Fluke 8840a/af  4-wire resistance inoperative - Page 1
Forum section: electronics / repair

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 16, 2017, 12:52:19 am
content: I have an old (35 years) Fluke 8840a/af bench meter that I purchased from an E-bay Fluke seller. (funny that I'd know of any, ha ha)As far as I can check, everything works (voltage (AC and DC) and 2 wire resistance) except the 4 wire resistance mode. I can't believe it's much, but I've not had any luck finding a service manual.Can anyone give me a direction in which to start? A 1.5 ohm resistor (confirmed using Dave's "poor man's 4 wire technique of using two meters") reads .5 ohms.

author: alm
author_rank: Super Contributor
timestamp: July 16, 2017, 01:27:24 am
content: Did the current from the force terminals match the specification? It is not a particularly tight spec, but it should not be 66% low. What voltage do you measure across the sense terminals?This manualhas schematics. I think the /AF version was quite similar (some options default and maybe tighter specs?).If the values read too low only in 4W mode, then that suggests to me that there is a low resistance path between sense low and sense high. Maybe one of the protection JFETs (Q301/302) is leaky?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 16, 2017, 05:23:16 pm
content: Correction: The 2 wire reading is 1.5xxx (correct) and the 4 wire reading is 1.2yyy. The error does not scale. Of course, it's so small for most measurements in the higher ranges, it appears that the 4 wire readings are correct.I removed Q301, Q302 and clamp Q309, with no effect.I checked Q303, and even substituted another part, but there was no effect.I've got to be missing something. I just can't figure out what could be causing the 4 wire reading to be wrong without affecting the 2 wire mode.

author: alm
author_rank: Super Contributor
timestamp: July 16, 2017, 10:55:19 pm
content: How do the voltage and current compare between 2w and 4w mode? I do not have the schematic in front of me, but what about any switching (e.g. mux) that is only used for 4w?

author: RobK_NL
author_rank: Frequent Contributor
timestamp: July 17, 2017, 02:56:16 pm
content: Have you checked the front/rear switch and U301D?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 18, 2017, 03:46:29 pm
content: Quote from: alm on July 16, 2017, 10:55:19 pmI can't see anything different so far.Quote from: RobK_NL on July 17, 2017, 02:56:16 pmI have bypassed the front/rear switch with no effect.I'll be honest, I messed up (got turned around and or backwards) when checking voltages at U301 and noticed that when I connect pin 13 to ground via my 10 Megohm Fluke 87, the reading is correct. I'll be honest again and admit I have no idea what that could mean.I've checked the Jfet switching between Q310 and Q301 via U305 and can't find any problems.

author: alm
author_rank: Super Contributor
timestamp: July 18, 2017, 04:17:40 pm
content: Quote from: Excavatoree on July 18, 2017, 03:46:29 pmSo current source switching is fine, and the problem is somewhere at or downstream of S301.Quote from: Excavatoree on July 18, 2017, 03:46:29 pmMessing up is an excellent way of finding a way out if you are stuck. Too bad it is often associated with extra work and expenses. Pin 13 is one of the input nodes to the ADC. Does it do the same if you ground pin 17 or 18? If not, then it sounds like a bad solder connection or break in trace/via.According to the table on page 93, U301C and U301D are involved in switching between two and four-wire mode. An open U301D or shorted U301C might explain the symptoms. If grounding the S/H input fixes stuff, then my money is on an open/high-resistance U301D (pin 12/13), either a broken IC or connection. In 4W mode, you should measure close to 0 V between R314 (C306 side) and Q314 pin 3.

author: xwarp
author_rank: Frequent Contributor
timestamp: July 19, 2017, 07:30:32 am
content: If you find any bad chips, send me a message as I have a couple 8840 boards that I have salvaged parts from.