Fluke 70 Series LCD Blotch Dilemma
 

Bought a Fluke 75 multimeter at a boot sale last weekend. Came with original polystyrene box, instruction manual and safety test leads. The low price asked reflected the fact that the meter was dead and had a blotch in the middle of the LCD. I was happy to buy it just for the test leads and had no realistic expectations for the meter itself. To my surprise (utter amazement actually), after a few hours work the meter is now fully functional and operating, as far as I can tell, within spec.

The dilemma relates to the LCD blotch. Although it doesn't seriously impair usability, as the digit segments are still relatively visible under the blotch, I'd rather like to get rid of it if I can. Please see attached photo which was taken before any digits were available to view. The blotch is quite colourful and casts shadows on the surface below. It is an integral part of the upper layer and so doesn't wipe off.

I've never messed around with LCD panels like this before so have spent some time researching the possibilities and it does seem that the blemish might be confined to the polarising film stuck to the surface of the panel. For a couple of pounds I can acquire some suitable polarisng film which would give me enough material for some practice runs. The risk is that the damage may not be limited to the film layer and any attempts to affect a repair might turn the LCD to scrap.

Replacement LCDs cost more to import from the US than used meters generally sell for in the UK. Bearing in mind that the meter effectively came free with the test leads but is quite usable I can't make up mind whether or not to try to banish the blotch. Any thoughts, opinions, ideas or advice which might help me make a decision would be very much appreciated.

By the way, I currently use an AVO 8 MkV and a cheap far eastern DMM. The DMM is tolerable but the Fluke 75 would be a welcome upgrade.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

Nice buy. The Fluke is well worth fixing, they knock cheap DMM's into the proverbial cocked hat. It is a bit of a dilema, to fix or not too fix? Have you tried dismantling the LCD? Not sure about the Fluke LCD's but some LCD's come with a removable glass on top with a polarising film on. Maybe a bit of gentle heating might help, hairdryer type thing.

Andy.

Re: Fluke 70 Series LCD Blotch Dilemma
 

I think I'd continue to use it as it is, while keeping an eye out for another, faulty, example with good display. I know that Flukes suffer from deteriorating displays, but good LCDs attached to faulty meters do turn up cheaply sometimes.

Re: Fluke 70 Series LCD Blotch Dilemma
 

I'd be inclined to do the same - use it as long as it's legible - once it's gone too far, try the repair. At that point you have nothing to lose.

In the mean time set up the appropriate eBay & Gumtree searches for either a faulty Fluke to salvage a replacement display from, or for the relevant LCD itself, whatever the Fluke (or generic ?) part number is, and just let them run. I've left searches running anything up to a couple of years, but often found the appropriate part I needed, for minimal effort.

Re: Fluke 70 Series LCD Blotch Dilemma
 

I'm with Goldie99on this. If it isn't spreading i would leave it alone until it's actually an intolerable nuisance- or until a spare surfaces- whichever happens first.

I find on average that a repair goes much better when there is a replacement waiting in the wings 'just in case'. Perhaps it makes my hands steadier knowing i'm not about to trash something.

Dave

Re: Fluke 70 Series LCD Blotch Dilemma
 

I have an LCD for the 75 somewhere, bought and never used in anger. If you have the patience to bear with me I will have a look for it but at this moment I have absolutely no idea just where I might have 'carefully' stored it!

It originally cost £38 from Holland but I'm sure I can do a bargain for you - that is if I can find it :-)

Cheers,
Steve.

Re: Fluke 70 Series LCD Blotch Dilemma
 

That's a surprise Steve and thank you very much for the kind offer. There's no rush to find the 'safe place' as I'm still at the research stage and will get in touch in due couse if the need arises.

Thanks as well to those who have contributed thus far. Further advice would be most welcome. In particular, I wonder if anyone has any theories about how the blotch came into being?

Would also be pleased to hear from someone who has attempted polarising film replacement on this or any similar LCD.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

Not sure how it might have started - maybe a splash of something too hot ? splatter of flux ? solder would probably have been too hot.

Try the following video - the polarising film looks pretty straightforward.

https://www.youtube.com/watch?v=39Gpu1umxhA

Re: Fluke 70 Series LCD Blotch Dilemma
 

Yes, have watched that one and he really makes quite a hash of it although he does get there in the end.

I'm coming to the view that changing the filter is worthwhile. Just a question of whether or not the top surface of the LCD itself is damaged.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

Plucked up courage to peel off the self-adhesive polarising filter only to discover that the panel itself is the cause of the blotch. Still no evidence to suggest how the damage happened in the first place. On a positive note I've managed not to cause further harm although I will need to replace the filter (film now on order).

For fun I'm attaching a couple photos showing the main reason for the meter's non-working status when I bought it. Actually think that the battery (zinc-carbon?) is the original, dating from circa 1986. The second shot shows the PCB after spending an hour or so carefully scraping away the crud left behind by the corroded battery. I've since improved things futher.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

I watched with interest the video as recommended by Goldie99 and thought could this fault be viewed using a pair of Polaroid sun glasses as a cheap check before buying the expensive polarised sheet. Not sure just how much the sheet actually cost. I remember using Polaroid sun glasses in the past and when they were moved around things became clear, can't remember the reason. Just a thought. Ted

Re: Fluke 70 Series LCD Blotch Dilemma
 

As an aside, for anyone who may not be aware, that video is by forum member Graham, 'Radiocruncher', who has his own website and youtube channel.

