Instrument Displays MDPM-3 nixie DPM (was Re: Service Manual/Schematics for an IBM 7861-015 modem?)

Ethan Dicks ethan.dicks at
Wed Feb 14 15:02:46 CST 2007

On 2/12/07, Ethan Dicks <ethan.dicks at> wrote:
> On 2/11/07, Brent Hilpert <hilpert at> wrote:
> > Ethan Dicks wrote:
> > > Had a good day at the Mansfield Hamfest today.  I now have [a]...
> > > nixie-tube panel DVM...
> >   ^^^^^^^^^^^^^^^^^^^^
> >  If that is a Weston 1294-series DVM (4.5 digits, TTL, fits standard 4"*1.75"
> > industrial instr. panel), I can supply the schematic.
> It _is_ 4.5 digits... and does fit a standard panel opening, but
> it's an MDPM-3 by Instrument Displays, Inc., of Haverhill, Massachusetts.

OK... I've taken it apart to have a look... the IC dates are late 1968
- there are a few DIPs and a number of 8-pin epoxy-covered
button-shaped ICs all over the innards.  The DPM came with a 36-pin
AMP connector with 4 wires loaded, two for power and two for input
voltage.  The power wires go right into a transformer at the back of
the unit, leading me to suspect that it takes 110VAC, but perhaps it
takes 24VAC.  I have not yet powered it up - does anyone have any idea
what it would take to fire it up?

The nixie tubes, for those who track such things, are 3 Burroughs
B-5750 7/8"(?) nixies, with a block next to them for "1", "+", and "-"
(3 individual neons).  The legend on the front proclaims it to be a
"mini-diget dpm".

I've never fiddled with a pre-LED DPM, so if anyone knows what voltage
it's expecting, I can fire it up to see if it works or is merely a
source of nixies.



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