[R-390] starting 390A restoration

[Roger Ruszkowski]

Bob,

You wrote,

also see if the PTO (Motorola) can be brought back to within specs without 
taking a turn off (doubt it, it's more than 10 Kc off end to end).

This range shold not be a problem to adjust.

Try to do an adjustment before you try to
modify it. Mostly you will just need to lube
the shaft to reduce the friction.

All the stuff you are reading about doing stack
adjustments are true.

If after getting the end to end good, you find the 
midpoints off by more than 100 Hertz you likly can 
not get them better by messing with the stack.

Unless the stack has come loose and is way out,
you can not get it better by adjustment.

The core materiel has not changed much over time.
The non linear adjustment is to compensate the 
taper of the coil as it was wound on the form.

The taper was machine wound. The core stack was
adjusted in a jig to best fit at a couple 10 points
along the travel.

Any displacement at mid points is then imperfections
in the coil wire and its lay on the coil form.

You trying to get it better than the the original 
factory setting is a loosing effort.

In the original, the magic stuff was sticky and slow
drying. The stack was painted up and stacked.
The parts were almost clamped tight end to end.
The zero was found at a 100Khz point, using a calibration oscillator.
The shaft was given 1 turn ( with a dial in a jig).
The stack was squished into a straight line and scooted over to
zero beat at the alignment point. The shaft got another turn
and the stack was scooted from the last point to this point to
again zero beat. The stick was curved. Buy using the correct section
of the stick you could get the right trapper on the stack to fit 
between the two points of interest.

An experienced operator could slide the sticky painted stack
into best alignment and recheck it faster than you can read this 
paragraph.

Roger