During one of my time travels in
the 1970's I brought home a Nixie-based multimeter: Schlumberger 1240.
Three and half digits in a compact desktop case. It is the same
instrument of Weston 1240, and the Heathkit IM-102 shares a lot with
them.
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| Schlumberger (Weston) 1240 multimeter from circa 1972. |
It was given away as working except for the 200mA scale.
Not a big deal if you want ot make a clock out of it, no? The first
power up confirmed both its working state and my suspicion that the half
digit neon bulb was broken. Just feed a variable voltage in the 20 V
scale and let it go beyond 9.99 V. Time to open it up, without a
manual/diagram/parts list to be found online.
Well, Weston
made also model 1242, a 4.5 digits multimeter that is aestetically
similar, and the manual is available online (not complete and some pages
were poorly scanned). At least it shows how to extract the circuit
board.
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| Top of the board with discrete logic. |
Two notches later, I had the two-layer through-hole
board on my desk. Meet another 1970's hand-drawn PCB, with charming
curvy traces and no ground plane! There are three Burroughs B5855S Nixies.
The "half digit" was a
25 mm tall neon lamp with an illuminated bar of about 15 mm and long
leads. Initially I suspected the driver transistor was gone but I begun
removing the lamp first: only two solder points to redo in case it works
rather than three short leads of the transistor. Well, one lamp leg broke in
the process and I couldn't lit it with my high voltage DC source.
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| Bottom of the board with curvy traces! |
Looks like it is not easy to source a neon bulb with this size in 2017, and temporarily a shorter one will do the job.
But something else happened ...
