Macintosh 128k Video Issue

[rikkitikki]

I am restoring a Macintosh 128k and have an issue where the vertical position of the image is lowered to almost halfway on the crt. The image is clear and crisp and the desktop is complete, just compressed to halfway. I have researched and performed all of the basic checking I.E. voltages, capacitors, position rings on the yoke. Any ideas? I also have Larry Pinas book on repairing the Macintosh and have gone through his video problems sections.

Any help is appreciated. I have attached two photos showing the problem.

thanks,
Rick

 

[MacTech68]

Try checking page 56 of Larry Pina's 'The Dead Mac Scrolls".

 

[rikkitikki]

Try checking page 56 of Larry Pina's 'The Dead Mac Scrolls".

I did all of those adjustments... no luck

 

[rikkitikki]

Try checking page 56 of Larry Pina's 'The Dead Mac Scrolls".

I also tried adjusting the purity rings on the yoke and they moved the image up a little bit but nowhere near it needs to go. I am starting to think this is more of a mechanical issue rather than an electrical one. I don't have enough knowledge of tv circuits to understand how the upper left hand position is determined. Is it an RC circuit constant or something else that may be bad in the vertical circuit...

 

[MacTech68]

Page 56 mentions 2 components that may have failed.

 

[rikkitikki]

I already replaced those... and also replace R3 and Q1, no luck.

thanks

 

[chabig]

I don't know much about analog video signals. But you're getting a clear picture, just not the right size. That makes me think it's not frequency related, but that the amplitude of the vertical sweep is reduced. The most likely cause for this is a failing capacitor, think. Resistors and inductors don't fail as often. All you have to do is find the right one. I'd guess that it's one of the larger ones, maybe C1 or C2. Is there any evidence that either might be bad or have bad solder connections?

 

[rikkitikki]

I don't know much about analog video signals. But you're getting a clear picture, just not the right size. That makes me think it's not frequency related, but that the amplitude of the vertical sweep is reduced. The most likely cause for this is a failing capacitor, think. Resistors and inductors don't fail as often. All you have to do is find the right one. I'd guess that it's one of the larger ones, maybe C1 or C2. Is there any evidence that either might be bad or have bad solder connections?

yes I thought the same, the trick is figuring out which one is bad without just replacing all of them.

thanks.