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Fluke 8840A bench multimeter teardown

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Following Kibi's teardown and repair of a Fluke 8840A multimeter (, here are a few pictures of mine. I had opened it up to investigate a seriously fading VFD display. Sadly, there is nothing electrically wrong (filament volts are fine at 4.5V AC, drive signals are fine at 0V/30V), so I conclude its just worn out. I might see if I can boost the filament volts a little.

This multimeter was made in 1986.

Picture descriptions:
1. The front panel. The VFD seems bright, but it isn't really. It has a tendency to fade up and down - a sharp tap seems to recover it (this is some mechanical effect, not a contact problem). [Edit: On further investigation, there was a dodgy joint on the filament output of the PCB-mounted mains transformer. Reflowing the pins fixed the fading up and down - it is still a bit dim though, but perfectly usable.]

2. The rear panel. This unit is fully optioned up with AC measurement board and the GPIB board.

3. Initial view - the GPIB add-on is on the left, the AC board on the right.

4. The main 5V rail electrolytic. My first reaction was 'uh-oh', as a distinct bulge is apparent. However, it seems that a domed plastic disc has been placed over the safety relief valve - you can push it down very easily into the void underneath it.

5. The three 40-pin DIP devices, from top to bottom, are
8279 Keyboard and display controller,
Z8 microcontoller (mask programmed with Fluke's code I think)
Custom Fluke Analogue to Digital converter

6 & 7. The GPIB option board. Again, the big electrolytic exhibits the scary 'false bulging top'.

8 & 9. The AC RMS option board.

10. Top view with both option boards removed. Of note, both front and rear inputs pass through an RF suppression toroid. To the left of the micro are ROM and SRAM ICs.  Upper middle are the VFD driver ICs (TTL to 30V level conversion), lower right is the front end circuitry.

11 & 12. More close-ups.

13. A close-up of one of the precision resistor networks. This one is three 33k resistors in series.

14. Another precision network. Some unusual values in this one - it is a series network of 5.00M, 5.007k, 45.52k, 505.3k and 19.54k.

15. Every rail and other significant reference voltage has it's own test point. Lovely! :)

Careful about the caps in the last picture, they look slightly bulgy

-Imagine the time taken to design this, BACK IN 19eighty-something!

Those caps are fine. It does have a few tants though - I'm more worried about those  ;)

The dipped caps are a lot more reliable than the SM ones in my experience. Only ones i have seen fail were due to overvoltage or reverse voltage, and that for long periods. I accidentally wired one in reverse one day, and it took around 2 minutes to blow up on a 5v rail.

I have three of these multimeters and they are definitely 'keepers'. Excellent accuracy and performance despite their age.

Your unit is 'fully loaded' (RMS + GPIB) so worth some effort to restore to a decent display brightness. Mine have nice bright displays indicating quite low usage. As you probably know VFD's do fade/dim with hours run and you can actually start to get a metallised film forming on the interior of the front face if they have very high hours on them. It would be worth searching ebay for a half decent unit that is faulty but has a good display. I have seen several come up over the years. Fluke will probably still sell you a display, but sadly the price could be unpleasant. They were willing to sell me a new handle when I asked last year but it was GBP40  :(



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