More circuit help required please

Adrian Graham witchy at
Sun Jan 8 12:11:46 CST 2017

On 08/01/2017 17:21, "Tony Duell" <ard.p850ug1 at> wrote:

>> 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.
> OK, probably some kind of luminance signal. Was there ever a monochrome
> version of this unit?

The whole unit is monochrome, it only has a little 5" B&W TV as its output
device -

However, two of us bought units at the same time and the other one has a
little bridgeboard containing a 74LS244 that sends the RGB signals to a DIN
socket on the back of the case for an external monitor. I also can't see a
luminance line on the connection to the TV board on mine, just
red-green-blue with sync and inverse sync which go to the horizontal driver
chip and vertical driver chip respectively. You may remember I was asking
questions about these chips a couple of months ago because that TV board
suffered some component rot and I thought the wavy picture was because of

>> 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.
> I doubt it.... Why would there be?

Clutching at straws :)
> Is the MR9735 doing anything? Is it accessing memory, for example?
> I still wonder if it has to be initialised (say to vewdata rather than
> teletext mode) and if this is not occuring.

Its address and data buses are active as is the pair of 2114s that are
acting as a page store. There's a 74LS240 buffering the data bus and that's

You could be right though, I'm not convinced the unit is actually running
any code yet despite the ROMCS lines working and I can see activity in all
16 RAM chips. Data bus and address bus at CPU and ROM chips looks happy from
a 'doing something' point of view. All the control lines on the CPU are
pulsing with the exception of IO/M but if all the IO the chip is doing is to
memory then that's to be expected.

I've disassembled the ROM code so I'll have a look through that for an init
routine, also the datasheet should tell me what its expecting at powerup.


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

More information about the cctalk mailing list