Re-installing a TouchTone keypad in an ASR-33

[Chuck Guzis]

On 18 May 2009 at 12:28, Dave McGuire wrote:

>    I'm not a telephony geek, but this comment prompted me to go look 
> at that schematic.  That's a really neat little circuit.  What are 
> the four odd components?  The symbols sorta look like DIACs.

Varistors.  Telco stuff was usually silicon-carbide rather than metal 
oxide.  Although they were used for circuit protection, and amplitude-
limiting (look inside the receiver of any 2500 series desk set), they 
were also used for circuit stabilization, which is, I think, the 
application in the dial circuit.

--Chuck