Author Topic: Fluke 77 II - incorrect readings on all ranges  (Read 11952 times)

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Offline retiredcaps

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Fluke 77 II - incorrect readings on all ranges
« on: October 07, 2012, 06:44:28 am »
I have a Fluke 77 II.  I got it used or more likely abused.  The 77 arrived with the rubber yellow holster caked in some sort of black ash/fine dust.

OBSERVATIONS

1) When shorting the probes, the ohms range does not zero.  I get something like 30 ohms and it slowly goes down to about 6 ohms and no lower.
2) When shorting the probes, the DCV range does not zero.  It shows 0.024V.
3) When shorting the probes, the ACV range does not zero.  It shows 0.024V.
4) When shorting the probes, the diode setting does not zero.  It shows 0.014V.
5) When measuring mains, the AC voltage fluctates between 118 and 146V AC.  A known working Fluke with the same probes and same AC socket measures a solid 123.1V AC.

STEPS DONE SO FAR

1) I cleaned the range switch (S1) and input jacks as best I could with slips of paper and q-tips dipped in isopropyl alcohol.
2) I tried a new fresh battery and a known set of good working TL75 test leads.
3) I can't see any obvious bad components or battery leakage, but there seems to be that black dust on the pcb and probably underneath S1 when viewed through a 10x jeweler's loupe.
4) The fusible resistor, R1, measures 1008 ohms.
5) On page 3-10 of the service manual, it looks like S1 and U1 are common as possible culprits to all my observations above.

QUESTION

Before I give the pcb and range switch (S1) an isopropyl alcohol toothbrush scrubdown/bath/wash, is there anything else that I should check for?

service manual

http://transmille.net/ProCal/Procedure%20Library/Fluke%20Digital%20Multimeter%2023%20MKII%20%5B1.10%5D/Technical%20Data/Fluke%2070%20Series%20MKII%20(PN%20896204%20July%201991%20Rev%201,%2009-92)%20Service%20Manual.pdf
« Last Edit: October 07, 2012, 07:09:47 am by retiredcaps »
 

Offline ModemHead

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #1 on: October 07, 2012, 05:53:05 pm »
The cleaning sounds like a very good idea since high levels of dust are involved.  A lot of the input circuitry is very high impedance and easily affected by the smallest of stray currents. Before scubbing with IPA, carefully drag a dental pick in between all adjacent pins of U1 to loosen any residue.  Blot the dirty IPA up with a paper towel to keep residue from re-depositing itself on the board.  Fluid will collect under U1, be sure to blow it out using the hole in the PCB during the drying phase.

S1 looks pretty good in the photo (great PCB photos, BTW).  The back side is power and function select, and is probably OK. With the symptoms you describe, any problem with S1 would probably be on top.
 

Offline AndyC_772

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #2 on: October 07, 2012, 06:03:35 pm »
Is that a fried track on the underside of the board, left side just below the round hole about 1/3 of the way down?

Offline SeanB

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #3 on: October 07, 2012, 06:13:03 pm »
Certainly looks like it, could be worth a check for continuity.
 

Offline ModemHead

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #4 on: October 07, 2012, 06:25:46 pm »
Is that a fried track on the underside of the board, left side just below the round hole about 1/3 of the way down?
Good eyes! Does look suspicious.
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #5 on: October 07, 2012, 11:18:22 pm »
Before scubbing with IPA, carefully drag a dental pick in between all adjacent pins of U1 to loosen any residue.

No dental pick, but I will use a sharp multimeter probe.

Quote
Fluid will collect under U1, be sure to blow it out using the hole in the PCB during the drying phase.

Thanks for that tip, I would not have looked under there.
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #6 on: October 07, 2012, 11:22:02 pm »
Is that a fried track on the underside of the board, left side just below the round hole about 1/3 of the way down?

There is continuity at the above area.  I think that black spot is the dirt/fine dust caked on.

The other dodgy areas/tracks around the input jacks also all have continuity.

By continuity, I mean 0.2 ohms.
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #7 on: October 07, 2012, 11:30:30 pm »
This is how dirty the holster is after I did an initial quick rinse (with no soap or cleaner) to get rid of all that black dust/ash/dirt.

Cleaning the holster will be done a bit later.
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #8 on: October 07, 2012, 11:31:58 pm »
By continuity, I mean 0.2 ohms.

That's awfully high.
 

Offline Tube_Dude

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #9 on: October 07, 2012, 11:40:11 pm »
That's awfully high.

That's the usual short circuit multimmeter leads resistance...  ;)
Jorge
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #10 on: October 07, 2012, 11:40:56 pm »
That's awfully high.

That's the usual short circuit multimmeter leads resistance...  ;)

Maybe on your meter..
 

Offline Tube_Dude

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #11 on: October 07, 2012, 11:45:39 pm »
That's awfully high.

That's the usual short circuit multimmeter leads resistance...  ;)

Maybe on your meter..

A less than one year old Fluke 179, with original probes. Not good enough?   ::)
Jorge
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #12 on: October 07, 2012, 11:48:32 pm »
That's awfully high.

