CRT Fuzziness, Getting Worse Moving From Center, On A Plus

[Paralel]

The image of the desktop on my CRT is clear in the dead center, but gets progressively fuzzier as one moves from the center to the edge of the monitor in all directions. I figure I just need to adjust one of the parameters for the tube. Just not sure which one and where the adjustment is located. Any insight into how to fix this would be appreciated.

 

[sadmanonatrain]

Paralel,

Macintosh Plus Service Manual: http://tim.id.au/laptops/apple/legacy/macintosh_plus.pdf

Within the manual there is a chapter called 'Adjustments' which has a section about focus.

Maybe the focus dial needs some adjustment?

I hope this helps!

 

[Paralel]

Thanks. Sounds like exactly what I need.

 

[techknight]

if its sharp in the middle, and fuzzy on the edges? thats 2 things. 1 the CRT is weak and has low emissions. This will cause beam divergence on the edges. 2, Low HV will cause this as well.

 

[onlyonemac]

I think focus is an overall adjustment, and won't obviously help here.

 

[tomlee59]

Without a more detailed description, it's hard to say exactly what's wrong. But large focus variations across the screen are rare without an accompanying symptom, such as incorrect raster size (or symmetry problems), or a brightness problem. If the brightness is set too high, focus variations will be exacerbated.

If you cannot increase brightness to normal values without focus degrading massively, that's usually a sign of a worn-out CRT gun. However, it can also be caused by a too-low voltage on the 12V supply (the CRT filament power comes from the +12V supply). Measure it and correct if necessary.

If the raster is too large, that is generally a sign that the HV supply has a problem and the voltage is too low. That, in turn, will usually be associated with a dim raster and poor focus.

If you could post a pic, or provide a more detailed description, that would help us narrow things down a bit.

 

[Paralel]

Brightness is fine and has no impact on the severity of the focus issue. No symmetry issues. The raster looks maybe a little small, but I haven't measured it to find out.

I'll see if I can convince my camera to take a decent pic.