Fluke 17B won't zero on Ohms

[crashanium]

I recently acquired a Fluke 17B. It was described as defective with missing display segments and flashing and beeping when switched on. I figured fixing the display should be simple so I took a chance and bought it.

When I received it and opened it, it became clear it had had a hard life. Both fuses were blown, one battery terminal was broken and there was evidence of significant internal damage due to battery leakage.

I cleaned/scrubbed it with contact cleaner and window cleaner and it started to more or less work. A simple cleaning of the contacts got the display back to normal. There were some strange anomalies though like the VDC giving different positive and negative readings (e.g. 11.69V vs -11.59V) and shorted probes on the resistance giving 200 Ohms. Further cleaning with contact cleaner got the resistance reading down to 80 and finally 10 Ohms or so.

I ordered some IPA and cleaned the PCB with that. The results were mixed. The positive negative DC offset had gone and shorted probes on VDC now gives 0.0V. The mV range typically does not go to zero, but shows 1.8-2.0mV with shorted probes. The resistance shown with shorted probes is up to 16 Ohms or so and has remained at this despite a second IPA bath.

The current ranges seems to work fine once I put a new fuse in. The temperature readings are very dubious, but I haven’t much hope for them as that part of the PCB was probably the worst attacked.

I have measured the input impedance on VDC scale and get 11Meg. The mV scale increases this to 17meg or so. Looking at the schematic, it looks more like the input impedance should be 10Meg. I’m guessing R64 can be adjusted to make it exactly 10Meg, but I haven’t tried that yet. I don’t think I will be able to trim it down to 10MOhms anyway. I just wish there was a zero offset adjust option somewhere!

I’m out of ideas. The meter seems to be fine in most respects except for the low resistance range.  What have I missed?

Thanks

[4thDoctorWhoFan]

Considering you had battery leakage, there might still be some damage to some vias or traces.  What happens is the acid will eat away the copper and the via will not have connectivity from one side of the board to the other.

For example, the via & trace by C25 & C26 looks suspicious.  It's hard to tell from pictures.

Otherwise, give it a good cleaning again.  I would soak the pcb in a bath of IPA for an hour or so while scrubbing it every now and then.  Of course remove stuff like the lcd and elastermers.

Good luck.

[retiredcaps]

I ordered some IPA and cleaned the PCB with that. The results were mixed. The positive negative DC offset had gone and shorted probes on VDC now gives 0.0V. The mV range typically does not go to zero, but shows 1.8-2.0mV with shorted probes. The resistance shown with shorted probes is up to 16 Ohms or so and has remained at this despite a second IPA bath.

I have seen this before.  With any sort of battery juice leakage, you may have to do repeated rounds of IPA with a toothbrush scrub to get rid of any leakage that have worked into vias, components, etc.

I suggest a thorough dunking of the entire PCB into an IPA bath with a clean toothbrush scrub followed by thorough drying using a fan for at least a few hours. Obviously remove the lcd, fuses and battery first.

I have measured the input impedance on VDC scale and get 11Meg. The mV scale increases this to 17meg or so. Looking at the schematic, it looks more like the input impedance should be 10Meg.

11M ohm is correct for the VDC.  17M ohm is not for the mV.  I don't have the 17B, but it's usually 10M on Fluke meters for the mV range.. 

I’m guessing R64 can be adjusted to make it exactly 10Meg, but I haven’t tried that yet.

I suggest not touching any trim pots yet until you do more rounds of IPA bath.

I also circled in yellow areas where I think the trace is gone/corroded or vias may not be making contact to the other side.  Carefully wash these areas with IPA and test the traces for continuity.

Examples from modemhead

http://mrmodemhead.com/blog/fluke-23-iii-dmm-repair/

http://mrmodemhead.com/blog/fluke-77-iv-copper-corrosion/

https://www.eevblog.com/forum/repair/fluke-77-iv-repair-no-backlight-or-continuity-buzzer/

Also, if you search the repair forum, you will see many Fluke and other meters that have this offset problem fixed by IPA baths. I can't guarantee you will have your fixed, but you have reported that each successive IPA bath has given better results.

[crashanium]

Thanks for the suggestions.

I'm not holding out too much hope as I agree that the corrosion has gotten into the internal pcb layer. I have checked most of the vias and amazingly all the ones I checked seemed ok for continuity. I'll have to try checking again and check for the actual resistance instead of just connectivity.  I have now found some thin enough wire to fit through the vias so I could try soldering them. However, there are a lot of them and I'd prefer to narrow down the likely culprits.

I've tried an IPA bath twice already and the first put the resistance offset up and the second did not change it. I'll try a third overnight, but I am not hopeful.

I did some probing tonight and  I can get the offset down to about 5 ohms by bypassing some of the input circuitry - connecting directly downstream of R20. This doesn't make a lot of sense to me as the input resistance is 10 met, so bypassing about 2k should not make a difference.

I can console myself that whatever happens I now  got a nice set of test leads!

[retiredcaps]

I would take a very close look at CR8.  All 4 vias in that are look very suspect in the photo.

The via closest to R34 silkscreen looks like there may be a track problem?

The two vias closest to the CR8 silkscreen has some dried up battery juice?  Maybe take a stainless steel needle and poke through these two vias to clean them out?

I can't read a schematic to save my life, but it appears that CR8 and CR9 might be part of the mV/ohms path?

[retiredcaps]

A remote possibility, but you might want to reflow the solder on CR8?  Just in case the battery juice has some affected the readings in some way?

