ZX-81 Corrupted text display

[Jelvis]

Hi,

I recently picked up a ZX-81, issue 3 board. After introducing myself it was suggested I post here to get further advice. The only thing I have done is replaced the keyboard, no other mods. Keyboard inputs seem to work but what is displayed looks corrupted as shown in the image below, almost as if each 'character' is split in two and then doubled, rather than the whole character being display correctly. Any advice which way to fault find or advice for resolution would be appreciated,

Mike

[XavSnap]

Hi Mike,

Something's wrong on the A3 memory address line, on the ULA or the ROM ship.
If you type "R", is there 3 displayed characters ? (to check if the BASIC/RAM is ok)
Type "V" then newline, to display "0/0"...

Check R18-R26 resistors. (A3'=R25)

[Jelvis]

Thanks, I'll take a look at this tomorrow at work as I've access to more test gear there. Nice to have somewhere to start.

[XavSnap]

Hi Mike,

If the Basic is ok, the UDG (characters table) is located at the end of the ROM ($1E00).
Check the ROM solders or clean the ships connectors.

The address line A12 can be cut or short-cut by the new keyboard.
A12= "67890" keyboard line (D3 diode).

And have a special look to the keyboard connector, may be a shortcut on A12-A10 or A12-A9 !

[Jelvis]

Hi Xavier,

thanks for even more helpful information. I typed 'R' and got the image as shown below, followed by V then Newline and got what is shown in the second image. I checked the resistors from 18 to 26, they all read similar values around 1K. I also checked the diodes from the keyboard, again all ok. All components were checked 'in circuit'. I removed the keyboard from the connectors and checked for shorts, again all ok.

[Moggy]

Any chance of a decent picture of the board?

Be interesting to see if it has a standard issue three ROM (649) or if someone has planted an EPROM inside.

[Jelvis]

Hopefully this is clear enough

[Moggy]

Thanks Mike, perfect.

Well Motorola ROM not an EPROM so that's my guess blown out of the water, at least the picture can be studied by the gurus and if nothing looks obvious then it's possibly chip replacement time which on an issue three is a bit daunting as they are soldered not socketed.

[Jelvis]

Thanks for taking a look. I take it I'm probably looking at eprom or ula?

[Moggy]

I'm not the most technically minded one here so couldn't say with any certainty but if I had to take a chance on replacing something then the ROM first if only because it has fewer legs to de-solder and also may be easier to get hold of than the 2c210 ULA.
I know from painful experience what a pig of a job is is to de-solder a ULA from an issue three board then reverse the procedure with another chip,
(no sockets in my toolbox at the time!)

Sorry I can't be of more help.