[R-390] PTO Rebuild

[Miles B. Anderson]

A couple of clarifications:

The PTO "tunes backward." That is, the PTO frequency is lowest (2.455 MHz)
at the high end of the counter dial (999 or +000) and highest (3.455 MHz) at
the low end of the counter dial (001 or -000).

I used the terms "startpoint" and "endpoint" in my original post with
reference to the alignment procedure in Paragraph 81 of TM 11-5820-358-35.

You can tweak the "startpoint" at 07+000 by slipping the Oldham coupler with
respect to the PTO shaft. This sets the low frequency end to 2.455 MHz with
the slug pushed most of the way into the main tuning coil L702.

As you tune lower in frequency (per the counter dial), the slug is gradually
withdrawn from L702; and the PTO frequency increases.

You adjust the "endpoint" at 07-000 by tweaking L701 so that the PTO
frequency is 3.455 MHz. L701 is in series with L702. Since the tuning slug
is most of the way out of L702 at the "low end" of the dial, its inductance
is smaller and L701 has a proportionally greater effect on the frequency.

To use a mechanical analogy, the "startpoint" (how far into L702 the slug
starts its travel)is the point where you put the end of your "ruler." The
"endpoint" (the setting of L701) establishes the "length of the ruler."

The shunt capacitors C701 (370 pF), C702 and C703 (10 pF each) affect the
frequency at all points. The little 10 pF capacitors are naked mica squares
with silver plates deposited on each side and very thin wire leads. They
corrode easily and in my case they failed. The result was to create an
"error bulge" midway down the dial after both 07+000 and 07-000 were set
dead on.

Miles, K2CBY