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Fluke 8840A bench multimeter teardown

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reagle:
Thanks for the pictures!
That's what I assumed when I was trying to get mine out- they just looked like they "grew" into each other with no dividing line :)


--- Quote from: Aurora on August 03, 2012, 06:29:21 pm ---You pull the 'toggle' top.

Its attached to a center pin that holds the three retaining spurs open so when you pull the toggle the spurs can move inwards.

Warning, when I removed mine for an inspection I found the plastic to be very brittle and some of the spurs broke off.

I'll take a picture of a broken one and upload it.

Aurora

--- End quote ---

reagle:
I finally got around to check why my 8840A has a display that comes and goes.
( Just posted a brief writeup  on my blog http://kuzyatech.com/fading-display-repair-on-fluke-8840)

It turned out to be the same exact problem as what you had. Bad solder joints of the mains transformer making intermittent contacts with the filament traces. They actually looked soldered until heated with an iron. Then you can clearly see oxide film preventing solder from sticking.
In my case the unit may have been dropped, since the corner board support bracket was broken, and the board was flexing a lot. This unit also has both GPIB and Option 9 AC boards, so I am keeping it with its new rock stable but dim display.
Has anybody come up with a way to wake these screen up like the used to do with CRTs? As in a brief capacitive discharge to "shake" some stuff off the filaments? Or alternatively, do we want to make an LED or LCD  based replacement front board? That may be fun ;)



SeanB:
The problem is not the filament ( it is uncoated so will not lose emission much with time, it is bare tungsten wire) but the actual anodes. Try getting the anode voltage up to the max allowed, probably the supply is at the low end of the range Check for ripple on the supply there.. If you want to liven up the filament run it from a power supply ( disconnect the one transformer lead) and take it up to a dull red ( view in subdued light and while looking at the display direct with no filter in place) for a half hour to warm it up and redistribute the surface. Dull red only, bright red or white will destroy it in seconds.

reagle:
Oh, good point! "Though shall check voltages" as Dave says :)

nukie:
How long is the memory rentention of the chips in the 8840? I have one and I have very little faith in it due to old age.

tapatalk

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