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From: jbn@FORD-WDL1
Newsgroups: net.ham-radio
Subject: telephone innards
Message-ID: <11046@brl-tgr.ARPA>
Date: Sun, 26-May-85 13:12:39 EDT
Article-I.D.: brl-tgr.11046
Posted: Sun May 26 13:12:39 1985
Date-Received: Thu, 30-May-85 00:45:09 EDT
Sender: news@brl-tgr.ARPA
Lines: 13


     One of the less-known capabilities of the old WE 600 series telephone
instruments is that automatic compensation is provided for losses in the
local loop; a varisistor driven by the DC level as seen at the instrument is
used to adjust the signal level.  This is not an audio compression circuit;
it's the long-term DC voltage (``battery'' in telco terminology) as
seen at the instrument that does it.
     Since a varisistor is a non-linear component, this makes a telephone
an RF detector in theory and sometimes in fact, and there was a fix kit for
RFI available for the model 600 at one time; later models had it built-in.
     This gives a little more insight into what's inside that potted network.

						John Nagle