The AM Forum

So this thing is coming up on the list. It's a 1959 auto-ranging digital AC/DC voltmeter! It spent 40 years in someone's garage, was rescued from the curb, and has worked its way to the top of the stack here.

It is most interesting. This must have been a very earnest reach towards accuracy. The book says 0.1% or two digits on the 5 digit display.

A comparator checks the input against a standard. Depending on the size of the discrepancy it sends pulses to one of 5 stepping switches (one for each decade) connected across voltage dividers. On each switch there are contacts for each resistance increment and for each display digit. So the unit rapidly clacks the stepping switches to balance the system and in doing so it displays the correct voltage. I'm guessing that, in 1959, 7 stepping switches (one for +/- and one for range) were cheaper and used less power than a couple hundred #1 quality germanium transistors.

For reference voltage stability it uses a thermostatically controlled oven with a zener diode inside.

The 'bridge balancing' is done by phase comparator and chopper.

It'll be on the list of fix-it projects and all we can hope for is that the precision resistors in the comparator loop have not changed value much and that the chopper is still good and that someone with hands like a pig has not been in it over the 58 years since it was lovingly made by hand in San Diego.

If it can be made to work I'll have to post a video demonstration.

Digital living vicariously thru analog! :D

73DG

And the manuals are heavy  :D