[R-390] Stinkin' PTO !!

[John Kolb]

On Sat, 28 Jun 2003, Phil Atchley wrote:

> Good afternoon all.
> Well, the odyssey of the Cosmos PTO lives on.  I repaired the end point coil
> this morning and was able to set the end points F.B.
>
> However, earlier when I checked the linearity at the 100KC points it looked
> 'pretty good, running typically 200 CPS to 1 KC off.  However, the 600 KC
> point was OVER 2 KC out and I figured I could correct this with the
> correction adjustments.  WRONG!

I should repeat a tale I've relayed once or twice before -

I once had a National HRO-500 which was rather beat up.

Some previous owner had adjusted the tuning linearity until
it was perfect NOT. Indeed it was on on the 100 kHz calibrator
points and 25? 50 kHz? However between calibration points
it was as much as 5 kHz out. Looking at the slotted end plate
of the tuning cap, it was bent up to look like a saw blade
/\/\/\.

To tweek it back into shape, I built a neat little crystal
calibrator box. Starting with a 4 MHz crystal oscillator,
I went through a chain of divide by 10 digital dividers,
with a switch to select a tap on the chain, and then a
chain of divide by 2's, also switch selectable. Thus I
could select 4 MHz, 400, 40, 4 kHz, or selecting 400 kHz
switch in divide by 2's to get 200, 100, 50, 25, 12.5 kHz
or selecting 40 kHz, get 20, 10 5, etc. This gave me a
real multitude of spot frequencies to check the dial at.

A perfect square wave would have only odd hormonics in
the output. If you instead of a square wave out of a
crystal calibrator, generate a very narrow pulse, the
od and even hormonics will be almost even in amplititude.
The narrower the pulse, the higher higher frequency output/
Using 74S logic, the output went to almost 100 MHz before
starting to fall off.

With as many adjustment screws as the Cosmos PTO has, it's
necessary to check at freqs other than the 100 kHz xtal
calibrator spots. Such a unit as I've described would be
ideal. With the high speed digital logic available nowadays,
it could be made flat to very high freqs indeed.

John



>
> Taking my time and after tuning on this thing for perhaps 3 hours in a radio
> room with an ambient temperature of 95 degrees (WITH the Air Conditioner on
> full bore) I got up to the frequency of +570 KC (that is with adjustment
> screws centered in the window).  Up to this time the tuning had gone pretty
> good with most adjustment screws only requiring a turn or two in one
> direction or the other. At this point the PTO tuning became very stiff and
> the adjustment screw had NO affect on frequency when tuned either way.  I
> even backed the adjacent screws off to make sure they weren't binding or
> dragging.  So I checked the next frequency, same way for the next couple
> steps.
>
> Stiff tuning and the corrector has NO affect!  At this point I stopped and
> threw in the towel.  (The screws adjusted earlier were rechecked and are
> still good).
>
> As I said before, though it looked nice on the outside, this PTO was no
> virgin, at least around the end point coil.  I suspect that either the
> Teflon disk or the "screw disk" is damaged.  That or there is another
> problem in that area.  I have no intention of going that deep into the PTO.
> The man I got the R-390A from said he has another PTO and I'm going to give
> that one a try.
>
> In the meantime, I have the time to pull the RF deck and check it out
> thoroughly.
>
> 73 de Phil, KO6BB
>
> ****************************************************
> *  DXing THE WORLD WITH THE FOLLOWING EQUIPMENT.   *
> *  RECEIVERS:                                      *
> *    '67 EAC R-390/URR Surplus Receiver with a     *
> *            HAMMARLUND HC-10 Sideband adapter.    *
> *    YAESU VR-5000 Wideband,  REALISTIC DX-392.    *
> *    Home built Longwave "Hybrid" CW receiver.     *
> *    Others which pass thru or are waiting repair. *
> *  ANTENNAS:                                       *
> *    Coil Loaded 18.5 Meter Long Sloper.           *
> *    Roof mounted Hustler 6BTV 6 band Vertical.    *
> *    Scantenna Scanner antenna at 7 Meters high.   *
> *  LOCATION: Merced, Central California.           *
> ****************************************************
>
>
>
>
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