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From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: net.video,net.analog
Subject: Re: Adding Video Input to a TV Receiver
Message-ID: <590@ttrdc.UUCP>
Date: Tue, 19-Nov-85 18:46:13 EST
Article-I.D.: ttrdc.590
Posted: Tue Nov 19 18:46:13 1985
Date-Received: Wed, 20-Nov-85 08:32:51 EST
References: <6263@amdcad.UUCP> <743@hou2e.UUCP>
Organization: AT&T, Computer Systems Division, Skokie, IL
Lines: 60
Keywords: TV VCR
Xref: watmath net.video:1677 net.analog:528

In article <743@hou2e.UUCP>, gv@hou2e.UUCP (A.VANNUCCI) writes:
>> I would like to add a Video input to my Sears 19 inch TV receiver,
>> to obtain better pictures from my VCR.  Is there anyone out there
>> who knows if this is possible, and if so how difficult is it? Is
>> it worth the trouble in terms of picture quality ?
>> Thanking you in advance for your suggestions
>> ----Ramesh
>   The main reason why TV sets don't have a video input is that they
>don't have an AC transformer in the power supply.  In order to save
>money, weight, etc. TV sets rectify the AC from the outlet directly
>and, therefore, have a hot chassis.  If you connect a coax cable to
>the video circuit input the shield of the cable must go to the chassis;
>however, the chassis may be connected to the 117 V live wire, so that
>you will get an electric shock when you touch it, you will blow fuses,
>damage other equipment, etc. etc.
>   From what I understand, you just need an isolation transformer to
>power the TV set and then you can add a video input with no problems.
>   An isolation transformer is just a normal transformer with a 117 V
>primary and a 117 V secondary, so that the load is not directly
>connected to the power grid.
>		Giovanni Vannucci

Mightn't it be possible to isolate the video connection, rather than
the entire set, as through, say, an opto-isolator?  (I do not know offhand
if the bandwidth is high enough.)  Then it wouldn't matter whether the
set were "hot-chassis" or not, and would perhaps be less clumsy than a big
bulky transformer (and there would be no chance for harm such as what
would occur if someone plugged  the set directly into the wall outlet
by mistake).  The issue of powering the phototransistor on the set side
of the optoisolator needs to be taken into consideration--does the set
possess a suitable internal supply voltage?  (It should probably be
well filtered between where it is "stolen" from the set and where it is
used to power the phototransistor circuit, to avoid unwanted feedback.)
The wiring from the phototransistor (and switch, if you use one to switch
between normal reception and the VCR input) to the video circuit should
be both as short as possible and well shielded.  (Note that your video
detector circuit is probably inside or under a little metal can on the
circuit board.)  Impedance matching (anti-ringing) may be a bit of a
problem.  Watch the video polarity, too, if you don't want negative
pictures.

[Like others have mentioned, if you can't get the set schematic from the
manufacturer, (or even if you can) Howard Sams puts out a nicely done,
if a bit pricey, set of documentation called "Photofact" (available from many
electronic parts dealers who sell to radio and television service firms)
on most television sets sold in America since WAY back.  The parts dealer has
the index, and you just bring your set's model number (and other
identifying information just in case) and he/she will look it up.  If it
is not in stock you can send directly to Sams for it.]

I would not suggest any video hookup not already provided by the manufac-
turer, however, for in-warranty television sets; it would likely void the
warranty.
-- 
 -------------------------------    Disclaimer:  The views contained herein are
|       dan levy | yvel nad      |  my own and are not at all those of my em-
|         an engihacker @        |  ployer or the administrator of any computer
| at&t computer systems division |  upon which I may hack.
|        skokie, illinois        |
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