Fluke 8024B won't zero

[SingedFingers]

Just started looking at the Fluke 8024B I picked up the other day for a reasonable £10. Came with a thermocouple which was the primary interest. The display is knackered but readable just about. I have a spare one in stock from another dead one. Fuse hack from hell discussed elsewhere so ignore that here

So, on power up it won't zero. It sat at -.455 volts on any DC volts range.

Did the usual things to kill off any dielectric effects around high Z areas i.e. clean the boards with IPA, wash with distilled and, dry throughly, reseat the Intersil ASIC. This has improved it to -.270 but alas no further so I can't cal DC volts. If I can't solve this I will stash it for parts. It is drifting a couple of mV every ten seconds towards zero.

Any ideas where to look next?

State of affairs. I did all measurements in the case with shielding attached for ref.

[retiredcaps]

So, on power up it won't zero. It sat at -.455 volts on any DC volts range.

Check your electrolytic caps especially the bottom bung.  It won't be obvious until you remove them as per modemhead's blog.

http://mrmodemhead.com/blog/gallery/fluke-8024b/

Multiple IPA baths might also be required.  I bought an used/abused Fluke 179 that showed -0.02 ohms with probes shorted.  I had to clean the pcb with IPA twice before it showed +0.02 ohms.

[retiredcaps]

PS. Modemhead says this is the fastest Fluke handheld multimeter in terms of continuity speed in his unpublished unofficial tests.  Faster than the Fluke 87V.

He keeps it solely for continuity checking.  So even if your display is bad, you might use it for the same purposes in locating shorts?

He also says the beeper is nice and loud.

[SingedFingers]

Thanks will check those caps right now. Not 100% sure I can be arsed with the IPA baths however!

I've got an 8021B with continuity and it is absolutely amazing. Like nothing else I've used. Bought some Pomona 5019A probes for it and it is simply a perfect meter.

[SingedFingers]

Bingo! Bastard has pissed out electrolyte and eaten the pad. Time to clean it up. Green legs:



Thanks for your pointer. Annoyingly I don't had any minature/low profile caps in stock

[SingedFingers]

Found some Wun Hung Lo capacitors in the junk box and installed them and we're at -.087 volts and falling at ~0.5mVsec. Wondering if it will settle at zero and there's some crud somewhere I missed. Will leave it for 30 mins and come back

I've managed to eliminate the main board as the thing does zero properly if you pop all the range switches. So this is the input or attenuator and hold section that needs investigating.

That's it for tonight though. It's 00:18 here and my eyes have given out so snack and bed

[SingedFingers]

This is looking hopeful...



Going to bed now. Boo hiss.

[retiredcaps]

Bingo! Bastard has pissed out electrolyte and eaten the pad.

In almost 85% of these older 8000 series, the caps leak from the bottom bung and can't be spotted by visual inspection.  About 50% of the people who ask for help are skeptical about the suggestion that the caps have leaked.

Modemhead and I have seen enough of these and virtually helped enough people to know the caps are always suspect.

[SingedFingers]

Indeed you're spot on. Thanks again.

It has settled at -16.2mV now which is still bad enough to be useless as a DVM on low ranges. On higher ranges it makes no difference really as the error is tiny. I'm going to scrub the attenuator board again today when I get a few minutes. There are a couple of high impedance points on it I can influence the zero reading with by touching them very easily so this makes me suspect there is some crap under the switches somewhere.

I popped the spare display I had in this morning quickly but it turned out to be knackered as well so I've ordered a similar display from RS and will do a Kynar job on Monday night when it arrives.

Also trying to find a new HRC fuse for it that isn't in a 10 pack here. Don't mind paying for one but min 10 pack is a bit much (£73).

[ModemHead]

Been offline a few days and missed all the fun...

Sounds like you're on the right track.  Just thought I would remind you to check the  ribbon cable.  I pulled my hair out looking for current leaks on one of these and eventually discovered adjacent conductor leakage in that cable.  Flipped the cable around and the problem was gone.

[SingedFingers]

Cool. Thanks for the tip. I will try that this evening. I hadn't considered the ribbon cable. I assume it is just a pressure fit with the plastic clamps?

Also thanks for detailing your experience with these on your web site. I probably wouldn't have these Fluke 802x meters if it wasn't for the helpful repair info on your site

It's floating at around -9mv offset this morning which is greatly improved since yesterday. On the higher ranges this isn't even visible.

[ModemHead]

Yes, just a pressure fit with little squishy tubes to even it out.

Note that usually the current leakage that causes these non-zero readings is like a very high-impedance voltage source.  Once you connect something to the input jacks, the external source will dominate.  So what we casually refer to as an "offset" is not necessarily a hard offset on the measurement.  Depends on where the leak is as to how much influence it has.

