Thanks Don. I was hoping that this was no the case. I have taken a few of
these things out and adjusted them. I t is a real pain in the butt to do and
was hoping that someone would come up with something else that might cause
the problem that I described. I just do not feel like doing it to this unit
as it belongs to friend and it is a very late model. The wiring under the
chasis is pristine and the is also very clean there. I am a bir
apprehensive about doing anything to the radio that involves major
surgery!!!!! I guess I will have to try though. Thanks for the advise. This
is exactly the kind of info that I was looking for. Someone else that had
the same thoughts on the mattter as I did!!! Thanks
Joe K4TR
----- Original Message -----
From: "D. Chester" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Saturday, July 14, 2007 3:11 PM
Subject: Re: [AMRadio] R 388
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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