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

http://minuszerodegrees.net/oa/OA%20...%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.

Now I notice the original transformer was a solid metal screened thing with an extra grounding wire.

I have twisted and moved the secondary wires away from the tube and also moved away and screened the transformer temporarily to no effect. Various things to try next include ferrites on primary/secondary wiring, a UPS and maybe some extra caps and also jumpering across from a working monitor.

The environment is terrible being a retro museum on an industrial estate with a railway depot over the road.

Though generally experienced this baffles me somewhat so I'd appreciate your thoughts.

Is the problem likely to be common mode noise ?

Ideally I'd like to keep this transformer and not have to seek out a screened one.

Thanks.