Multimeter repair time again! Fluke 177, reads -.8 ohms

[Robomeds]

So I have a new repair project.  It's a Fluke 177 meter.  I've tested the AC and DC voltage and all is good.  However, when I short the leads the meter reads -0.8 ohms.  It seems to work well in higher ranges.  When compared to my BK meter they agree to withing one count on the 177.  However, they are clearly off by a lot at the very low range.  I don't think the 177 was miscalibrated but I don't think it has a calibration count indicator (I could be wrong).  The MOVs and diodes look good but I was wondering if one had been previously removed and perhaps put on backwards.  Then again I don't think those affect resistance measurements. 

Anyway, any suggestions? 

[retiredcaps]

Sometime in 2012, I bought a "working" 179 and when I received it, it read -0.2ohms when shorted. 

The previous owner was also a heavy smoker as the 179 reeked with smoke.  I did the usual standard cleaning and IPA bath (twice) and the meter finally read 0.1 to 0.2 ohms.

Modemhead has listed some products he used

http://mrmodemhead.com/blog/dirty-multimeter-clean-up/

but I used gojo for the yellow parts and standard IPA for the pcb.  Make sure to clean the input jacks with a qtip and IPA.  Unfortunately, I was never able to get rid of the smoke smell.

I have noticed that the 17x series with firmware revision 1.x doesn't bootup saying F17x.  With firmware 2.x, it boots up saying F17x where F17x denotes the model number.

edit: fixed some typos.

[Robomeds]

That would probably make sense.  This one was listed as not working and appeared to have some chemical spilled on it.  I'm do another round of cleaning and see what happens. 

[Robomeds]

Well I've cleaned the board with IPA.  No luck.  I checked the calibration and the unit shows only one calibration.  Any other suggestions?

Thanks

[retiredcaps]

Q1. Did you check the PTC?  It should be around 1k ohm.

Q2. What is the DCV being sent when the 177 is in 600 ohm range mode?  Is it a stable reading?

Q3. What is the current being sent when the 177 is in 600 ohm range mode?  Is it a stable reading?

Q4. Check the fusible resistor. It should be around 1k ohm.

Q5. Check the MOVs. They should all read open circuit.

[Robomeds]

Thanks,  I will check those things today or perhaps Monday. 

[Robomeds]

Q1. Did you check the PTC?  It should be around 1k ohm.

Q2. What is the DCV being sent when the 177 is in 600 ohm range mode?  Is it a stable reading?

Q3. What is the current being sent when the 177 is in 600 ohm range mode?  Is it a stable reading?

Q4. Check the fusible resistor. It should be around 1k ohm.

Q5. Check the MOVs. They should all read open circuit.

OK, I've finally had a chance to check a few things.
0.  The device was only calibrated once thus we have the factory calibration (I don't think that should surprise anyone).
1.  The PTC is 1.051k 
2.  DC voltage is 1.255V, stable
3.  DC A is 312.4 uA DC, stable
4.  1.060k
5.  All three are over range. 

It seems the under range is only in this 600 ohm range.  When I test say a 1k resistor with this meter vs a known good meter they are spot on.  Only in this 600 ohm range do I see the issue.  I'm starting to wonder if I don't have some small corrosion related failure. 


Thoughts?

[retiredcaps]

I'll have to haul out my 175 to see if those readings are the same ballpark or not.

I'm in the middle of preparing taxes, switching from XP to some other OS and getting ready for a road trip coming up, so I won't be able to check for a few days.   

[Robomeds]

Thanks!  I will be interested to hear what you find.  I'm still wondering about your contamination comment.  I did clean the PCB (twice) with IPA and that seemed to have ensured reliable operation but hasn't fixed the error in resistance.  The small error that goes away as resistance goes up (it still reads lower than my reference meter but once I'm off the 600 ohm scale the difference shifts from a consistent .8 to something like one count lower.  Seems like just a bit of current leaking somewhere.

[Refrigerator]

Isn't there any callibration pot to adjust ?

[retiredcaps]

Isn't there any callibration pot to adjust ?

None.  All Fluke 17x series are closed case calibration. The offset for the 600 ohms range is stored somewhere in a chip.

[Refrigerator]

Isn't there any callibration pot to adjust ?

None.  All Fluke 17x series are closed case calibration. The offset for the 600 ohms range is stored somewhere in a chip.

Damn, maybe there is an ISP port on it somewhere ?
Anyone know how to fix a multimeter if it's reading DC voltage 2x bigger than it really is ?

[retiredcaps]

Damn, maybe there is an ISP port on it somewhere ?

You need a lot of calibration gear to calibrate a closed case multimeter.  In most cases, you cannot just calibrate one range and quit otherwise the calibration is invalid.

Anyone know how to fix a multimeter if it's reading DC voltage 2x bigger than it really is ?

Start a new thread on your specific problem and we can take it from there.

[Robomeds]

I got some new info.  I was messing with it and noticed a few things (most have since slipped my mind) but the big one was the current reading.  Initially I thought the current readings were fine because by default the meter is in AC range and the AC ranges were fine... or at least 0 which is what you expect when you have no current applied.  When I checked DC the meter shows -.5A in the mA range but 0 in the 10A and 0 in the uA range.  That's at least something.  Too bad Fluke doesn't release schematics anymore.  Time to figure out what is in a current reading range. 

[retiredcaps]

Too bad Fluke doesn't release schematics anymore.

I'm still learning how to read schematics, but I find the text based theory of operation information more valuable at this point of my knowledge level.

