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Bell Telephone DTMF Dial and Phone Schematics Compiled, HTML'd and Maintained by Mike Morris WA6ILQ

The 35-type dials are TOUCH-TONE® dials having twelve pushbuttons. Ten are used in number-letter calling and two are used to give "special service" connections (Mother Bell's terminology).

The 66-type are the 16-button AUTOVON dials... they had a 4th column.

Below is a schematic of a 35C3A 12-button dial (anybody have one for a 66-series dial?). The original transistor is a Western-Electric packaged and labeled 2N404 (PNP germanium 24v 100ma). Each numbered pushbutton actuates two different sets of contacts that make coil tap closures for 2 of the 7 output frequencies of the dial. Contacts 1-1 through 1-4 select the row tone, contacts 2-1 through 2-3 select the column tone.
When any button is pressed the "common" switch is activated. I remember one piece of Bell literature referred to it as the "any button down" switch.
The common switch has four sets of contacts:
a) Set y-z un-shorts R3, a 5.1K resistor that is in series with the earpiece. This resistor reduces the DTMF pad sidetone volume to the earpiece.
b) Set e-v opens the microphone path to prevent any sound in the room from being mixed with the DTMF audio.
c) Set w-x applies voltage to the transistorized audio oscillator.
d) Set k-u removes the DC path from the tank coil, which starts the audio oscillator.

If you have one of these and want to use it as a stand-alone DTMF generator, this article from Ham Radio Magazine March 1973 may be useful.

A simpler connection, courtesy of the 1986 ARRL Handbook, can be found in this diagram.

The schematic below shows how the dial pad connects into a telephone set (a Western Electric 2500D Telephone Set from Bell System Practices, Plant Series, Section 502-510-450, Issue 1, June 1967, AT&TCo Standard).

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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This page created Aug 2006

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.