Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.BERKELEY.EDU
Path: utzoo!decvax!decwrl!ucbvax!telecom
From: telecom@ucbvax.UUCP
Newsgroups: mod.telecom
Subject: Re: Three Questions
Message-ID: <8512221556.AA29714@RDCF.SDC.UUCP>
Date: Sun, 22-Dec-85 10:56:55 EST
Article-I.D.: RDCF.8512221556.AA29714
Posted: Sun Dec 22 10:56:55 1985
Date-Received: Mon, 23-Dec-85 23:01:40 EST
References: <8512121712.AA25461@aero.ARPA>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: System Development Corporation R&D, Santa Monica
Lines: 47
Approved: telecom@mit-xx.arpa

In article <8512121712.AA25461@aero.ARPA> kremen@aerospace writes:
>My first question relates to a problem I am having with my telephone.  I
>am able to make local and long-distance calls fine. When I call, I can
>hear the touch tones in the receiver. But as soon as my call is
>connected, any touch tones I press are muted. I just hear a "click."
>...when a call was connected in most cases, the polarity of the line
>reverses. According to this guru, my phone is an ATT Touch Tone and is
>unable to switch polarities on step central offices. He indicated two
>choices: buy a polarity suppressor at around $70.00 or get a new phone.
>My first question is can I make a polarity suppressor myself?
>ARPA: kremen@aerospace

The transistor (or IC) circuitry in every phone needs DC of the right
polarity to function.  Older ATT phones "ensure" the right polarity by
having a single diode in series with the electronics and wiring the
telephone office and your telephone jack to a consistent polarity.  Most
other telephones use a rectifier bridge to make them polarity independent.
When ATT originally did this, they probably saved $3 per phone ($1 per
diode), but now diodes are a nickel each.  It's certainly possible to modify
the network, but it's too hard to perscribe details.  The general idea is:
Old:
  |-------------|
--| tone circuit|--->|---
 +|-------------|-
New:
--|---------------|------
  |AC  Bridge   AC|
  |   +     -     |
  |---------------|
      |     |
     +|    -|
  |--------------|
  | tone circuit |
  |--------------|

There are a couple of possible locations for the bridge: the easiest to find
may actually be between the hook switch and the entire non-bell circuitry
rather than isolated to tone circuit.
-- 
Darrel J. Van Buer, PhD
System Development Corp.
2525 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
                                                            !sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA