Specs of R21 in a Fluke 87 III - Page 1
Forum section: electronics / repair

author: giosif
author_rank: Frequent Contributor
timestamp: September 24, 2017, 11:37:58 pm
content: Hi,I am trying to restore a Fluke 87 III whose R21 has been eaten by corrosion (among other things).I tried measuring the resistance from one end to the other of the trimmer, but corrosion seems to have eaten some of the carbon part too (i.e. not only its metal bits).I've also looked at the only manual with schematics that I could find (i.e. that of the original 87 series) and R21 is listed as a .3W 100kOhm +/- 20% trimmer.However, for series III, the trimmer is different even from a physical format perspective (i.e. lower profile, ceramic base), so I am not sure R21 in this version is supposed to have the same specs.Is there someone that could confirm the specs for R21 on a Fluke 87 III for me, please?For reference, it is the trimmer at the top of the picture below, right next to the yellow cable coming from the battery.Thanks!

author: Toasty
author_rank: Regular Contributor
timestamp: September 25, 2017, 01:56:11 am
content: deleted

author: retiredcaps
author_rank: Super Contributor
timestamp: September 25, 2017, 05:20:10 am
content: Quote from: giosif on September 24, 2017, 11:37:58 pmMy guess is that both the I and III series use R21 to adjust VDC to 3.500 +/- 0.001 that they use the same value trimmer.If in doubt, for test purposes, why not just use a 100k ohm resistor in place of the trimmer and see if the meter works. You might find more corrosion problems, but at least you can proceed temporarily with a 100k ohm resistor in place?

author: giosif
author_rank: Frequent Contributor
timestamp: September 28, 2017, 09:15:15 pm
content: Quote from: Toasty on September 25, 2017, 01:56:11 amThanks, but that is the manual for the original 80 series, not the 80 III series.Could still be the same value of resistor, but I personally doubt it since the 80 III series has a different physical format for the trimmer.I mean, it could still be a 100kOhm trimmer, but not the same power rating.

author: giosif
author_rank: Frequent Contributor
timestamp: September 28, 2017, 09:21:23 pm
content: Quote from: retiredcaps on September 25, 2017, 05:20:10 amYes, R21 is to calibrate DC readings.Good point with soldering in a temporary 100k resistor.I've done that and initial testing indicates there are other issues with the meter.In the meantime, however, I put this troubleshooting on hold since I realized I could use the rotary selector contact "fingers" from this Fluke 87 III on my Fluke 89 IV which is missing its contacts (have tried various ways of obtaining "original" Fluke 89 IV contacts, but without success up to this point).The Fluke 89 IV seems to be working fine with these contacts, although they are different in physical format.

author: Robomeds
author_rank: Frequent Contributor
timestamp: September 29, 2017, 12:00:20 am
content: Quote from: giosif on September 28, 2017, 09:21:23 pmI was able to buy a 187 contact (same as the 89 IV) from a seller on ebay. I don't recall which but he was one of the ones selling other Fluke 189 parts. I just asked a couple of the sellers until one said yes.

author: giosif
author_rank: Frequent Contributor
timestamp: September 30, 2017, 10:22:31 am
content: Quote from: Robomeds on September 29, 2017, 12:00:20 amYes, I think I know who you are referring to.And, yes, I've already contacted them and they were supposed to get back to me, but that didn't happen (also sent a few follow-up messages, but no response).The person is usually quite responsive and helpful (I've used their service before), so they must be caught up with other things.

author: giosif
author_rank: Frequent Contributor
timestamp: September 30, 2017, 10:45:37 am
content: Quote from: giosif on September 28, 2017, 09:21:23 pmAnother update: in order to advance my troubleshooting of the Fluke 87 III, I used some thin wires and Kapton tape to temporarily bridge the contacts on the board where the rotary selector contact fingers would normally make contacts.After some going back and forth, I finally identified the source of the problem: a leads input socket, particularly the common/"ground" socket.It seems both metal half-tube bits inside the socket had their lower end part broken off from the rest of the metal body.By how clean the area around it looks, I think this problem happened due to mecanical stress rather than corrosion.In any case, I am now in the process of frankestein-ing the input jack receptacle using parts from another broken receptacle I have.Next step: get an SMD 100k trimmer to replace the temporary resistor and look to source some rotary selector contact fingers.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 30, 2017, 06:44:28 pm
content: Quote from: giosif on September 30, 2017, 10:45:37 amCommon problem. Seehttp://mrmodemhead.com/blog/fluke-80-series-jack-assembly-rebuild/

author: giosif
author_rank: Frequent Contributor
timestamp: September 30, 2017, 10:43:47 pm
content: Quote from: retiredcaps on September 30, 2017, 06:44:28 pmYes, I remembered that article while I was working on my input jack.