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-   -   Fluke 87 display fix (https://www.vintage-radio.net/forum/showthread.php?t=183913)

6SN7WGTB 17th Sep 2021 4:45 pm

Fluke 87 display fix
 
I have had a very trusty Fluke 87 since 1990.

After about 10 years, the display started 'losing' segments - they were actually on but hard to see anymore.

I took it apart and found how the LCD display is 'connected' to the PCB by means of what must be some sort of conductive flexible strip - but simply held against the copper tracks of the PCB.

Initially, cleaning it and the PCB and reassembling solved the problem.

However, after 6-9 months it started playing up again.

Trying all manner of contact cleaners, conductive oils and so on did not work any better - the solution now seemed to last less and less time.

Meanwhile, removing the thing had not broken off one of the fairly brittle clips - meaning that it was held even less securely to the PCb.

The solution I found that as worked now for TEN years is as follows:
1. Having removed the LCD, clean the flexible conductive pad with a mild solvent (alcohol e.g.)
2. Ditto the board
3. Apply a VERY VERY thin film of silver loaded heat sink compound to the edges of the flexible 'conductive' strip. The right result is that you have wiped it gently with a vey clean cloth and there remains a visible silver film (well - looks more grey).
4. Reassemble.

It all sounds counter-intuitive applying something conductive to a strip which must have many small adjacent contacts, but it doesn't seem to 'short' them out

ajgriff 18th Sep 2021 8:13 am

Re: Fluke 87 display fix
 
Fading LCD segments are a common problem with older Fluke DMMs. I use a soft pencil rubber to clean the contacts (PCB, zebra strips and LCD) before wiping with IPA. I'm fairly sure that this method was recommended by Fluke at some point but can't now find the particular reference.

Another tip is to maintain the original orientation of the zebra strips as they tend to adopt the shape of the PCB solder pads over time.

Overall this approach seems to be effective for a period of many years.

Alan

Leon Crampin 18th Sep 2021 3:27 pm

Re: Fluke 87 display fix
 
Relevant to the post, but differing in application, I have a 30 year-old Drayton programmer controlling my boiler. It is fitted with an LCD display slightly larger than that used by Fluke, and developed the "missing segments" problem.

Dismantling, cleaning the tracks with a rubber (Drayton just left them solder coated...) and finishing off with IPA on a cotton bud resulted in a fix - but only a temporary one. After a couple of sessions, the segments still eventually faded.

The real fix was to file down the edge of the display holder by about 0.5 mm and add a corresponding shim to the other side of the pc board to maintain a tight fit. This has the effect of increasing the preload on the "zebra" strip connecting the pc to the display panel.

As with most elastomers, especially when heat cycled near a boiler, the strip had clearly relaxed and consequently there was inadequate contact pressure to give a reliable connection to all conductors.

Increasing the strip preload effected a lasting fix. I "rewarded" the controller by fitting a new set of electrolytics at the same time.

Roll on next Winter - if I can afford the gas.

Leon.

ajgriff 18th Sep 2021 4:43 pm

Re: Fluke 87 display fix
 
Quote:

Originally Posted by Leon Crampin (Post 1407717)
As with most elastomers, especially when heat cycled near a boiler, the strip had clearly relaxed and consequently there was inadequate contact pressure to give a reliable connection to all conductors.

Increasing the strip preload effected a lasting fix.

Yes there needs to be adequate pressure applied to the zebra strips and the broken clip may have been part of the problem in the OP's case. I'm more used to the earlier 70 series Flukes where the LCD holder is screwed to the PCB thus maintaining pressure as long as the mask remains undamaged.

Alan


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