[AMRadio] R 388

kk4tr 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

Joe K4TR

----- 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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