|[AMRadio] R 388|
kk4tr at hotmail.com
Sat Jul 14 15:48:02 EDT 2007
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
----- Original Message -----
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
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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