Re: Fluke 70 Series LCD Blotch Dilemma
 

It's true that the faded display fault described in Graham's (Radiocruncher) video could have been diagnosed with polaroid glasses or similar providing of course he'd been aware of the potential problem. Clearly Graham wasn't until someone who had watched the preceeding video contacted him. The glory of hindsight.

The reason for my 'hash' comment relates to the fact that in the video the filter is initially fitted upside down ie, sticky side up. This is then corrected but it's not clear exactly how this is achieved. Works in the end but not really ideal as a demo. Sorry Graham.

Taken together the two videos were very useful to me as they demonstated that it was possible to replace the filter although in my case the fault is the blotch and not a faded display. I'm grateful to Graham for making the videos at all and for the effort involved.

As to the cost of polarising sheet, the self-adhesive variety seems quite difficult to source economically in small quantities. However, for this application plain sheet is fine as it can be trapped between the display shroud and the panel itself. Got mine earlier and it cost £1.99 for two 6cm x 10cm sheets which is enough for four Fluke LCDs. They are intended for repairing the screens of certain models of iPhone.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

Would it be worth salvaging some filter sheet from a broken TV?

Re: Fluke 70 Series LCD Blotch Dilemma
 

Definitely. You never know when it might come in handy and at least it wouldn't take up much storage space!

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

Ha! Just like the thousand and one other things which might come in...!

Re: Fluke 70 Series LCD Blotch Dilemma
 

Alan, i think the board mounted PP3 terminals from a late 20th century smoke alarm could replace the rotted ones you have there. Would be much preferable to a floating connector such as those from the memory circuit of a clock radio.

Dave

Re: Fluke 70 Series LCD Blotch Dilemma
 

What a good idea Dave. I have fitted a flying battery clip for testing purposes but I think I've got a long defunct smoke alarm with suitable terminals in a junk box somewhere. Just a case of finding it as ever. Thank you.

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

To round off this saga I feel the need to confess my sins to those who, like me, suffer from occasional bouts of 'perfection syndrome'.

Having established that the blotch is a flaw in the LCD panel rather than a problem with the polarising filter I made the foolish error of seeking perfection. Having previously done a fairly thorough job of removing the battery contamination using various of the usual concoctions and after much careful scraping the meter didn't always zero quite as it should. In particular, it consistently read 0.2 Ohms with the test leads shorted. Technically this was just within spec but seemed intrinsically unsatisfactory. I therefore decided to undertake a more thorough cleansing operation concluding with baking the PCB in the oven on a low heat in order to drive out all traces of moisture. Didn't think it would do any harm to adopt a comprehensive approach.

To do all of this it was necessary to remove the LCD assembly and since I'd already had this apart a couple of times it also seemed sensible to clean the contacts at the same time. This was a big mistake as when I attempted to remove the LCD mask/shroud again it snapped into two pieces. They weren't even clean breaks so there was no chance of using superglue. Luckily there was nobody else around to hear the stream of expletives.

New masks seem to be unobtainable now so I was forced to enter Heath Robinson mode. The result is shown in the attached photograph. The tricky part was arranging some gentle downward pressure on the panel to ensure good electrical contact with the elastomer connectors. This was achieved with thin strips of anti-slip drawer matting fixed to the panel with double sided tape, The necessary pressure on the flexible rubber strips is generated when the case screws are tightened.

Despite my clumsiness the meter now performs really well. A thorough comparison with the AVO 8 (which I trust) across all ranges revealed no perceptible differences. I also used the Fluke to set up the output conditions for a Roberts R505 after replacing a faulty AC188. I then checked the output on the scope and obtained a perfectly symmetric sine wave with no sign of cossover distortion, further confirming the meter's accuracy.

By the way, apart from the battery corrosion the only other functional issue that needed attention was a leaky (and leaking) tantalum electrolytic which caused low readings on the DC current ranges. Eagle eyed readers may have spotted evidence of the fault in the out-of-focus area of the second photo attached to post #10. Well done if you did because I didn't at the time of taking the shot.

I have to say that the meter is an absolute pleasure to use and I now understand why Flukes became the meters of choice for so many professionals and affluent amateurs alike. Just leaves the 'blotch' to gnaw away at the perfectionist side of my personality!

Alan

Re: Fluke 70 Series LCD Blotch Dilemma
 

I had a similar attack of 'why isn't it working perfectly' on an avominor based low resistance ohmmeter. Repeatability of reading and repeatability of zeroing was not as it should be and it turned out to be a combination of 3 factors, bad continuity from the adaptor block used to make the AA cell fit, a little bit of static from the acrylic i used as a replacement for the cracked glass, and finally the movement showing it's age.

I replaced these parts, using a substitute movement scavenged from a contemporary avominor which i had confidently (and wrongly) calculated to be of the same sensitivity....All had to come apart again as the movement is unique to this particular meter and somewhat more sensitive than the 2mA standard article. More like 1 or 1.25mA...

Eventually... i got away with it. The glass seemed to improve the repeatability of readings so i shan't be using plastic again for this purpose! Twice i had to walk away from the job to avoid damaging anything through rushing..