martian Tech]

I'll bet this thread would be happier in the https://www.eevblog.com/forum/repair/ forum...

[AVGresponding]

I'll bet this thread would be happier in the https://www.eevblog.com/forum/repair/ forum...

There's a lot of crossover. It's fine where it is afaic, and I wouldn't have seen it there, so wouldn't have mentioned the 5V rail mod. Ultimately it's up to the OP and the mods (who are not heavy handed on this).

[tggzzz]

Eeeek, a TANT ! 



Just because it's lollipop red don't think it won't throw a curveball somewhere down the track.
Get that sucker outta there for a new and higher voltage rated one.

Two tants, actually.

One is cheap and nasty, and it might be worth replacing it. If it is operating at half its voltage limit, I would be less concerned than having 13V on a 15V tant bead (thanks for nothing, Tek!)

The other (TRW) is very expensive (>£50 replacement) and probably still good. Only replace it if it has spewed acid across the board

To the OP: when measuring power rails, measure the DC value and the ripple. The ripple can give a clue as to whether an electrolytic has dried up.

With that in mind, I'd look at those blue Philips electrolytics.

[NoisyBoy]

I have two of those. One thing that has not been mentioned is cleaning the push switches, use some DeOxit to clean them out whether this is related or not. I have seen switches that have not been operated for a long time cause various measurements issues.

After you applied DeOxit, operate each switch 20-30 times to clean them out.

Those caps are very old, I did purchased all new caps for the two 8000A I own, but when I pull them out and measured them, they were all within spec. But given the age, it wouldn’t hurt to replace them.

Good luck.

[joeqsmith]

First time poster here. 
...
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.
...
Thanks in advance for your time.

A better option may be the free meters from HF.  They are smaller, have a long battery life and ... free.   The 8000A was my first DMM and while it did do a major overhaul on it a few years back, the only reason was sentimental value.   There are much better meters available today for little cost.