I recently bought a new video machine, a Thomson, one of the
new multi-standard jobs. It plays, according
to the documentation, PAL, NTSC and SECAM VHS videos. Uhuh. Yes, it does, but there's a catch.
Unless a video machine incorporates a scan converter
(Note: this is a large, complex device) you can't get
real PAL video out from an NTSC tape.
My TV set is a Sky TV, it has a Phillips chassis. The main
do-almost-everything chip is a TDA4503. By
hit and miss I determined that the horisontal hold control is the 220k trimmer connected to pin 1. There's
a cap from somewhere (power?) to pin 1, and a 220k trimmer + 470k fixed resistor from pin 1 to pin 7,
which looks like it's ground. (Why didn't I measure what's power and what's ground? Because I switch off
and unplug the TV before I touch the PCB. And remember about all the capacitors, switched-off TVs have
killed people before.
So, I added a second trimmer, with a switch in parallel, into the
circuit. Close switch, set NTSC, open
switch, set PAL. First mistake: the long wires to the switch pick up hum, causing the image to do strange
things. So, I moved the (new) trimmer to the other side of the 470k resistor, reasoning that the impedance
is lower over there. Worked, too.
Then, while performing the final tweek, my screwdriver slipped
(HINT: insulate the shaft, as I belatedly did)
and shorted the trimpot rotor to a nasty big resistor - poof. Ah well, a new TDA4503 is < R30, so out comes
the old one, in goes the new one, tweeking gets done with an insulated screwdriver, and I can watch NTSC
So, remember to drop me a line if you find this useful. And, if your TV never works again, tough shit :-)
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