[Heathkit] IO-12 Vertical Drift

[Harry Vaught, KT4AE]

I've got an IO-12 I built in the '60's that I'm trying to put back into 
general 'scope service.

The problem is that over a half-hour to forty-five minures, the trace 
will drift up to where you can't bring it back down to mid-screen.

I've replaced the 12AT7 and all the resistors and caps that would seem 
to have an effect on vertical position.  Nothing seemed to make any 
difference.  Voltages are good - measurments after the trace drifts 
don't show an imbalance.

The direction it faces on the bench also affects the trace position. 
Putting it right next to my SX-101A (a large hunk of steel), pulls the 
trace up about 1/4 inch.  This almost sounds like a floating ground, but 
I can't find anything.

Anyone run into anything like this?

Harry Vaught, KT4AE
Maryville, Tennessee