More circuit help required please

Adrian Graham witchy at
Sat Jan 7 17:37:11 CST 2017

Evening all,

I wish I had the ability to take a board layout and turn it into a logically
laid out schematic but as yet I don't. Video sync on my Executel 3910 is
still running me round in circles so could one of you fine folk take a look
at this board layout drawn as best I can:

...and let me know what it does please? The chip on the left is a Plessey
MR9735 teletext video driver wired to run permanently in 'off hours' mode -
you can see 'Sync In' is just pulled high so the chip doesn't detect any
incoming sync and is supposed to generate its own, which it doesn't even
though all external clocks are present.

Resistor values are correct as far as measuring them with a DMM goes, the
diodes aren't actually 1N4001s but I can't read the printing on them. Both
transistors are BC548 NPN. The area labelled 'MON' is a 14-way ribbon cable
that goes off to the TV driver board.

I'm feeling pretty dumb at this point. Logic and truth tables are more my
thing, current flow is something I'm learning slowly.


Binary Dinosaurs creator/curator - the UK's biggest private home computer

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