A question about CRT scan problem

[clh333]

Newly acquired AT&T PC6300 boots and appears to be fully operational, but observing the display the letters appear taller at the top of the (monochrome) screen and shorter at the bottom. This is confirmed with the Customer Diagnostics tests which show a pattern of concentric figures; they are definitely foreshortened bottom vs top.

The question is: What is the likely cause, monitor electronics or display card electronics? Yoke position? Haven't seen this one before...

Thanks,
-CH-

 

[Chuck(G)]

Monitor electronics, very likely. There should be a pot on the PCB for adjusting vertical linearity--that would be my first target. Could be nothing more than a dirty pot.

According to the service manual, that would be RV6.

 

[clh333]

Certainly appears to be the fault of the monitor: Swapped out the monochrome for a 318H color monitor; character size is even top to bottom.

Thank you for the advice - and for SSTAT.COM, btw.

-CH-