FLUKE 87 IV - drifting, negative-ohms, cant go past 3.3Meg

[gamerpaddy]

Just saved a 87 IV from the dumpster, its in a rough condition and has some issues.
besides corroded battery terminals which i cleaned, it has some electrical problems.

i cant seem to calibrate it. yes i dont have a calibrator, but i got a known good meter to roughly compare to when setting a bunch of pots to get the mV and Resistance values i need.
the calibration manual states, getting the Millivolts calibrated first, since everything DC depends on it. which i did.. several times.

after setting 50V, 500, 3000mV it still not reading 50 ... 500... 3000
when i short the inputs together, it shows -20..-50Ohms depending on how accurate i "calibrated" the mV range. but i never got it to read 0.
Temperature measurement allways shows "open". i tried with a k-type and resistive temperature probe.
in Ohms mode, i cant make it go past 3.3Meg with 5x1Meg resistors in series.
When its open leads, its very erratic. Overload and -5 Meg showing up   bargraph going wild.
it never shows 0V in VDC, VAC, mVDC mVAC etc.
it take a while to settle after giving it a new voltage/resistance.


just Capacity measurement seems bang on, no complaints here.

looked on the board, nothing obvious.  all resistors in the input section seem to measure fine. (1meg, 100k, 1k and such.
 


Anyone got an idea where to look?  this kind of stuff is not my usual bench-guest in for repair.

ignore the horrible fuses, ill give it some proper bussmann as treat, once it starts behaving well.

[coppercone2]

I have a feeling it might not be as clean as you think. Not being able to achieve a maximum resistance is a hall mark of damaged PCB.

easy way might be to give it a ultrasonic vinegar bath followed by a distilled water bath

if that does not work, you can try lifting IC's up to check under neath them. That is alot of work.. but corrosion damage can be a total bastard to fix.

you could have crystals under a SMD part. Sometimes the only way to fix that is to lift all the parts and scrub under neath.

The cleaning power / reach of a ultrasonic acid bath should not be underestimated. Nothing else works vs those 4 sided SMD chips really. And you want that sucker hot, like 60C. The cleaning power of acetic acid really goes up with temperature. But again its going to start damaging platings and stuff . and bake it good after.


 Probobly you can use other dilute acids too if you have really strict cleaning protocols. but that is like a last ditch before you throw it out (no support, no one has the balls to do it lol)

also I noticed something, in some cases, if you spray something with deoxit and there is residual corrosion causing substance, you will see green spots there. slow though, like 1 month. I have seen it show up on some stuff like harnesses that got exposed to chemicals.


and not sue how that meter works exactly, but have you checked the big rotary switch to make sure its contacting good?

[gamerpaddy]

There are absolutely no signs of corrosion or liquids on the PCB itself. just check every part of it.
didnt lift the chips, but couldnt see any hints from the side. all pads are looking shiny, nothing dull.
contacts on the rotary switch are fine as far as i can tell. wiped it with a dry cotton stick.


Ohms range is climbing.. with a 5Meg connected to it, it started at 3.5 and now is at 4 after a few minutes. 

When i enable Min/Max it is instantly oscillating between  4 and 2.7 with an avg. of 3.8  and constantly beeping on both min and max settings as it moves around a little.
as if some supply rail is oscillating.

offset seems out too, quite a bit.
i measured a old 9V battery i had laying around right way around, it was 8.12V, but in reverse it was 8.55V


i hooked it up to a 6V supply to measure with the lid open
noted some voltages and waveforms i saw by randomly picking a via / pad.

1.) 1.2Vpp with a chopped ramp waveform going 0.6v in either direction.
1.1.) is after the red component above the 102 resistor. 87mVrms, 40mVpp with a different waveform.
2.) is before the squiggly line resistor. 10.8Vpp same chopped waveform as 1.)
3.) is 3.21V
4.) 449.85khz at 3.44Vpp resonator
5.) 3.1V
6.) -5.7V
7.) 3.1V
8.) 5.6V
9.) 2.35Vpp at 7.372MHz
10.) 5.99V
11.) 1.25V
12.) 3.13V
13.) 153.6 Khz at 3.25Vpp
14.) 3.13V
15.) 0.465V
16.) 80hz at 2Vpp
17.) 1.34V

[coppercone2]

hmm not sure but I saw a connector once that wicked up battery electrolyte, the front looked almost fine, the back had a thick ass forrest of crystals growing there. its damn evil

[Hamelec]

what is going on here:

[gamerpaddy]

noticed that too, just some dust / residue from the corroded battery terminal above.
wiped it off, nothing left.  via is fine.

[gamerpaddy]

tried factory reset
measured -2ohms but it slowly drifted back to -30..-50 after a while.

also measured the ptc, sparkgap and resistors. all seem fine. solder joints not cracked.
i think the fluke asic or the precision resistor network is toast
gotta go on fleebay for parts soon