Apple //e dotted grid screen?

[ScanDisk]

In order to try to take care of and do some preventative maintenance on my Apple //e, I bought a freshly made power supply replacement.

It appears to work, though the socket for the plug from the power cable seems a little tight, and it is a battle to get the power cable plugged in right, and sometimes it will go "out of place" on it's own.

Anyway, when there is good connection the computer turns on and functions like it should, at least it did, though recently it seems to have encountered a problem where a dotted grid screen appears, though everything else works fine.

I don't know what the cause is, maybe something to do with the power getting "out of place" while it was on, or something?
Either way, what things should I try to remedy the situation and get the grid off my screen?

Here's a photo of what I mean:

 

[VeryVon]

Interesting. Curious, have you tested your memory recently? Using Apple-Cillin, RAM test Utility or the like.

 

[HoJoPo]

Check if it has dots in the high resolution graphics modes (HGR command from BASIC), if it doesn't, it's likely the character generator ROM that isn't seated or is damaged.

 

[daver2]

If you have an oscilloscope - obtain the schematics and work your way back from the VIDEO DATA signal with the initial boot screen active.

Since there is a nice, regular pattern there, it should be easily possible to identify where it is coming from.

I can't easily see how it can be from the character generator or the character parallel to serial shift register. I would look round the graphics part of the video logic.

Dave

 

[Hugo Holden]

It looks characteristic of a stuck bit in video memory, though that more often results in vertical lines, not dots, so there could be a problem between the memory output and the video shift register perhaps. It should be fairly easy to find the origin of the signal (with the scope)

 

[ScanDisk]

So, I just got to it. I put the old original power supply back in, and it works good, and turns on every time, but I still get the dot grid on the screen, and it also happens in the HGR mode.

But just for trouble shooting, what chips should I look at pulling and re-seating first if any?

 

[daver2]

As I stated previously, work backwards from the signal VIDEO DATA on the schematic.

Dave