Fluke 8840A/AF repair issues - Page 1
Forum section: electronics / projects

author: Slh
author_rank: Regular Contributor
timestamp: November 25, 2023, 05:31:37 pm
content: I'm trying to fix my 8840A/AF. It's got a couple of intermittent faults. They are common to all measurement options so it appears to be on the main board. I've removed the RMS and GPIB board as they seem to just get in the way.1) Generally when it turns on, the unit struggles to choose a range. It keeps flicking between them. If I select a range or press <RATE> then it will select one and give a reading. All of the ranges show incorrect measurements. 200mV quite often shows overrange and 20V and 1000V often shows something close to the right answer but with around 400-600mV offset.2) Occasionally when it turns on, 200mV, 2V and 200V show the correct value but the 20V and 1000V have significant offsets. This makes sense as the 20V and 1000V ranges appear to have similar configurations apart the input divider so if one is wrong then the other is wrong,Has anyone ever seen these faults before? I'd like to get to at least only having one issue (fault 2) as I think I can actually debug that one with the manual.My suspicion at the moment is that something is not perfect on start up and perhaps there's a calibration routine or a latch that is often not set properly. Maybe something to do with the power sequencing. But sometimes I get lucky and it works.Things done so far:1) All the voltages look good. They all appear to be in spec.2) lots of the solder joints have been retouched. When the fault first occurred I only saw fault 1). After a significant amount of resoldering, fault 2 turned up.3) I am slowly going through some of the trouble shooting in the manual (https://assets.fluke.com/manuals/8842a___imeng0300.pdf). Given that I have two different errors depending on how it starts makes this a lot harder to work on.4) I have desoldered and measured all of the electrolytic caps. They appear to be ok (capacitance and ESR). I haven't replaced the electrolytic caps (mostly because I don't have any the right size at the moment).

author: bdunham7
author_rank: Super Contributor
timestamp: November 25, 2023, 05:51:57 pm
content: Have you run the self-test? Do that several times and see if you get consistent results.

author: Slh
author_rank: Regular Contributor
timestamp: November 25, 2023, 06:36:50 pm
content: Good point.Self test throws up errors for lots of the tests. The errors are consistent for each of the faults but they are different between them.

author: Slh
author_rank: Regular Contributor
timestamp: February 04, 2024, 04:08:28 pm
content: It's fixed!I noticed that when the second fault occurred the 7.5V rail was being pulled down to around 6.5V. It's just got a simple zener regulator so one of the chips was drawing a few mA too much. This suggested that one of the unobtainable analogue switches (Fluke 700013 U301/302/303) was broken.The chap who sold me back in 2021 noticed that I was having trouble with it and offered to send me one.The dodgy chip was U302 (tried U301 first). All appears to be working now.Many thanks @giosif for sending the replacement chip!Note for people who are interested in this sort of thing. the old one was made in 1986 and the new in 1987. The old one has larger print on it. See attached photo.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 04, 2024, 05:41:49 pm
content: AFAIK there are some replacement circuits available for the Fluke chips. This are small PCBs with some 3 chips to provide the same function with standard parts.

author: Zman
author_rank: Contributor
timestamp: January 08, 2025, 03:20:38 am
content: Quote from: Slh on February 04, 2024, 04:08:28 pmIf you still have it, would you be able to send me the _faulty_ IC? Postage paid! :-)I am doing some testing with these chips, just wanted to play with diagnostics of faulty ones...

author: Slh
author_rank: Regular Contributor
timestamp: January 08, 2025, 05:59:51 am
content: I'll have a look. If I did keep it then I probably put it in a safe place that I can't remember any more.