16 December 2017

A missing trace in the Schlumberger 1240

Alright. I've gone back and forth the diagram, read the comprehensive manual, looked for possible leaking capacitors both on paper and on the circuit. Nothing.

Then I checked the "modern" 0.1 ohm resistor on the lower side of the PCB fitted in place of a broken trace. One side is grounded. So I followed ground traces to nearby components suspecting a ground loop but this is what I saw:

Almost 5 mm of missing (ground) trace! And it is on the biasing network around the dual-slope integrator. The result was an impedance mismatch causing stray currents and disturbing the integrator.

A piece of resistor leg was promptly used to rebuild the exposed trace and I anxiously powered up the multimeter (still without the AC converter board). Numbers were shown, didn't flicker or runaway but did not make sense. OPS! I forgot to press one of the "mode" buttons. So I got a stable, not null reading.

I shorted V/ohm input and turned the zero calibration pot as described in the manual. I had moved it around when troubleshooting. VoilĂ ! The Weston Schlumberger 1240 multimeter is with me again!

I have no idea how I managed to pull away that piece of track. Perhaps with time it has "glued" to a floating cable coming from the front panel, or .. well, it's fixed now!

02 December 2017

New instrument in the lab: Schlumberger 1240 multimeter

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.

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.

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.

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 ...