More circuit help required please

Tony Duell ard.p850ug1 at
Sun Jan 8 12:20:16 CST 2017

On Sun, Jan 8, 2017 at 6:11 PM, Adrian Graham
<witchy at> wrote:
> 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 -

OK, was there ever a colour version :-)

> 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

And in a sense there was...

> 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

Do you mean that the common cathode connection of those diodes goes to
the monitor connector but nowhere then on the monitor PCB?

>> 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
> active.

OK.. My dislike of 2114s is legendary, but probably not the problem here....

> 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

What are the RAMs? If they're more 2114s I wonder if they have problems
that are confusing the CPU (perhaps corrupting the stack?)

> 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.

What's the CPU? 8085? I've had those fail in odd ways.

> 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.

Personally, I'd stick a logic analyser on it and start tracing the code. See
if it is executing anything correctly.


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