Another Fluke87 problem

[bdunham7]

So there was another long thread on Fluke 87 repair featuring users Manul and Multiple Cheese Slices, you should find and read it.  The way the power supply is set up, the battery negative is below V_ss.  I'm not sure how it all shakes out internally, but the fact that the indicated negative voltage was actually present on the terminals when you checked seems to indicate that somehow negative battery voltage is leaking back into the input somewhere.  One thing to try would be to put the meter in the ACV position (battery installed) and then measure with another meter with the black lead going to the com jack and the other lead to terminals 3 and 4 (one at a time!) of the front switch--if that's feasible.  If your negative voltage shows up there, then you have a better idea where it might be coming from.

[bdunham7]

Thank You very much, that exactly what i have. So my R2 is out of spec.

Are you sure?  What sort of meter do you have that measures 882 megohms?? 

[planenut2000]

fluke 88

[planenut2000]

crap, i missed the decimal point. it was .896mega ohms. so the R2 is good

[planenut2000]

So there was another long thread on Fluke 87 repair featuring users Manul and Multiple Cheese Slices, you should find and read it.  The way the power supply is set up, the battery negative is below V_ss.  I'm not sure how it all shakes out internally, but the fact that the indicated negative voltage was actually present on the terminals when you checked seems to indicate that somehow negative battery voltage is leaking back into the input somewhere.  One thing to try would be to put the meter in the ACV position (battery installed) and then measure with another meter with the black lead going to the com jack and the other lead to terminals 3 and 4 (one at a time!) of the front switch--if that's feasible.  If your negative voltage shows up there, then you have a better idea where it might be coming from.

I dont see a way to do the test you suggested

[bdunham7]

With the meter disassembled and just the board laying on the bench?  Just make sure you know which position is ACV, and don't worry about the display not being installed. 

[planenut2000]

i put in both the ac volt scale and checked all the taps on the selector switch, nothing near the -4 volt. also did the same thing in the dc volt scale same result

[Manul]

R1-reads 882mega ohms

Maybe it was 0.882M? What kind of meter you used to measure it 882M? Does that meter really go up to GigaOhm?

So as I understood, there is current injection happening somewhere. 4 volts is "real" volts. The fact that it goes zero after shorting probes and that it drops to 3V under load of 10M (?) may suggest the likely point of injection.

Edit: I just saw that megaohms question is already cleared. Nevermind.

[bdunham7]

i put in both the ac volt scale and checked all the taps on the selector switch, nothing near the -4 volt. also did the same thing in the dc volt scale same result

OK, so under those conditions (in DCV) are you still seeing the voltage at the input jacks, but not at terminal 3 or 4 of the front selector switch?

[bdunham7]

Another thought that might be an easy fix--is is possible there is some board contamination or a dead spider or whatever right in the area where the battery clip attaches to the board and the input jack is?  Perhaps under the input jack assembly?  You could try some IPA and then blowing it out with compressed air to see if that makes any difference.

[planenut2000]

I do see what looks like a very crappy solder job on the volt ohm tap of the input jacks, i will re-solder it tomorrow

[planenut2000]

well re-soldering the cold joints on the input jacks and i re-soldered the 9 volt battery to to circuit board connections didnt help. now i have warning beeps of having leads in wrong positions

[bdunham7]

If it was me, I'd take the input jack assembly off and have a look.  I realize that you had the problem before they replaced that, but I'm having trouble imagining that this issue is likely to be anywhere else.  I don't know how difficult getting that off would be for you, but if you have desoldering ability, I'd try that. 

[planenut2000]

i agree, i was thinking about doing that. Are there any good quality input jacks on the market for this model. I installed the jacks previously on my fluke 88 before.

[bdunham7]

Fluke p/n 826214.  eBay seller 'Fluke-USA' hasn't sold me any bad or fake parts yet.  Apparently this part is discontinued from Fluke but surplus ones remain in the market plus there are fakes.

[planenut2000]

Only this i see is the volt/ohm printed circuit is torn, might have happened in the removal

[planenut2000]

a lot of solder to remove with a radio shack solder sucker

[bdunham7]

Well, now you can power it up and see if you still have the negative voltage at the input jack.  I like test equipment, but sometimes I find problems by prodding things with an insulated stick.  This may be what happens here!  I would look closely at the area where the two green battery wires go around the input area.  Is there some gunk or corrosion there?  Is it possible that the insulation is cracked on those wires or they have residue from previous battery leakage? 

[james_s]

I cringe at the thought of somebody working on a high end meter with a Radio Shack solder sucker. That's a good way to mangle the PCB beyond repair. I lived with one of those sucker bulb irons for many years before I finally got a vacuum desoldering tool, would never go back.

[planenut2000]

I cringe at the thought of somebody working on a high end meter with a Radio Shack solder sucker. That's a good way to mangle the PCB beyond repair. I lived with one of those sucker bulb irons for many years before I finally got a vacuum desoldering tool, would never go back.

Thanks for the help!

[planenut2000]

I think i made some progress on the 87.  With the input jacks removed i still had the same readings. I did notice that the 2 green wires from the 9 volt battery area looking a little fuzzy, so i re-soldered them.
That made a big difference on the readings. now the ACvolts with settle at 11milli volts, dc volts show -1.1 volt,  dc milli volt will show -OL  ohm scale is toggling into OL and the top of its auto ranging.

[planenut2000]

i soldered the input jacks back in. I re- soldered the R1 and R2 and RT1 and E1. now ma AC settles at about 3.2 milli volt. the DCV is showing -.495 volt and is OL or sometimes shows 495milli volts. the Ohms
scale is 3.15 mega ohms.
Sometimes when i first switch onto ACV the meter will sound alarm like the leads are in the wrong position until i turn the dial selector back and forth. Do you think at this point it just needs cleaning?

[bdunham7]

It looks like you're in the area.  Perhaps an IPA bath and a bit of scrubbing?

[BMK]

Can confirm I had this exact issue. Weird readings after a sight slight battery leak(Caught early, before any corrosion visible). IPA did not work to clean, eventually used electrolube safewash and another 1-2 washes with IPA and a bake off at 90'C for a couple of hours. Always stuck with me as a lesson in how tiny leakage currents can appear with contamination.

[james_s]

About 30 years ago I built a Heathkit DMM, it was roughly equivalent to the meters you can now get for free at Harbor Freight, maybe a tad better but it was expensive at the time. I still remember the manual was very clear about keeping the PCBs clean and avoiding even fingerprints, and had a procedure for cleaning them.