wrote:

I have partially repaired an IBM mono monitor whose mains transformer had
failed.


http://minuszerodegrees.net/oa/OA%20...y%20(5151).pdf

Driven from a genuine PC of the day which drives another such monitor
perfectly well the picture now sort of moves around but at a rate of
1-2Hz. Turning up the brightness shows a wavy left/right edge implying a
few hundred Hz effect. But the effect is also vertical.

Note I say moves. It does _not_ contract or expand which would indicate
insufficient headroom after the rectifier before the 15v regulator. I am
happy this isn't happening.
Most likely it is magnetic radiation from the transformer. osme
transformers have steel shields, others have copper shorting straps to
reduce the external magnetic field. It REALLY doesn't take a lot of
external field to affect the picture. You might be able to help by changing
the orientation of the transformer, or getting it farter away from the neck
of the CRT. Mu-metal shields might be fashioned to help, but if you bend
the mu-metal, you have to re-anneal it.

Jon