More circuit help required please

Adrian Graham witchy at
Sun Jan 8 13:05:41 CST 2017

On 08/01/2017 18:20, "Tony Duell" <ard.p850ug1 at> wrote:

>> 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...

Yep. I'm pleased we found that mod in the other unit because I made my own
bridgeboard and hooked the display up to a Microvitec CUB, I got exactly the
same picture (in colour) on that as I did in mono on the 5" TV so I knew the
analogue board and TV were OK.
>> 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?

No, in trying to trace some of the lines along the bottom of the board I
took the monitor cable off and traced the pins. They are:

1 - 5V
2 - NC
3 - looped to 5
4 - Sync out
5 - looped to 3
6 - emitter of V326 via R336
8 - RED
9 - Inverse sync (which also goes via R332 to the base of V333)
10 - BLUE
11 - GND
12 - GND
13 - 12V
14 - 12V
>>> 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....

They're OK too, surprisingly. I took them off the board and put them in a
PET8032 as video RAM.

>> 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?)

4116-2 wired as 2 banks of 8.

>> 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.

Yeah, 8085A. Thanks to list members I have some spares to try.

>> 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.

I've only got a cheap-ish Saleae compatible 16 channel analyser. Watching
code is a bit above my current level of self-paced learning :)

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

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