View Full Version : Adjusting geometry on IBM PS/2 Model 25 CRT

May 21st, 2017, 05:20 AM

The CRT on my IBM PS/2 Model 25 is producing a distorted image. The image was fine until I turned on the computer last October and it's been the same since.


The only external controls are brightness and contrast. How would I go about adjusting the geometry? Could this be fixed by adjusting the deflection yoke? Is this a sign of capacitor failure?

Thank you.

August 1st, 2018, 01:08 AM
Does anyone know? It's mostly been stored away since I last posted. The image distortion isn't any worse than it was before.

August 2nd, 2018, 08:51 AM
That CRT has problems, it's not out of adjustment. There is not going to be any setting that fixes that distortion, it needs to be repaired.

August 2nd, 2018, 09:06 AM
Geometry adjustment is sometimes patched with magnets on the sides, maybe one or more fell off?

Barring that, capacitor replacement is generally a good first step that fixes many issues.

August 2nd, 2018, 10:24 AM
I'm thinking magnets also. There are several glued to the yoke that can produce the effect shown.

The other aspect is that the vertical axis of the picture seems to be compressed. That could point to a power supply issue as well.

Hugo Holden
August 3rd, 2018, 12:22 PM
I doubt if a geometry magnet has anything to do with this.

The internal SMPS power supply output voltage is low with ripple on its output affecting both the H & V scan. The usual cause is the main filter capacitors on the rectified mains input will have gone high ESR and low. uF value.

THere could always be another fault affecting the power supply, but less likely. Anyway it is definitely a low power supply output problem with ripple on the supply voltage.