Adjusting focus (R57) on an international Mac Plus analog board does nothing

[pfinn]

I have an international Mac Plus with a very blurry screen towards the edges. Center pixels seem okayish. When I adjust the R57 pot meter (focus) nothing happens on the display. Using my multimeter I can confirm that the pot meter varies resistance between about 0 to 2M ohm, so it seems to be fully functional. Does anyone have any ideas what could be the problem?