Fluke 87 Series I calibration issue - Page 1
Forum section: electronics / repair

author: Axtman
author_rank: Regular Contributor
timestamp: May 17, 2019, 09:42:09 pm
content: I bought a cheap broken Fluke Series I (1986 Revision H) multimeter and am attempting to repair it. I found that the fusible 1k resistor was open so I replaced it with another 1k fusible resistor. The meter appears to be working but the readings are off. I downloaded the Fluke 83, 85, 87 Series Multimeters Service Manual and followed the procedure for calibrating volts ac and volts dc. The problem is that I cannot get the readings low enough and the PCB pots don't appear to vary the reading much. For example when I put a 3.5 volt source into the meter I get a reading of about 5 volts.Any advice would be greatly appreciated.

author: Axtman
author_rank: Regular Contributor
timestamp: May 18, 2019, 10:56:33 pm
content: The service manual is located here:http://assets.fluke.com/manuals/83_85_87smeng0500.pdf

author: retiredcaps
author_rank: Super Contributor
timestamp: May 20, 2019, 02:55:43 am
content: An open fusible resistor suggests that more parts of the input protection could be bad. Check the MOVs and PTC.A good MOV should read "0L" in resistance. A good PTC should read around 1K ohms. Both measurements can be done in circuit.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 20, 2019, 02:56:42 am
content: Quote from: Axtman on May 17, 2019, 09:42:09 pmNo calibration will ever trim it from 5V down to 3.5V.

author: floobydust
author_rank: Super Contributor
timestamp: May 20, 2019, 04:54:14 am
content: For voltage readings to be too high - that is worrisome.If the 1k fusible resistor popped, the 1.5k PTC might be damaged, or the 909k resistor for mV and V. I'd guess it was overloaded on ohms or diode-test.The diode/transistor clamps are switched-in for ohms and diode test functions.On DCV the Fluke 87 is relying on the 909k input resistor - so leaky clamp diodes/MOV/transistors etc. I don't think can make it read higher?I would check pin 2 AGND compared to input COM(-), the analog DMM IC has a few grounds. I would check the 1.235V reference near pins 1,3.

author: Axtman
author_rank: Regular Contributor
timestamp: May 21, 2019, 02:45:50 am
content: The 1.5k PTC (RT1) measures 1.24k.The 909k input resistor (R2) measures 918k.The two MOVs (RV1 and RV2) measure OL. Interesting enough one of the MOVs was not soldered to the board. It looks like it was a factory mistake. I soldered the MOV back in.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 21, 2019, 03:00:51 am
content: I forgot to mention to check the spark gap. It should also read 0L in resistance mode.

author: Axtman
author_rank: Regular Contributor
timestamp: May 21, 2019, 03:05:38 am
content: Spark gap measures OL.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 21, 2019, 03:05:54 am
content: Put the Fluke 87 into DCV mode.Using another meter, what is the input impedance of the 87? It should be around 11.11 M ohm.

author: Axtman
author_rank: Regular Contributor
timestamp: May 21, 2019, 03:42:11 am
content: I get 10.28 M ohms.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 21, 2019, 03:56:12 am
content: It should be 11.11 M ohm. Seehttps://www.eevblog.com/forum/repair/fluke-87-i-repair/msg390462/#msg390462I suggest a complete IPA bath of the entire PCB. Remove the lcd. Let it dry completely and recheck the input impedance.

author: Axtman
author_rank: Regular Contributor
timestamp: May 21, 2019, 04:24:16 am
content: The pcb is very clean as is the rotary switch. There is no evidence of carbon, solder splatter, or other contaminants.

author: Axtman
author_rank: Regular Contributor
timestamp: May 21, 2019, 04:26:17 am
content: I checked the Pin 2 AGND with respect to COM (-) and got a reading of -1.728 v. Floobydust said it should be 1.235 v.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 21, 2019, 07:11:13 pm
content: What about all the other readings in table 2-1? Please report them all.