Modifying a B&W TV for composite video input

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Jul 1 16:31:00 2003

> I have read a lot about the SWTPC TV Typewriter, and I am interested in
> how people modified a black and white TV for a video monitor. I have an
> old black and white set that I thought would be fun to modify for use
> as a video monitor. Now, granted, I don't really need another composite

It's fun until you start working on a live-chassis set and you suddenly
apply mains across your video interface circuit and yourself....

Seriously, many large-screen (non-portable) B&W TVs have a live chassis
connected directly to one side of the mains (or worse to one side of a
bridge rectifier connected to the mains, so the chassis sits at half
mains voltage on average). Don't even think of adding a composite input
to such a set unless you connect the mains input to the secondary of a
double-wound isolating transformer. (OK, it is possible to isolate the
video input rather than the mains -- Radio Shack did on the original
Model 1 monitor, which has a hot chassis. But this is a lot harder to get
working right, and a lot more dangerous to experiment with).

Portable TVs that can also run off a 12V battery _normally_ have an
isolated PSU, but please check to be sure...

A circuit diagram of the set is _essential_. I am not sure where you are
-- if you're in the States, get the Sams Photofact for the set if you
can't get the original service manual. If you're in the UK, see if it was
covered in 'Radio and Television Servicing'.

Then all (!) you need to do is find the video detector stage and feed in
your composite signal at the output of that stage. Hopefully the sync
separater takes it's feed after that point (it's almost certain you can
find a point where this is the case) so you really can feed in a
composite signal and expect it to work.

-tony
Received on Tue Jul 01 2003 - 16:31:00 BST