Fluke 87 - DC reading very slow

[chrsm]

We have an old Fluke 87 which show DC reading very slow. If we e.g. measure a 1,5v battery then it takes about 5 sec. to stabilize the reading. AC reading works fine.
We have tried to remove the varistors VR1 and VR1 and the surge protector E1 without any change.
Any ideas ?

[Axtman]

Which version? Fluke 87, 87 iii, 87 iv or 87 v?

[chrsm]

The first one Fluke 87 without any version

[mzacharias]

Measure the 87's input impedance and confirm about 10-11 MegOhm.

I have an 83III that for some reason the input impedance is is around 14 MegOhm, and is slow to settle.

(Actually that 83III mainboard is retired for now for this reason)

[retiredcaps]

If we e.g. measure a 1,5v battery then it takes about 5 sec. to stabilize the reading.

Show a 7 second video of the above.

What happens if you measure a 12V DC power supply instead of an AA cell?

[chrsm]

With the Fluke set to DC reading it has a imput impedans of around 10MOhm

[Electro Detective]

I go for the usual suspects first:

Dirty sockets, crusty lead plugs,
black smudge cross contamination on switch selector tracks,
cooked input components
or intermittent combo of the above   

[chrsm]

If we e.g. measure a 1,5v battery then it takes about 5 sec. to stabilize the reading.

Show a 7 second video of the above.

What happens if you measure a 12V DC power supply instead of an AA cell?

I have uploaded 2 videos to google that show reading of an 1,5V AA cell and 12V power supply. The reading of the 1,5V AA cell is actually to high
https://photos.app.goo.gl/SNW6oj6LpGMe2zfv6

[The Soulman]

With the Fluke set to DC reading it has a imput impedans of around 10MOhm

Do you have another meter so that you could measure the voltage
simultaneous with the faulty fluke?
Just to make sure the fluke isn't affecting the source in anyway?

If only dc voltage is affected (and dc current is fine?) it might be as simple as a bad
contact on the range switch.

btw, How did you conclude ac voltage is fine, just stuck it an outlet?

[retiredcaps]

I have uploaded 2 videos to google that show reading of an 1,5V AA cell and 12V power supply.

There is definitely something wrong with your meter as shown by the video.  It should take the 87 less than 1.5 seconds to provide a stable reading.

Regarding the input impedance, see my pictures at

https://www.eevblog.com/forum/repair/fluke-87-i-repair/msg390462/#msg390462

With the 87 set to mV, input impedance should be exactly 10M ohm.  At DCV, it should read exactly 11.11M ohm.

If you already removed the varistors and spark gap, leave them out for now.  Check your 87 PTC.  It should be around 1.5k ohm.

With Electro Detective's suggestion, see

http://mrmodemhead.com/blog/all-posts/

for several Fluke related repairs that involved giving the entire pcb an IPA bath, cleaning rotary switch tracks, removing carbon buildup, etc.  Lots of good case studies and suggestions to try.

http://mrmodemhead.com/blog/ideal-carbon-footprint/

[retiredcaps]

I have an 83III that for some reason the input impedance is is around 14 MegOhm, and is slow to settle.

At Mark, I have a working 83 III if you would like to make some comparison readings.  If yes, start a new thread and I will try to help you pinpoint what is wrong with your 83 III pcb.

[retiredcaps]

We have tried to remove the varistors VR1 and VR1 and the surge protector E1 without any change.

BTW, all the varistors and spark gap should measure 0L resistance.

[mzacharias]

We have tried to remove the varistors VR1 and VR1 and the surge protector E1 without any change.

BTW, all the varistors and spark gap should measure 0L resistance.

Well, I already robbed the LCD from that one. Works fine in an original 83, by the way, just the digits are larger.

[chrsm]

Do you have another meter so that you could measure the voltage
simultaneous with the faulty fluke?

btw, How did you conclude ac voltage is fine, just stuck it an outlet?

Yes I have another Fluke 83 III that I can use for messurement.
I have used a tone generator and set it to 5V AC, and that works fine.

[chrsm]

With the 87 set to mV, input impedance should be exactly 10M ohm.  At DCV, it should read exactly 11.11M ohm.

If you already removed the varistors and spark gap, leave them out for now.  Check your 87 PTC.  It should be around 1.5k ohm.

I have made 2 videos. One with DC reading, the impedance is round 10,3Mohm raising to 10,6Mohm. And another video with mV reading where it is more stable around 10,11M ohm.

About the PTC it is very strange. When I measured the first time it started to show 1,25k ohm and dropped slowly to 1,174k ohm. When I read it a second time it is stabile around 1,174k ohm. The PTC are unmounted from the PBC.

https://photos.app.goo.gl/wMYbye3VsYL41L8C7

[chrsm]

Problem solved

It was some "carbon dust" on the PBC, near the Z1, that caused the problem. After mechanical removing the "dust" the reading is stabilize within 1 sec.
Thanks to everyone and specially retiredcaps that mention the carbon problem .

[retiredcaps]

Glad you got it fixed.

Is your input impedance now a solid 11.11M ohm for DCV and 10.00M ohm for mVDC?

[chrsm]

Yes they are - thanks