[AMRadio] R 388

D. Chester k4kyv at charter.net
Sat Jul 14 15:11:14 EDT 2007

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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