CMB PET 2001 restoration tips?

[Roberto Bazzano]

> 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