25 Feb
1998
25 Feb
'98
1:57 p.m.
Power on -
BEEP
That should be a two-tone beep. It's the standard BBC power-up sequence.
In mode 7, 1 byte of video memory corresponds to 1
character on the
screen. So it looks like you have a problem with address line MA6 from
the 6845. If you can borrow a logic probe, look at pin 10 of the 6845
(look at 8, 9, 11 as well, in case I've miscounted) - there should be a
square wave (pulsing) there, and then trace it through IC36c (again,
there should be pulsing at pin 8 of this chip).
Will be a while before I can borrow a logic probe. However, removing and
reseating the 6845 has had no effect on the problem.
To be continued...
A