In article , Bob Chilcoat wrote:
Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off of
runway heading at departure. Today a new partner was up with an instructor

I see you've already found the problem, but the local soaring club's
Grob has problems with the front-seat compass, and it turned out to be
related to a weld on the cabin-air vent. The mechanic managed to magnatize
it when he was welding it, and the compass swings 20-30 degrees off,
depending on the position of the cabin air vent!

So I'd check things that had been welded on recently.

Tina Marie