That's the usual short circuit multimmeter leads resistance...  ;)

Maybe on your meter..

A less than one year old Fluke 179, with original probes. Not good enough?   ::)

Apparently not.
 

Offline Tube_Dude

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #13 on: October 07, 2012, 11:55:00 pm »
That's awfully high.

That's the usual short circuit multimmeter leads resistance...  ;)

Maybe on your meter..

A less than one year old Fluke 179, with original probes. Not good enough?   ::)

Apparently not.

So, I rest my case, you look so experienced, that I fell incapable of discussion with you at the same level...  8)
Jorge
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #14 on: October 07, 2012, 11:56:59 pm »
So, I rest my case, you look so experienced, that I fell incapable of discussion with you at the same level...  8)

The Fluke 179 is a basic 6000 count meter. It is, apparently, incapable of measuring such low resistances without being zeroed, a function it lacks. Other, higher resolution meters will show significantly lower readings on dead shorts.

But it's okay, you have a Fluke. Everything it tells you is gospel.
« Last Edit: October 07, 2012, 11:58:47 pm by Monkeh »
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #15 on: October 07, 2012, 11:57:17 pm »
By continuity, I mean 0.2 ohms.

That's awfully high.
The Fluke 177 with TL75 leads was right next to me and that is what I used.  As you know, it doesn't have a relative function.

I do have a Fluke 85 III and I can remeasure it, but the pcb is now drying after its IPA scrubdown and wash.
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #16 on: October 08, 2012, 12:00:11 am »
By continuity, I mean 0.2 ohms.

That's awfully high.
The Fluke 177 with TL75 leads was right next to me and that is what I used.  As you know, it doesn't have a relative function.

I do have a Fluke 85 III and I can remeasure it, but the pcb is now drying after its IPA scrubdown and wash.

I would expect that little trace to read 0.00 if it's intact, if you can make good contact with the pads.

However, that's not the cause of your problems anyway, as it's the 300mA jack.

I would check what I assume is a resistor network under the black cover.
« Last Edit: October 08, 2012, 12:04:07 am by Monkeh »
 

Offline Tube_Dude

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #17 on: October 08, 2012, 12:05:12 am »
Retiredcaps.

If you short your Fluke 177 probes and you get 0,2 Ohms, the same as in my multimeter, and if measuring the traces you get the same 0,2 Ohms. Its obvious that the trace is not adding any resistance, so they are OK... ;)
Jorge
 

Offline Monkeh

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #18 on: October 08, 2012, 12:16:37 am »
A meter with a resolution of 100 milliohm is not quite the tool for measuring the resistance of a PCB trace.

But it's a Fluke! It must be right!
« Last Edit: October 08, 2012, 12:20:33 am by Monkeh »
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #19 on: October 09, 2012, 04:46:46 am »
Thanks to AndyC_772, ModemHead, Monkeh, SeanB and Tube_Dude for all the suggestions and participation.

I used a sharp multimeter probe to loosen the dirt out on the ICs and then followed with an IPA wash and firm toothbrush scrubdown.  I probably would not have done the first step if it wasn't for ModemHead's suggestion.

After waiting overnight for the IPA to dry, I reassembled everthing and got a solid 0.2 ohms when shorting the probes.

I don't have a precision reference standard, but I compared the 77 II to its true RMS successor, the 177 with unknown calibration, using an ATX power supply, AC mains, and 250 ohms 0.1% precison resistor.

The inner screen on 77 II is perfectly solid and black.  The outer lens is a bit scratched which makes it look dim next to the 177.

PS. When I used the 77 II's touch hold, I thought something was broken because I wasn't accustomed to seeing no display until a stable reading was measured.
« Last Edit: October 09, 2012, 04:49:47 am by retiredcaps »
 

Offline Tube_Dude

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #20 on: October 09, 2012, 05:14:27 am »
Glad you have you have sorted out and now it is working like a charm.

Agree with you, ModemHead suggestion was absolutely spot on.

Cheers...  8)
Jorge
 

Offline retiredcaps

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #21 on: October 09, 2012, 06:00:53 am »
Well, with one pot to turn, I decided to tweak the 77 II.  I don't have a precision 3.0V supply, but the green pin on an ATX power supply was pretty close.

The used 177 and used 187 have unknown calibration history.
 

Offline ModemHead

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Re: Fluke 77 II - incorrect readings on all ranges
« Reply #22 on: October 09, 2012, 11:30:29 am »
Great job!  Another 'oldie but goodie' rescued from the rubbish heap.  A good cleaning can do wonders. Your cal job looks pretty good, too.  Those old 70-series with one or two trim pots are really easy to calibrate.

P.S. Some plastic polish will probably do wonders for the lens.  It takes a few repeated applications and some elbow grease. A easy source for polish is to go to the auto-parts store and get a product made for polishing modern plastic headlight covers.  The lens in a 70-series is removeable, so be sure to pop it out first.

Oh and @Tube_Dude: your post #17 advice was also spot-on.
« Last Edit: October 09, 2012, 01:29:51 pm by ModemHead »
 


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