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

From: Ethan Dicks <[email-masked]>
Date: Sun Mar 30 15:54:01 2003

Tony (and others ;-),

How does one go about looking for the right place to install an NTSC
video input jack into a B&W TV without schematics?  I have the "Cheap
Video Cookbook" which describes the technique, but I'm trying to do
this to a $20 K-Mart 4" B&W TV set (Spectra Model 52-BWR).  Among
other appeals, it's battery/auto/AC powered and should work great with,
say, an 1802 with an 1861 (Elf, VIP, et al).  Good portable demo set
for low-res graphics.

If it helps, the largest IC appears to be marked as follows...

  CD5151CP
   EP3 II
  DE028

Thanks.

-ethan

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From: John Lawson <[email-masked]>
Date: Sun Mar 30 16:13:01 2003

Given nothing but the set; no doc of any kind (have you Googled for a
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.
That will get you in the neighborhood of the video amps - then some simple
signal injection [even a metal needle using 'hum' injection from your body
capacitace] might show up some traces where demodulated video lives. You
can then experiment with the proper place , right coupling capacitor,
polarity, etc., to apply the computer video.

  If you have a scope, this can also be done by inspection, rather than
injection.

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

  Cheers

John

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From: [email-masked] (Tony Duell)
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...)

> 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

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From: Ethan Dicks <[email-masked]>
Date: Sun Mar 30 17:30:01 2003

> I assume there's no schematic stuck inside the set (stranger things have 

Nope.  I'd be nice, though.

> displaying such a signal. The problem comes when this point only exists 

If it helps, the tuner is on a seperate board with a 4-wire cable to
the main board.  There are a couple of tunable elements (cap, coil),
a few caps, diodes and a single IC - CD2003GP

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

Naturally.
 
> Be very careful if you work on mains-only sets...

This one is internal batteries or 12V car plug or a 12VDC wall wart.

Of course, the internal voltages are still typical for CRT use, but
no hot-chassis here.

-ethan

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From: [email-masked] (Tony Duell)
Date: Mon Mar 31 17:31:00 2003

> the main board.  There are a couple of tunable elements (cap, coil),

The output of the tuner will be at the IF frequency (around 30MHz), and 
it is not the signal you're looking for.

> a few caps, diodes and a single IC - CD2003GP

What's on the main board? How many chips, and are there any discrete 
transistors? You really need to find the video detector stage -- the 
output side of this is an almost perfect place to feed in composite video.

> Of course, the internal voltages are still typical for CRT use, but

OK, I think it's safe to assume this set doesn't have a live (hot) chassis.

-tony