14 Jul
2006
14 Jul
'06
7:38 a.m.
One strange thing: I pulled the character generator
ROM, expecting that
doing so would either give me a completely white or completely black
screen. It doesn't - I get some white blocks and some black blocks at
random on the display. I admit to not yet understanding what the PET's
doing with its display circuitry, but I had expected that having no ROM
there would result in every character having the same value, so regardless
of what happened to be in the video RAM at power-up I'd get the same
character at every screen location. Not sure if that sounds indicative of
a particular fault...
I'll try to replace 2114 video ram: on three pet I repaired they were
defective.
Also, put an oscilloscope on the CPU data and address bus and look if
signals are all 0 or 5 volts: if you see some strange value like 2-3 volts
or attenuated square waves (near 0 or near 5 volts) then there is some
defective chip. Then you can try to remove chips one at times to identify
it: you'll find it when the strange wave value goes away.
Roberto