ESI 300 PVB meter/bridge detector

[trobbins]

A friend's 300 PVB had a non-operational plug-in detector module, which he carefully decapitated to then inspect the internal sealed circuitry.  I picked up the 300 PVB a few weeks ago to do some comparison testing of its standard 0.1 to 1M decade precision 0.005% resistors against my HP 34401A, as each reference resistor allows 4-point connection through front panel terminals.  Strangely, the detector decided to work again on the day I visited, so perhaps the issue was a poor contact with the 14-pin interface connector, or a chopper relay contact recovered, or the circuitry thought it was better to continue working again than risk more invasive actions.

Anyway, I decided to reverse engineer the detector module circuitry as I couldn't identify any schematic or detailed description on-line.  The schematic may have some errors, and its layout may be a bit unconventional as I couldn't identify any existing circuit layout to align it to.  The chopper relay part had no markings.

I also couldn't identify any detailed design descriptions to elaborate on the circuits operation, apart from some early simplistic chopper discussion in transistor manuals and app notes, and 'The Contact Modulator' series by Airpax - so would appreciate any operational insight or similar circuitry references.

Attached is schematic, photo of detector internals, and an example measurement scheme of the versatile instrument which was advertised in Electronics Design 1966 by 'For $750 it does the work of 9 devices'.

Ciao, Tim

[trobbins]

I've been able to identify a closely related detector circuit schematic in ESI's 250DE maintenance manual, which aligns with the 300 PVB detector schematic - so nice to see.

As an aside, the mid-1960's 300 PVB reference resistors appeared to be still all within their 0.005% tolerance, based on an un-cal HP 34401A, but supported by transfer measurements on a 10R and 10k reference resistors with a Fluke 8588A and similar test temperature, and accounting for connection offset with the 10R.