Hacking an RS-170 tap into a B&W TV set?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Mar 30 17:13:00 2003

> Given nothing but the set; no doc of any kind (have you Googled for a

I assume there's no schematic stuck inside the set (stranger things have
happened...)

> Sams Photofact of it?) I would get the number of the CRT, find the
> control grid pin, and trace that wire from the socket back to the board.

Be careful. In many TVs sold in the UK (and I guess elsewhere -- I am not
only talking about sets designed in the UK), the control grid is used for
the brightness control only, and the video signal is applied to the cathode.

Most small TVs, particularly those designed to run off a 12V car battery,
use a 7 pin CRT. The pinout is almost standardised at :

1 g1 (control grid)
2 cathode
3 heater
4 heater
5 control grid (linked to 1)
6 first anode ('screen control')
7 focus (second) anode

> That will get you in the neighborhood of the video amps - then some simple

If you're lucky, the video output stage will be discrete components and
easy to hack. If you're unlucky it'll be hidden inside a 'jungle' IC...

You also need to get the sync signals in somehow. If you're lucky
(again!), you will find a point at the input of the video amplifier that
also feeds the sync separator -- which means you can feed in composite
video with no problems. After all, the signal at the output of the video
detector stage _is_ composite video, so the set must be capable of
displaying such a signal. The problem comes when this point only exists
within an IC.

> The usual Cautionary Parables about working on naked TVs apply. zzzzap!

Be very careful if you work on mains-only sets. Many of these had a live
chassis (connected to one side of the power line) with no transformer
isolation. Connecting one of those to your classic computer can apply
mains voltage to places where you don't want it, and give you a nasty
(and possibly fatal) shock, or worse still do serious damage to said
classic computer.

This is unlikely to apply to a set that runs off batteries too (in 99% of
the cases, the mains is stepped down to 12V using a conventional
transformer/retifier circuit), but it can't hurt to check. If in doubt,
run the set off an isolated 12V supply connected to the battery
terminals, rather than using the internal mains PSU of the set.

-tony
Received on Sun Mar 30 2003 - 17:13:00 BST