While there are no public schematics of the 170 series, the original Fluke 80 service manual does a good job of describing how the ohm measurement is done on the 400 ohm range.  See page 2-3, section 2-8 of

http://assets.fluke.com/manuals/83_85_87smeng0500.pdf

In reading section 2-8, I suggest checking

i) switch/rotary switch contacts - give them a good IPA clean
ii) give the main IC a good examination using a 10x loupe to see if you can see any contamination
iii) maybe take a dental pick or sharp probe and drag it in between the main IC or ICs to possibly dislodge something that an IPA flush cannot?
iv) check the thick film resistor for dirt/contamination and measure the value especially the 1.0001k resistor
v) check the solder joints on the COM and V jack - I remember modemhead had some issues with this

PS. What revision board is your Fluke 177?  I know they have a rev 12 and 15.  The two are completely different internally.

[retiredcaps]

2.  DC voltage is 1.255V, stable
3.  DC A is 312.4 uA DC, stable

Sorry, after a very long 2 months of stuff, I forgot about this thread and probably a lot others as well. 

Anyway, using my BK5360, the Fluke 175 outputs 1.2331V and 303.1 mA when set to 600 ohm manual range.

edit: This is the only 17x I have access to nowadays.

[mzacharias]

2.  DC voltage is 1.255V, stable
3.  DC A is 312.4 uA DC, stable

Sorry, after a very long 2 months of stuff, I forgot about this thread and probably a lot others as well. 

Anyway, using my BK5360, the Fluke 175 outputs 1.2331V and 303.1 mA when set to 600 ohm manual range.

edit: This is the only 17x I have access to nowadays.

I'm thinking your Fluke should probably source 1 mA, not the 300 or so uA (yeah - I think you had a typo there).

Something in there may still be open circuit... especially if the output current is also low on the diode test function.

[Robomeds]

Hey all,
Thanks for the replies.  I did find something else that may be of some help.  With no input on the DC mA current mode the meter reads ~ -.54mA. I might have messed up what I posted earlier because the meter defaults to AC amps, not DC like I'm used to. 

Anyway, what I've been wondering is if the DC reference for the low range might have been damaged.  It would explain problems with both resistance and DC amps at the same time.  A big fear would be a damaged trace on one of the middle layers of the PCB (this is a 4 layer board).  However, I haven't seen any visible evidence of such a problem.  Would I?

Thanks all!

[retiredcaps]

I'm thinking your Fluke should probably source 1 mA, not the 300 or so uA (yeah - I think you had a typo there).

The Fluke 175 I have is rev 12 board.  When I turn it on, it doesn't display the F175 at startup like the newer ones.  The current manual for the 175 indicates the short circuit is less than 1.1mA.

However, since my 175 is more akin to a Fluke 77 III design, it sources 300uA (as documented in the 77 III manual).

[retiredcaps]

Re-continued from

https://www.eevblog.com/forum/testgear/fluke-187-reads-2-ohms-high/

For the reference of others, the 177 reads something like -0.9 ohm when shorted (auto ranging) and in the DC mA range reads a -530 mA current and, I just checked, draws 40mA from it's battery when on.  The board looks good so I've been at a loss as to what is wrong.  Sadly, I think it might be a short of some sort between the layers of the board but that is just a wide guess.  I simply can't find any visible damage.

Depending on which pcb version of the 170 series you have, the documented battery life could be 200, 300 or 400 hours as per Fluke manuals.  Assuming a 9V battery is roughly 500mAh, that would imply a 2.5mA, 1.67mA or 1.25mA draw respectively.

Does your 177 draw 0mA when it is off?  See

http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=264761

Using 40mA when the meter is on suggests something is really wrong.  What does it draw if you turn on the backlight?

[Robomeds]

I think it was drawing ~2mA when off (I didn't have time to really verify I was getting a legit measurement).  It does appear to draw current when off.  Time to pull out the bench power supply to test things. 

I think this is one of the earlier PCBs.  It has the older style switch rotary switch vs the traces on the PCB like we see on the newer Fluke meters.  I might get a chance to do some more work tonight.

Edit:
Similar to this one:

[retiredcaps]

I think it was drawing ~2mA when off (I didn't have time to really verify I was getting a legit measurement).

If it draws 2mA when it is off, that implies a fresh battery will be dead in about 11 days (500/2 = 250 hours, 250/24 = roughly 10.4 days). This is just like the Mike C. meter in the antique radio forum thread.

I might have some ideas later on, but I have some research to do first.

For the ultrasonic bath, did you dunk the entire 177 into it so that it cleaned the rotary switch as well?

[Robomeds]

I put the stripped PBC in.  So everything that could be removed easily was off but the switch went into the bath.  The grease on the switch was mostly washed away. 

[Robomeds]

OK, I think I'm at the bottom of this.  The Fluke 642998 ASIC is bad.  So with just the bare PCB I connected power and measured almost 40mA of current at 9V.  The backlight ads 10mA to that.  Well anything that's drawing that much current might just get warm.  The 642998 was getting just a bit warm to the touch.  Warm enough that I have no doubt that it is the bad part.  I'm not sure if I can source a replacement.

[retiredcaps]

I'm not sure if I can source a replacement.

I recently opened an aliexpress account and got my order this week.  I haven't checked my order yet to confirm the mosfets are good, but this seller sells a replacement.

http://www.aliexpress.com/store/product/642998-FLUKE-QFP/130723_1861821775.html

It is $50.76 USD for five + free shipping.

Send an email to the seller and ask if you can buy just one (1).  Offer $15 USD + free shipping and the seller might do it?

I have no financial affiliation with the seller or aliexpress.