Early digital voltmeter

[Tractorfan]

Hi,
Just thought I'd share a couple of pics (I know we all like photos) of an early digital voltmeter that I acquired many years ago. It's intended for panel mounting, and runs off 240 volts mains. The input is 20 volts DC, (well 19.99 volts according to the label), and it's interesting to see the amount of circuitry within compared to a modern pocket sized digital multimeter.
Cheers, Pete.

[G0HZU_JMR]

I have an old DataTech DMM that (I recall) looks quite similar inside. I don't know how old it is but it is probably >35 years old. See the image below. I bought this at a radio rally in a lucky dip box maybe 25years or so ago. It was easy to see inside the lucky dip box so I picked out two of these meters and made one working one from two faulty ones. I think it was two (broken) items for a pound.

It has two standout features. The first is that the 5V and 0.5V ranges have an ultra high input impedance. I removed the 5V from the meter over 5 minutes ago and the meter is still displaying 4.72V with no connections at the input. It seems to go down maybe 1mV every 2 seconds or so.

The other standout feature is that the Ohms function spits out something like 15V as the test voltage. So this function is best left alone for modern electronics work!

I never tried to 'properly' calibrate it back then and it's amazing to see it showing 5.005V when compared to a Keithley bench meter (showing exactly 5V with lots of zeroes). I used this old meter as a companion to my trusty Maplin Gold DMM for many years but I think this is the first time I've powered it up in ages. It still works!

[Refugee]

I got a good one out of two 1970s flukes.
They were expensive in their day.

[G0HZU_JMR]

To give an indication as to how high the input impedance is of the Datatech DMM (on the 5V range) I redid the 5V test via a 10Meg ohm series resistor. It still reads 5.005V although it takes several seconds to fully charge/settle at 5.005V on the display. So the input impedance must be insanely high on this range.

On the 50V range the meter reads 2.78V via the series 10Meg resistor so the input impedance must be fairly typical for a DMM on the 50V range. i.e. 10-12Meg ohm?

[TonyDuell]

Actually, to me all those are relatively modern digital voltmeters. The oldest ones that I have seen (but do not own -- yet) used a servo amplifier and potentiometer (rather like the circuit of a pen recorder) driving a mechanical turns counter.

As far as I know the oldest all-electronic DVM was the Blackburn Instruments BIE2114. All valves with trochotrons as the counters. I have attached a photo of the front panel and chassis

[The Philpott]

Perhaps the earliest (most primitive) are the US made ones which have 10 sheets of etched glass per digit stacked in a line, each with a downlighter filament bulb which fires thus lighting up the appropriate number.

I have some tiny numicator modules, each with an array of wheatear lamps in the rear, illuminating the front panel with a white digit, via a tiny lens and microfilm.

Dave

[TonyDuell]

Those edge-lit transparent sheet displays were used well into the IC era. I have a Trend datacomms test set that uses them for some display function and said instrument is packed with DTL ICs.

I also have a Dynamco discrete-transistor DVM (apart from some valves in the null detector IIRC) that uses them.

I beleive the BIE2114, the first all-electronic DVM (from the early 1950s) used nixie tubes for the display driven by trochotron counters.

[G0HZU_JMR]

I used some fairly old DMMs at my first job (at Marconi) and these had nixie tube displays. I suspect they were Solartron or Advance models but I can't really remember. I'm guessing they were made in the late 1960s or so. However, I suspect that there could also have been some older models in the darkest, dustiest corners of the test gear storage area as there was a lot of abandoned and unloved test gear there dating back to the 1950s.

Does anyone know how old my Data Tech Model 30A DMM is? It is probably a similar age to Tractorfan's meter as the PCB and components look to be similar vintage. It was tired and battered when I bought it in the early 1990s so I'm assuming it is from the 1970s. I can't be certain but I think the other Model 30A meter I got in the lucky dip had a Racal logo on it so maybe it was rebadged.

[Martin G7MRV]

Originally Posted by Tractorfan

Hi,
Just thought I'd share a couple of pics (I know we all like photos) of an early digital voltmeter that I acquired many years ago. It's intended for panel mounting, and runs off 240 volts mains. The input is 20 volts DC, (well 19.99 volts according to the label), and it's interesting to see the amount of circuitry within compared to a modern pocket sized digital multimeter.
Cheers, Pete.

Those exact same meters were used for the temperature readings of the auxiliary Met reporting station at Emley Moor. I would read them, and do a cloud observation and visibility check, every 3h, with precipitation measurements and readings from the min/max and wetbulb thermometers in the Stevenson screen every 12h.

I acquired the met station kit when we ceased reporting (after 48 years!) and later passed it on to a school somewhere down south

[Lloyd 1985]

I’ve got a nice old Fluke digital multimeter that uses Nixie tubes for the display,got it off eBay sometime in 2012. It works, but has an annoying problem where certain digits begin to glow across all 4 tubes making it difficult to read the display. Shame I haven’t got any photos of it all lit up on my phone, but there is a thread about the meter over on VRAT somewhere with photos of inside it and with it lit up.

Regards
Lloyd

[Julesomega]

My earliest DVM is an anologue electro-mechanical counter type, a Sapphire 851 dating probably from the late 50s. The earliest electronic/digital DMM is a Solartron, type LM 1420.2, a 3.5 digit cold-cathode numeral instrument which included a Weston cell for a calibration reference and just happened to include a counter/timer function. There is no date on my photocopy of the service manual so I would guess ca. 1964 because a later model, LM 1450 has a manual dated 1966

I can't claim any Digital Multimeters from this era, interesting to hear that there were any - there can be very few.