Modifying a B&W TV for composite video input

From: jpero_at_sympatico.ca <(jpero_at_sympatico.ca)>
Date: Tue Jul 1 17:46:00 2003

Tony wrote:

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

Yep. Nasty. Even with isolation still can get that ZAP!.
 
> 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).

Every one you described for non-isolated sets are like this. Every
one, I'm referring to cheap color sets old or new. The - side of
rectifier bridge is "hot ground" in those. Very rare few I seen had
one diode and that was in old citizen sets usually 13" to 19". This
is really downright cheap. In those with non-isolated sets that
does have inputs, always have small signal isolation transformers and
or optical isolation. The others that is both hot/cold chassis be it
via flyback transformer like this one hitachi and about 7 yr old.
Other kind via SMPS is very common. Using mains isolation
transformer is *extremely rare*. In so many sets in 1
and half years of sets (cranks out about 30 sets every 5 days) I
worked on, only ONE did by design and that was really ancient Samsung
32" tv. So do your self a favor buy a newer used or new sets and
hook up things without modifying that set or be hackish on old stuff,
splice it on that internal mains wires a isolation transformer
and bolt it inside that set.

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

13" is considered portable also and almost always are hot chassis by
design. In RCA 13" AC/DC tv it's hot chassis, to run on 12V RCA
puts in an DC to DC converter! I'm talking about those that is made
from 1991 onwards. ( chassis TX8xx series) But what nice about this
13" RCA has no B+ regulator and the pix is very stable because of
direct control of horizontal pulses to regulate power in flyback
itself. Hence very little horizontal size swelling or jaggies with
bright parts. The later 13" RCA tvs gained mono audio and video
input after it got the SMPS but basic design of pervious 13" RCA
mostly unchanged.

By contrast I have seen so many other sets that doesn't have good
handle on the HV regulation the pix width swells with bright parts
and shrinks with darks. This drives me nuts. Those have same
chassis but has pincushion feature added (it is a module in RCA) had
better handle on this but not 100%.
If the vertical edges isn't straight (more of a flared or jaggies)
that what I mean by this poor HV regulation.
 
> A circuit diagram of the set is _essential_. I am not sure where you are
Received on Tue Jul 01 2003 - 17:46:00 BST