The soldering on CR8 looks dull in your photo, but it just could be lighting?

[retiredcaps]

I did some probing tonight and  I can get the offset down to about 5 ohms by bypassing some of the input circuitry - connecting directly downstream of R20.

Since this meter was abused, it only takes a minute to check the input protection.

Your PTC should measure around 1.1k ohm.  The MOVs, RV1, RV2 and RV3 should measure 0L resistance.  The fusible resistor should be around 1k ohm.

[retiredcaps]

One other question, what is the output current when the Fluke 17B is set to 400 ohm range?

For example, in 600 ohm manual range, my Fluke 87V outputs 1mA.  I verified with another meter.  Picture attached.  The probe is in the 300mA jack of the 77 III.  So 1.00 is 1.00mA, not 1.00A.

The 1.00mA reading matches what is in the 87V manual.  I cannot find the output current value in the 17B manual.  It does say something about diode test: "short circuit current is < 0.6 mA (typical)."

7.9 ohms are the shunt resistors value inside the 77 III.

[crashanium]

 Thanks for the suggestions. I soldered wire links in the vias round CR8 although they all checked out electrically ok. I also made sure there was no gunk left in any of the vias as well. No change.

After that I took it apart again and left it in an IPA bath overnight. It's currently drying out so I'll check the current when it's back together later today.

[retiredcaps]

After it dries, does it show 0 ohms in the following manual ranges? 400, 4k, 40k, 400k?

I'm just wondering if that ohm offset is in all ranges not just the 400 when you short the probes?

[crashanium]

Hello Retiredcaps,

I measured the input protection and it seems fine. RT1 measured 1.31k; R20 was 1.05k so in spec and the three RV's were all OL as you said.

Some more info. The dried out meter is no better than  before - a little worse in fact. The ohms readings with shorted probes are as follows:
400R - 28 ohms;
4k - 28 ohms
40k - 00.09k
400k - 0.009k

Things get more interesting when I measure the current in ohms mode. With the meter in 400R mode the indicated current on my old HP3465 is 0.127mA. interestingly the 17B then shows a reading of 128.3 ohms. In 4k mode it is again 0.127mA, but the display is now 0.126k. In 40k it is 0.0354mA and displays 00.19k and finally in 400k mode the readings are 0.0042mA and a display of 0.008k.  I think there is some leakage somewhere.

Shorted probes in VDC gives a reading of 0.004V and millivolt mode gives 003.9mV.

The more I clean it the worse it seems to get! A real puzzle as I have no idea where to look next. Let me know if you need more data.

[retiredcaps]

I measured the input protection and it seems fine. RT1 measured 1.31k; R20 was 1.05k so in spec and the three RV's were all OL as you said.

All the above readings suggest nothing wrong.

I think there is some leakage somewhere.

I have to agree.  The current reading for 400 ohm range seems really low to me.  Of course, the manual doesn't specify, but I'm guessing that is low?

Shorted probes in VDC gives a reading of 0.004V and millivolt mode gives 003.9mV.

Is the input impedance still 11.11M ohm for VDC and NOT 10.00M ohm for mVDC?

The more I clean it the worse it seems to get!

Try drying the pcb more with a pc fan and see if the ohms readings gets better.

For some reason, my brain is nagging me that I have seen this before somewhere, but I can't recall what fixed it or where the thread might be in eevblog.

I think it was a Fluke meter, but I can't recall the circumstances on how it was fixed regarding the ohms offset.

Let me sleep on it and see if I can remember in the next couple of days?

Another question/test.  Does diode work properly?

[retiredcaps]

1. Take the batteries out of the 17B. 

2. Turn rotary switch to ohms mode.  Using another meter, what is the input impedance?

3. Do the same for mV and diode.   Report all 3 readings.

Hopefully someone with another 17B will do the same and we can do comparison readings.

[crashanium]

Input impedance on VDC is 10.6/10.7 Mohms. With power off it is 14.9-15 meg.
On mVDC it is around 17 Mohm.  With power off it is 3.3-3.4 initially climbing slowly to 5 or so
Diode test mode works.
Power off in ohms mode is 2.8 climbing slowly to 3.1 Mohms.

Note that the input impedance readings are very meter dependent and also slightly polarity sensitive.

[neikalo]

IPA is not useful against alkaline battery leakage. I soaked a 17B PCB in a mild solution of vinegar and detergent followed by a bath of just distilled water. This seems to have fixed the ohms problem.

[Wallace Gasiewicz]

You said the fuses were blown. Perhaps the MOVS are damaged also.Maybe disconnect them and see.

[bdunham7]

Things get more interesting when I measure the current in ohms mode. With the meter in 400R mode the indicated current on my old HP3465 is 0.127mA. interestingly the 17B then shows a reading of 128.3 ohms. In 4k mode it is again 0.127mA, but the display is now 0.126k. In 40k it is 0.0354mA and displays 00.19k and finally in 400k mode the readings are 0.0042mA and a display of 0.008k.  I think there is some leakage somewhere.

The fact that your resistance reading goes up about 100R when going from a short to the HP3465 current shunt (which is probably 100R, I didn't check) and the 28 ohm offset remains would indicate that the resistance test current is roughly correct but there is a voltage offset somewhere.  If it is using 0.125mA as the test current, then 28 ohms corresponds to only 3.5mV.  If you've thoroughly cleaned everywhere and the offset is completely stable, the fix is probably recalibration.  But I wouldn't rush into that.

You might try taking the input protection diode clamps off the board temporarily to make sure they aren't leaking.