Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: What's in a Line Test Set?
Summary: Not very complex...
Message-ID: <3707@kitty.UUCP>
Date: 11 Apr 90 03:54:01 GMT
References: <1723@argus.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 44

In article <1723@argus.UUCP>, ken@argus.UUCP (Kenneth Ng) writes:
> Hello everyone, the equipment budget for the department I work in has
> been slashed to zero, and our line test set got stolen recently.  
> I've been wondering if anyone knows what justifies the $200-300
> cost of such a beast.  Is it because it is a speciality market,
> or are there components in there that are really worth a couple
> hundred dollars?

	There is nothing sophisticated or expensive in a telephone
test set; the price is based upon the device being a "specialty item"
in a marketplace generally *willing* to pay.
 
> I've added up the cost of the obvious components
> (headset, tone pad, aligator clips, batteries, isolation transformer)
> and arrive at about $50.

	Ain't no batteries in a conventional telephone test set.  From
a circuit standpoint it is nothing more than a regular telephone with
(today) switchable tone-pulse dial and a monitor-talk switch.  In the
monitor position, the set is little more than the receiver element
in series with say, a .1 to .22 uF capacitor.  Some sets have a polarity
indicator - big deal, an LED along with one or two resistors, and
possibly a zener diode.

> The only thing I don't know about is the
> monitor option, which is what I use most to debug the modems.  What
> makes up this circuit?  

	Traditionally, nothing more than a receiver element in series
with a capacitor.  A better approach for data work is to use a higher
impedance connection made by an externally-powered amplifier bridging
to say, a 100,000 impedance.  However, only one or two handheld
telephone line test sets have such a battery-powered amplifier.

> One final thing, does anyone know if I can use the telephone
> isolation transformer on an old 300 baud modem to shave a couple
> dollars?  We've got some 300 baud modems around collecting dust.

	You don't need an isolation transformer for this application.

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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