If the pto is showing too much bandspread, you need to adjust the end
points. The natural ageing process of the Collins type PTO tends to do just
that. It is a routine adjustment in the R-390 series and the 75A-4.
Typically the end point adjustment is about 3 kHz off, from one end of the
band to the other. In other words, if the dial is right on the mark at one
end, you have to tune about 3 khz beyond the mark at the opposite end to get
zero beat with the calibrator.
There should be a small trimmer adjustment somewhere on the PTO can. This
is a small slug tuned inductor inside the sealed can wired in series with
the main variable inductor in the PTO. You adjust it until there is exactly
1000 kHz of bandspread over 10 turns of the PTO shaft, as indicated by the
dials. When the adjustment is accomplished, the entire kilocycle dial will
probably be off a couple of kHz. You correct this by loosening the shaft
coupler that turns the PTO and rotating the pto shaft or the tuning
mechanism shaft until the kilocycle dial reads exactly zero at the 100 kHz
calibration point.
Usually you will not be able to achieve perfect linearity. The dial may be
right on at the extreme ends, but be up to a kilohertz off at some points in
between. So you usually have to try for the best compromise. I have found
with the 75A-4 that I get better overall dial calibration by setting the end
points to match the dial at 100 kHz above the bottom end of the range, and
100 kHz below the top end. The nonlinearity tends to become more severe and
noticeable at the extreme ends of the range.
The instruction manual describes the procedure. Once you get the hang of
it, you can usually make the adjustment and get it right the first time.
Otherwise you may have to repeat the adjustment several times to get the
best linearity and dial calibration.
Sometimes, a PTO will age past the point where the range can be brought back
with the slug. Then you just have to live with it, and recalibrate to the
nearest 100 khz calibration point. It is possible to take the PTO apart and
remove some of the turns from the slug tuned trimmer coil, and be able to
attain proper adjustment, but I wouldn'n recommend doing that unless you
feel confident about disassembling the sealed PTO unit, working on it, and
putting it back together again. The procedure is described in the Electric
Radio "75A-4 compendium".
Don k4kyv
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