Fluke 8842A self-test 15 and 22 errors - Page 1
Forum section: electronics / testgear

author: zike
author_rank: Contributor
timestamp: March 30, 2025, 03:03:46 pm
content: I have a fully-optioned 8842A DMM with a perfect, bright VFD that boots fine and appears to be in cal on all ranges I can test *except* 20 Megohms (reads a standard 10Meg between 2% and 8% low, it varies).Internal self-test (hold down SRQ key for 3 seconds) *sometimes* fails Tests 15 and 22.These appear to implicate either the ohms current source or T&H amplifier gain range switching. The suggestion is a failed/failing proprietary Fluke 700013 analog switch IC.Before I dig in and see if I can harvest one of these from a donor unit, I wonder if anyone has experience debugging this type of fault? Any advice appreciated!Thanks,MikePS If there is a better place to post this please let me know. I found the Fluke DMM group on groups.io, but my attempt to post there hasn't been approved for weeks; indeed nothing new on that board January... Is that group dead?

author: AVGresponding
author_rank: Super Contributor
timestamp: April 01, 2025, 05:08:33 pm
content: If the fault is intermittent you might try looking for a dry joint around U303, and reseating any socketed ICs in the supporting circuitry. Have you measured TP302 yet? Also might be worth working the F/R terminal switch, they can get dirty and cause off-beam readings.

author: zike
author_rank: Contributor
timestamp: April 02, 2025, 07:07:35 pm
content: Quote from: AVGresponding on April 01, 2025, 05:08:33 pmThanks, this is useful advice. Did not open the case yet, I will inspect the board for corrosion or dry joints and then hook onto TP302 when I do.With intermittent faults, it feels like any intervention becomes a crapshoot between diagnosing "the" problem, and making new ones. Baby steps for me.

author: bdunham7
author_rank: Super Contributor
timestamp: April 02, 2025, 07:32:14 pm
content: There is an 8840A/AF and 8842A Facebook group. Or there's EEVBlog right here.I'll assume you have another decent meter and some sort of voltage source. Can you do four simple external tests?First, measure the current output in the 20M range. If you don't have a meter you trust to 0.5µA, then try measuring the voltage across a 100K resistor. Preferrably with a high-impedance meter, but 10M will only give you a 1% or so error.Next, if you can supply a fairly accurate 10mV and 50mV, measure 10mV with the 20mV range and 50mV with the 200mV range.Last, measure the input impedance with another DMM in each DCV range from 20mV to 1000V.After that you'll probably have to crack open both the case and the manual.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 03, 2025, 05:29:01 am
content: Quote from: zike on April 02, 2025, 07:07:35 pmYeah, if and when you do open the case, I recommend wearing disposable gloves, especially when handling high impedance stuff like the analogue circuitry

author: zike
author_rank: Contributor
timestamp: April 04, 2025, 08:52:05 pm
content: Thanks for the hints! Going on work travel for a few weeks, this interim post is partly to remember where I was at when I get back to the bench.Nothing in the house is accurate for sub-uA current, but I have a pile of known good 8840A and HP3478A and some 0.1% and 0.02% resistors.Very interesting, on 20 Meg range the 8842A reads 1.000 M accurately and spits out a solid 500 nA to accomplish this, but when measuring more than about 2 M the output current degrades and drifts around (always less than 500 nA). Thus it under-reports R, variably.--> Internal current source appears to have a voltage compliance problem above about half volt or so of burden (it is supposed to give 10V).Even more interesting, I see a similar behavior for high R's in the 2 M range (never happened to check this before); for example 1.500M reference resistor reads only 1.28M on the 2M range, and the meter is only putting out 4.25 uA (should be 5.00 uA). Here it appears to struggle with 6.3V of burden voltage.So I reckon it could be a bad supply rail to U404? or maybe leaky/damaged output protection Q402-407? OTOH it seems to work OK again on the 200 kohm range (10 uA test current).Anyway I have a good start, many thanks. Maybe not a bad 700013 after all.MikePS Almost forgot, I do have an Analogic AN3100 which I consider trustworthy to about ±100 uV (and agrees with several 8840A and 3478A units, though all came from dumpsters).10.0 mV input on the 20mV scale reads as 10.0145 mV, and 50 mV input on the 200 mV range gives reading of 50.000 mV. So mVDC seems to be within uncertainty of the calibrator. On higher DCV ranges, where the cal unit's offset is negligible, I pretty much get Fluke's specified accuracy.Also checked input Z on DCV as suggested; it's 10 megs for 20V, 200V and 1kV, and >30 megs (open) for 20 mV, 200mV and 2 V ranges. I believe this is correct ?Quote from: bdunham7 on April 02, 2025, 07:32:14 pm

author: bdunham7
author_rank: Super Contributor
timestamp: April 04, 2025, 09:13:01 pm
content: Quote from: zike on April 04, 2025, 08:52:05 pmActually it is not correct, the 20V range should also be high-impedance, <10G. This might lead you right to the problem. Can you take a meter with a 10M (or so) DCV impedance on the 10V range and set it for voltage, connect it to the 8842A in the 20M range and report what resistance the 8842A reads and what voltage the other meter reads.

