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

[Brent Hilpert]

Ethan Dicks wrote:
> > 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

The epoxy-button (with ceramic base) ICs sound like Fairchild's TO-5
alternative to the metal can TO-5/TO-99 package. They produced (at least) some
SSI RTL and Counting MicroLogic ICs in that package.

Considering the date and application, (many of) the ICs are likely Fairchild
Counter MicroLogic. This was a small family specifically directed at decade
counter/display strings. The technology can be described as MSI RTL, although
the 960 was sometimes used with TTL.

  9958 or 958: decade counter        TO-5 or 14-pin DIP
  9959 or 959: 4-bit latch           16-pin DIP
  9960 or 960: nixie decoder/driver  16-pin DIP
  9989 or 989: mod-16 counter        TO-5 or 14-pin DIP

 Supply voltage is 3.3V to 5.5V.

Are there only date codes and no part numbers on the ICs?


> 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?

..could fire it up at 24VAC and see if the logic supply level makes
any sense, then go for 110 if not, although my guess would be 110.

FWIW, the Weston Nixie DPM I have is 110VAC.


> 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.