Nascom 2 screen offset problem

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Lurch666
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Joined: Fri Mar 01, 2013 1:30 pm
Location: Stoke-on-Trent U.K.

Nascom 2 screen offset problem

Postby Lurch666 » Mon May 15, 2017 11:57 am

trying to fix a nascom 2 but a screen problem has me stumped.

https://www.youtube.com/watch?v=tsnGL1bOGXA

In the video I have replaced a faulty prom with an eprom and can now type into the nascom but the display is offset on character to the left.

it should say
NAS-SYS
but as you can see it comes up
AS-SYS
The squares on the right are in the margin of the screen and should not be displayed.
The schematics of the video circuit are here:
http://www.nascomhomepage.com/pdf/Nascom2.pdf

I've got quite a way fixing this following data paths to where they stopped to get it this far but this isn't missing data-it just seems to be maybe a timing problem to me.

I have a basic oscilloscope and multimeter but was wondering if anyone here has come across anything similar on an 8 bit system?

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