Fluke 87 - DC reading very slow - Page 1
Forum section: electronics / repair

author: chrsm
author_rank: Newbie
timestamp: November 20, 2019, 05:26:59 pm
content: 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 ?

author: Axtman
author_rank: Regular Contributor
timestamp: November 20, 2019, 07:19:55 pm
content: Which version? Fluke 87, 87 iii, 87 iv or 87 v?

author: chrsm
author_rank: Newbie
timestamp: November 21, 2019, 10:14:23 am
content: The first one Fluke 87 without any version

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 21, 2019, 05:59:14 pm
content: 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)

author: retiredcaps
author_rank: Super Contributor
timestamp: November 21, 2019, 08:39:40 pm
content: Quote from: chrsm on November 20, 2019, 05:26:59 pmShow a 7 second video of the above.What happens if you measure a 12V DC power supply instead of an AA cell?

author: chrsm
author_rank: Newbie
timestamp: November 21, 2019, 10:20:41 pm
content: With the Fluke set to DC reading it has a imput impedans of around 10MOhm

author: Electro Detective
author_rank: Super Contributor
timestamp: November 21, 2019, 10:58:30 pm
content: I go for the usual suspects first:Dirty sockets, crusty lead plugs,black smudge cross contamination on switch selector tracks,cooked input componentsor intermittent combo of the above

author: chrsm
author_rank: Newbie
timestamp: November 21, 2019, 11:08:20 pm
content: Quote from: retiredcaps on November 21, 2019, 08:39:40 pmI 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 highhttps://photos.app.goo.gl/SNW6oj6LpGMe2zfv6

author: The Soulman
author_rank: Super Contributor
timestamp: November 22, 2019, 12:37:29 am
content: Quote from: chrsm on November 21, 2019, 10:20:41 pmDo you have another meter so that you could measure the voltagesimultaneous 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 badcontact on the range switch.btw, How did you conclude ac voltage is fine, just stuck it an outlet?

author: retiredcaps
author_rank: Super Contributor
timestamp: November 22, 2019, 01:23:22 am
content: Quote from: chrsm on November 21, 2019, 11:08:20 pmThere 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 athttps://www.eevblog.com/forum/repair/fluke-87-i-repair/msg390462/#msg390462With 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, seehttp://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/

author: retiredcaps
author_rank: Super Contributor
timestamp: November 22, 2019, 01:24:50 am
content: Quote from: mzacharias on November 21, 2019, 05:59:14 pmAt 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.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 22, 2019, 01:55:08 am
content: Quote from: chrsm on November 20, 2019, 05:26:59 pmBTW, all the varistors and spark gap should measure 0L resistance.

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 22, 2019, 10:06:52 pm
content: Quote from: retiredcaps on November 22, 2019, 01:55:08 amWell, I already robbed the LCD from that one. Works fine in an original 83, by the way, just the digits are larger.

author: chrsm
author_rank: Newbie
timestamp: November 23, 2019, 02:07:53 pm
content: Quote from: The Soulman on November 22, 2019, 12:37:29 amYes 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.

author: chrsm
author_rank: Newbie
timestamp: November 23, 2019, 06:50:23 pm
content: Quote from: retiredcaps on November 22, 2019, 01:23:22 amI 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

author: chrsm
author_rank: Newbie
timestamp: November 23, 2019, 09:39:54 pm
content: Problem solvedIt 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 .

author: retiredcaps
author_rank: Super Contributor
timestamp: November 23, 2019, 09:51:56 pm
content: Glad you got it fixed.Is your input impedance now a solid 11.11M ohm for DCV and 10.00M ohm for mVDC?

author: chrsm
author_rank: Newbie
timestamp: November 23, 2019, 10:48:40 pm
content: Yes they are - thanks