author: zike
author_rank: Contributor
timestamp: April 04, 2025, 10:57:50 pm
content: Thanks you are correct, I evidently copied my notes wrong above! Very sorry.I'd just come to the same realization looking at the schematic and ran back down to the shop to recheck; saw your post on returning.On VDC I do indeed see high-Z looking into the input terminals for 20mV, 200mV, 2V, and 20V ranges whereas I correctly see 10 Megs only for 200V and 1000V (due to Z302 with K301 released).With 8842A set to measure 20 Megohms FS across the input of an HP3478A (10M nominal input impedance on DCV), the 8842A claims ~8.8 Megs (it drifts around) while the 3478A registers ~4.4 VDC.Thank you again for your help (and for keeping me honest ;^),MikeQuote from: bdunham7 on April 04, 2025, 09:13:01 pm

author: bdunham7
author_rank: Super Contributor
timestamp: April 04, 2025, 11:29:47 pm
content: Quote from: zike on April 04, 2025, 10:57:50 pmOK, so you definitely have not enough current there. The 8842A will have an OCV of >11V on the 2M and 20M ranges and about 6V on all the lower ranges. Obviously if you use a 10M meter, the voltage on the 20M range will measure at 5V, but if you shift down to the 2M range you should see "1" (overrange) on the 8842A and 11V+ on the other meter. If you don't see that, then there might be a compliance voltage issue and IDK exactly what the cause would be offhand. Can you read the OCV of the 2M and 20M ranges with another high-impedance meter? If they're both low and the same, I'll have to think about it. If they're both low but the 20M OCV is much lower, then you may have a leakage issue and I'd start by looking at Q405. Remove either Q405 or CR403 to test that theory.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 05, 2025, 08:14:03 am
content: Could be one of the analogue switches going bad. As it happens my 8840A has recently developed an issue with the 2k range, and everything points to U403D. I'd say it's possible that U402A might be going bad on yours. Naturally these are Fluke ASICs and unobtainium, but I have been pondering a replacement using DG200s and 74HC75s

author: zike
author_rank: Contributor
timestamp: April 05, 2025, 04:59:11 pm
content: For the moment I'm stumped, but I'm out of time for a while. Here's what I checked.- I think there's also a current deficit on the 200k and 2000k ranges, although the voltage range is less so the R inaccuracy is smaller (but still well out of spec).- With the measuring terminals fully open (very high Z voltmeter, e.g. 8840A) the output does indeed float up to 12.7VDC or so. It just won't supply enough current. So it doesn't seem to be a straightforward current leak (e.g., protection transistors Q402-404 or "diodes" Q405-7 going bad).- Current source U401/Q401 seems fine. Develops 2.5V across Z401 pins 3 and 4, independent of TP403 absolute voltage.- Shorting U402A or U403A does not change behavior (fails to develop 500nA across 10Meg in 20M resistance range). This suggests if it's one of these 700013's, it is more subtle than just failing to close.- Noticed that U404 pins 2 and 3 don't seem to be held at the same potential; they should always be. In desperation I swapped out U404 (happened to have some ancient LM308 in stock). Behavior was unchanged. This one's a head scratcher, very possible I am misinterpreting the feedback loop here.- Thinking it could be an open thickfilm in the U404 feedback loop, I checked sections of Z401; the low-R ones look spot-on. The hi R ones (pins 1-4, 5 Meg nom. and 7-6, 505k nom.) read low in-circuit, but would need to be desoldered to properly isolate.- Checked with a scope, nothing seems to be oscillating or otherwise unstable (other than drift).So, I'm stewing over what could rob current, including how a failing U402 or U403 could cause such behavior (even though it's unchanged with relevantsections shorted).There are two well-engineered 700013 subs I've seen online, one recent DIY with PCB artwork by TubeTime (Eric Schlaepfer) and another by a member here; the latter had some complete units and kits posted for sale on eBay. But neither is cheap/convenient/guaranteed pin-for-pin enough, particularly at very high Z, to make shotgunning these parts look sensible as a first line of attack. And my other two 8840A's actually work, so I'm reluctant to cannibalize, even though I really need an 8842A with GPIB right now.I will think more on the plane, hopefully a theory will emerge. Thanks all for your help; hope I can at least keep you entertained!Mike

author: bdunham7
author_rank: Super Contributor
timestamp: April 05, 2025, 05:20:55 pm
content: Quote from: zike on April 05, 2025, 04:59:11 pmHaving both of them high was't one of the options I had anticipated!If I was looking at it I might just take out CR403 anyway, partly because I've had a Q405 go bad before and partly because I'm thinking that if there was leakage of 300nA @ 12V then just maybe it would behave that way. Probably not, actually, but quick enough to check. Other than that, I suppose you might just put a 10M resistor on the terminals and select the 20M range and then start testing the points of the circuit backwards from the relay to see where the voltage drop is. If you can eliminate the entire ohms protection circuit from suspicion that will narrow down your options quite a bit.