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!