Re-installing a TouchTone keypad in an ASR-33

Dave McGuire mcguire at
Mon May 18 15:17:48 CDT 2009

On May 18, 2009, at 1:32 PM, Chuck Guzis 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.

   Oh, how thoroughly bizarre.  Thanks for the explanation.  I'd love  
to see a SPICE simulation of that.


Dave McGuire
Port Charlotte, FL

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