Glad you find my repair ramblings useful.  And yes I do keep a very nice 8021B on my bench just for tracing continuity.  You can't beat the analog comparator for that job. 

[SingedFingers]

Well I took the cable off which was nice and easy actually. Thanks for the instructions. And regrettably I found some more capacitor spooge that had seeped under the edge of the connector. I thought this was the end of it. The solder mask is pretty much gone around where the cap leaked:



I carefully cleaned this with IPA and a fibreglass pencil and it came out absolutely fine fortunately.

Now to the ribbon. This was a mess:



However the conductors are actually intact. So it got a coating of IPA, scrub with a paint brush (winsor and newton sable brush stolen from the wife temporarily - shhh!!!) and then lightly wiped with the fibreglass pencil to get rid of the remaining oxidation and it is as good as new. Phew.

Cleaned the squish tubes (which were covered in crap), reassembled everything and it zeroes perfectly!

Quick cal check against the big gun meter and we're in business:



Now ParcelForce has eaten my new display coming from RS so I'm going to have to wait a couple of days while they ship another one out. Next steps: clean the casing thoroughly and replacing the display.

A big thanks to everyone for the help so far - much appreciated!

[ModemHead]

Nice job.  Have some TEA and celebrate.  Good luck with the LCD replacement.

[SingedFingers]

RS are being dicks about the consignment with the display in so I've ordered another display from these guys for a reasonable £4: https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=172_182&products_id=2354&zenid=lnt4gmk4a21fra42olueq39ca0

Mechanically it's the same as the other display but half the cost which is a nice surprise.

The 8024B has an up and down arrow segment above and below the negative sign which I I'm leaving unwired obviously as an exact equivalent display is unobtainium so this is inevitable anyway.

[Gyro]

Doh, I've been rather enjoying the Comic Sans display, 7 segment can be a bit stark. 

[SingedFingers]

I love Comic Sans. It pisses off our graphic designers when I do a presentation in it

[SingedFingers]

Back again. Spent 2 hours with some fairy liquid, a paintbrush, a roll of kitchen paper, a scouring pad, a hair dryer and a pointy stick. It's clean. Still awaiting new display. ETA tomorrow.

Before (as advertised):



After! Looks good as new now!



Expenditure so far in total on the whole thing including the meter, new HRC fuse and display £20.50. Fuse cost £7.30 of that!

Edit: thermocouple that came with it works now as well! Needs calibrating. Trying to work out what I can do with a DIY ice bath in my tea flask in that space

[SingedFingers]

More fun. Got the display finally!

I got stuck on a 2 hour long conference call about ISO certification standards and business process modelling so out came the hands free set and I set about with the whittling and bending on the old display support while pretending to sound interested. Unlike ModemHead's version, this one was a little more difficult as it had thick plastic rips from hell on the back. This was modified with merely some crappy old Clarke side cutters and an X-Acto knife. Not the prettiest job but this all I have to work with. I have bent the pins round to clamp the display in place and have Kapton'ed underneath them ready for soldering (no wire wrap tool here!). Plan is to use oodles of Kynar and shrink the termination once complete.

Finished product...







On to soldering tonight, after a beer and the kids have gone to bed 

Have treated myself to some nice new Pomona B series banana leads as a reward for persistence

[SingedFingers]

Future reference for anyone else coming across this.

Display pinout of old display:



Display pinout of new display:

[SingedFingers]

kynar'ed and shrunk. Not the neatest job in the world as I'm half asleep but it'll do. Will connect to the main board tomorrow.

[ModemHead]

Looking good so far.  Especially the clean-up job!  Word of caution based on interesting tales that I've heard: make sure the LCD glass sits nice and flat on the support with no bumps or wobbles.  If there are any stress points on the LCD it can crack when put under a little pressure.

[SingedFingers]

It was a pretty tight fit and slightly scary I agree. I carefully whittled the plastic until it sat flush however so the thick bit of plastic isn't bent. The main risk was the two pins at each end on the bottom which had to be very carefully bent. Didn't fancy soldering close to the display itself if I broke one

I did blow my wife's heat gun up unfortunately so had to shrink a few of them with the soldering iron. Fortunately not a firestick or it'd be molten ick everywhere.

[SingedFingers]

And we're done. It didn't take too long to attach to the board and reassemble. In fact I got so into it I forgot to do any photos.

A big thanks to ModemHead for the help and the inspiration.

The only negative thing being I now have two more with bust displays on the eBay watch list

[bitseeker]

Great job, SingedFingers. Really looks like new.