Modifying a B&W TV for composite video input

From: John Lawson <jpl15_at_panix.com>
Date: Mon Jun 30 21:21:00 2003

  First - the TV itself is needed. When a unit is to hand, look up the
Sams Photofact for it - this will be the comprehensive data and
documentation package - numerous schematics, pictorials with all the
components marked, etc. I think they're, what - $15? Dunno, but it's
money well spent.

  Then - most BW TVs from the 60s and early 70s (if tube) were
series-string type - they had no power transformer, rather the incoming
mains current was tied to a resistor string to power the tube filaments
and to diodes for the various DC voltages - this meant that one 'side' of
the mains always was connected to the metalwork of the set - thus posing a
signifigant hazard to persons **and to any other gear hooked up to the set
that didn't share the same ground polarity!!**

  It is A Very Good Thing if you can score a simple mains<->mains
isolation transformer, just 110V in and 110V out, 1:1 windings. Be sure
its a two winding device *NOT* an 'autotransformer', which is typical of
most 'travel convertors' one buys for Journeying to dark and wild corners
of the world (Munich for instance). If you can't find a 100Watt (or so)
1:1 unit, you can make one by getting two identical power transformers and
and attaching thier secondaries to each other, thus a 1:5:5:1 for example.
Just be sure that the transformers will carry the load of the BW set
(total watts).

  Now, using the Photfact schematics and data from the Web, here, and
other sources - locate the video and sync-separator circuits. Determine
the polarities of the signals involved, and what relative levels the
circuits want.

  Buld up an interface circuit to accept TTL level (or standard NTSC
composite analog data) and feed it at the right levels and polarity to the
video input stage of the BW TV.

  Power up both units and test.

  After the Fire Captain tells you it's all right to go back inside,
salvage what you can from the sodden charcoal mess and try to find a new
hobby that carries less intrinsic risk.

  ;}


  No but, seriously - after you have the BW TV set data, it is not a
particularly onerous task to suss out the points in the circuit where one
could ostensibly inject an external video sig.

  I might also suggest that you look into opto-isolating the video feed
from your computer, just to be safe. Letting the Magic Smoke out of one's
vintage computers is not exactly the best way for further knowledge and
gain experience, though Mr. Wilde might have disagreed with me.

  But hey - he's dead.




 Cheers

John
Received on Mon Jun 30 2003 - 21:21:00 BST