More circuit help required please

Adrian Graham witchy at
Sun Jan 8 11:05:39 CST 2017

On 08/01/2017 14:06, "Tony Duell" <ard.p850ug1 at> wrote:

> On Sun, Jan 8, 2017 at 2:00 PM, Adrian Graham
> <witchy at> wrote:
>> You were right, I'd drawn out R335 wrongly and flipped V333. All the anodes
>> go via R312 to the collector of V333 along with R309-R311. Base for V333 is
>> the inverted sync output of the MR9735. R322 is 6.8k
>> (blue-grey-black-brown). All diodes are 1N4148.
> That makes more sense. I am surprised that the anodes of the diodes don't go
> to a +ve power line (maybe +12V), though. It could be right as drawn,
> but I would check it.

I photographed both sides of the board and merged the pictures together to
make a front/back composite. The trace for the cathodes of the 3 diodes
links together on the back of the board and doesn't go anywhere else. The
front trace from V302 cathode goes straight to R312. The anodes DO link with
the monitor driver board through the connection marked MON though.

> When you power up, what voltage and/or signal (use a 'scope) do you see on the
> anodes of said diodes?

Looks to be 4V on all 3 lines for RGB:

One of the reasons I'm asking these questions is I'm trying to see if
there's another source of composite sync given that the MR9735 itself
doesn't do it, even though it should.


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

More information about the cctalk mailing list