128K image centering

[decle]

Hi, I have 128K Mac that I've been restoring. Following a "new" analog board and some RAM fixes, I've more or less gotten it operational. The new analog board required some image tweaks, I've worked out how to change the size, however, the screen image is not centered horizontally:



I believe that the early analog boards don't have controls for centering the image, so presumably this is caused one or more components being slightly out of spec. Could someone provide some pointers for things to check or some documentation on how to adjust the image?


Cheers

decle

 

[cheesestraws]

Could someone provide some pointers for things to check or some documentation on how to adjust the image?

There are two rings towards the back of the CRT that rotate and center the image, though along two diagonal axes, which makes it a little painful to adjust. Also, normal warnings about working near the back of the CRT apply here.

so presumably this is caused one or more components being slightly out of spec

I think this is pretty expected when a CRT is used with an AB that it wasn't "originally" paired with: you need to redo the centering adjustment. Probably doesn't mean that the AB is out of spec in any noticeable way.

 

[LaPorta]

Agreed. One way to make this easier is to not reach around the back of the machine: do this in front of a mirror.

 

[cheesestraws]

One way to make this easier is to not reach around the back of the machine: do this in front of a mirror.

That is a really good idea and I'm annoyed I didn't think of it when I had to do it last! I shall do in future...

 

[LaPorta]

It certainly is not my idea: old-time arcade machines have built-in highly polished, chrome mirror panels built into them so that when you slide out the CRT, you can see the reflection in the mirror and make adjustments. Just borrowing from those before me!

 

[decle]

Cheers guys, that was easy, job done!