ramesh@amdcad.UUCP (N.Ramesh) (11/22/85)
I had recently requested information on adding a video input to a
TV receiver, and received quite a few responses. Thank you all.
This posting is in response to some requests (by mail) to post a
summary. It follows....
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RE:
"I would like to add a Video input to my Sears 19 inch TV receiver"
Try to find a SAMS photofacts for your set. It will contain schematics
and photos. I don't know if it would be difficult.
The first thing to do is to check whether the set's transformer isolates the
it from the the power lines. Some cheaper sets (maybe more these days)
have a direct connection between the chassis and the AC line.
If the set isn't isolated, you have about a 50/50 chance of getting the
chassis connected to the 'hot' wire. If you then connect the set to
a grounded piece of equipment, you will start frying things. If your
set isn't isolated, you might be better off forgetting the idea.
harold ravlin
{ihnp4,pur-ee}!uiucdcs!uicsl!hr
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
AT&T Bell Laboratories HOH R-207
Holmdel, NJ 07733
hou2e!gv
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.
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not have a video input provision use full wave bridge rectification of the
AC mains. Thus, the chassis is always 77 volts garbage-AC above the neutral
in NTSC countries. In addition, most consumer sets require considerable
level shifting and garbage logic (many Zeniths, for example, have a separate
second detector for sound-sync and composite video...)
David Anthony
DataSpan, Inc
I've done this with both isolated and unisolated televisions. Most TVs
these days are not isolated unless they are intended to be used as
video monitors. Of course, high-end video stuff these days usually
has a composite video input on the back (some Sonys even have seperate
leads for each color driver and for the sync input).
If your set is not isolated you will have to buy an isolation transformer
to place into the power line between the set and the house current. An
isolation transformer is a 1 to 1 transformer that only isolates the
output from the direct connection to the house power. The only problem
I've had with these is that they tend to introduce a screen ripple that
is difficult to get rid of.
J. Giles
Los Alamos
You can still add a video input to a hot-chassis set, you just have
to be careful. The easiest way around this is to add an isolation
transformer to the AC line, and make sure the set can not be accidentally
powered up without it. Alternatively, you could use an optocoupler
to isolate the incomming video signal. Just don't let the signal's
ground come into contact with the set's ground. I believe there was
a circuit in Radio-Electronics (or maybe Modern...) a few years back
that did it this way.
/Michael L. Ardai
...!seismo!columbia!cucca!mike
You might find it easier and cheaper to just buy one of those low-cost
composite compputer monitors now available... you can get a Commodore
1702 or CM141 for $169 if you catch it on sale, and it has the prettiest
picture on a VCR that you could ever want...
I'm sure there's other monitors out there that are equally cheap.
In any event, I found that just buying a monitor was well worth it
when compared to the choice of modifying a TV, when you considered
the amount of time and money you'd pour into the project...
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Thank you all once again
N. Ramesh
Advanced Micr Deviced, Sunnyvale, CA PH: 408 749 4726