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ajgriff · 17th Aug 2018, 10:14 am

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

17th Aug 2018, 10:14 am	#19
ajgriff Nonode Join Date: May 2004 Location: Halifax, West Yorkshire, UK. Posts: 2,587	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 Attached Thumbnails Last edited by ajgriff; 17th Aug 2018 at 10:23 am.