Fluke 26-3 current leakage? - Page 1
Forum section: electronics / testgear

author: windsmurf
author_rank: Frequent Contributor
timestamp: April 07, 2019, 08:33:47 am
content: Hello! I have a Fluke 26-3 which works but behaves a bit strange.The symptom:When the meter has been shut down for over 30 minutes or so, and I turn it on with no leads connected, by moving the dial to the Ohms reading, it reads about a megohm and the reading goes up gradually until it reaches O.L., which can take from 10 seconds to 15 seconds (haven't actually measured but feels that way).When I turn it off or move it to another position, then back to ohms, its instantly gives O.L. reading, as it should.Checking the 26-3 with my 87V, here is what I find:1. In the off position, the meter is drawing 4.35 microamps constantly from the 9V battery.2. After being in the off position for a long time, w/battery installed, it develops a voltage potential between the input terminals COM and V, of up to a 4 volts, which seems to "discharge" when measured by my Fluke 87V in this case, gradually dropping negative exponentially, as if a capacitor is being discharged.The readings are being affected by a phantom voltage somewhere for up to like 15 seconds, but only on startup after a long shutdown.Otherwise, except during this "warm up" like phase, the meter appears to be accurate. This isn't normal is it?Are there particular components I can check that might cause this behavior? I already checked the input protection circuit but it seems to be ok. Could a MOV be acting like a capacitor? I don't know enough about the meter to understand if it should be receiving any voltage from the battery, while in the OFF dial position...Any and all comments or suggestions would be highly appreciated!

author: coromonadalix
author_rank: Super Contributor
timestamp: April 07, 2019, 09:55:58 am
content: Is your pcb clean, no corrosion, no humidity etc ... i did not find any schematics, just a calibration procedurehttps://elektrotanya.com/fluke_79_26_series_iii_calibration.pdf/download.htmlhttp://assets.fluke.com/manuals/7926____cieng0200.pdfThe input protection device should be checked, fuses too

author: windsmurf
author_rank: Frequent Contributor
timestamp: April 07, 2019, 11:09:39 am
content: Quote from: coromonadalix on April 07, 2019, 09:55:58 amBoard looks clean with no visible signs of corrosion. I did notice dark sticky fluid on the rotary contacts. Is this old dielectric grease? Should there be any lubrication on these old rotarys? Would it be best to do a thorough cleaning of the board, maybe with an ultrasonic cleaner?Are these symptoms I am getting a common occurrence with dirty or humidity-affected circuit boards?

author: coromonadalix
author_rank: Super Contributor
timestamp: April 07, 2019, 11:32:19 am
content: well a good clean up could help on the dialUse some anti static wipe, or cotton swab on the dial pcb traces, and before cleaning with ipa or anything else do a recheck ?My flukes 189 dont have any lube on the pcb dial traces, just on the mecanical dial plastic, non petroleum basedHad problems with Fluke 83, stranges beeps, wrong ohms values, cleaned the dials and problems gone, no lube on them tooI found an Brake cleaner who doesn't damage the plastics (done some tests prior use)

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 07, 2019, 12:45:21 pm
content: The tiny current draw when off is normal. Probably a pull-up on the FET that turns the power on and off. I have a 79-III here and I measure a 6.50uA draw when it is switched off.Quote from: windsmurf on April 07, 2019, 11:09:39 amYes! You get R-C behavior when you combine a high-impedance A/D input, parasitic capacitances and leakage current, unsurprisingly.I usually clean boards by immersing them in isopropyl alcohol and brushing, after removing the LCD assembly. Sometimes I have to repeat the procedure several times. I've never tried using an ultrasonic cleaner though.I prefer to leave switches dry after cleaning. Grease attracts contaminants. It seems to me that the concept of lubrication and electrical contact are antithetical. The grease has to be displaced for contact, so the best it can do is help to prevent oxidation.

author: windsmurf
author_rank: Frequent Contributor
timestamp: April 07, 2019, 07:38:25 pm
content: Thanks Coromonadalix and ModemHead!