[R-390] VFO endpoint adjustment - stumped!

[Jim M.]

Probably a Cosmos PTO.  The little coil you say doesn't change anything is 
the end point adjustment.  Maybe a previous owner turned it too far in 
trying to set the end points and pushed it out of its threads.  The old PTOs 
cores are notorious for changing permeability with age requiring more and 
more aggressive end point tweaking until the "tweak" coil bottoms out, then 
you have to go in and remove a turn from the end point adjust coil.

Is the tuning repeatable from end to end, even though you can't reach the 
one extreme?  Or does it seem like there is a large amount (10's kiloherts 
worth) of hysterisis or backlash as you go up and down?  If so, the little 
disk that holds the linearity screws could have broken loose from the shaft. 
Then you need to read my article posted on Medley's site (it has some bad 
images, I can provide originals if desired).  One way or another it sounds 
like you are going to need to open it up and investigate.  Or buy a 
replacement off of someone on this list.  Goodluck.

Jim M.
----- Original Message ----- 
From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, January 29, 2006 3:38 PM
Subject: [R-390] VFO endpoint adjustment - stumped!


> OK, I'm stumped.
>
> I'm putting my yellow striper back together. Right now the VFO covers
> a range of 2.500MHz to 3.6MHz. At the low end it hits some sort of
> apparent mechanical stop. Didn't hit anything at the high end because
> I don't need to turn it that far :-). It is supposed to cover 2.455
> to 3.455 MHz (and with 25kHz of under-run it ought to go to 2.430 MHz)
> just as a reminder.
>
> Even though it doesn't span the right range, the linearity is pretty
> good, and in the first ten turns it is always within 0.5kHz of being
> on the money.
>
> I look in the Y2K manual, but the endpoint adjustment screw illustrated
> there does not match my unit physically. There is a plug-screw in mine,
> but it's on the left hand side, outside the triangle frame as seen from
> the front. The one in the Y2K manual is shown as being inside the
> triangle frame and a bit on the other side.
>
> Inside the plug-screw, I find some sort of slug that can be turned
> with a 3/8" screwdriver bit. The slug seems to be hollow and probably
> brass. Turning it does nothing, doesn't budge the frequency at all.
>
> There is another access hole, inside the triangle frame, that has a bunch
> of little set screws in it that slowly scroll past as I turn the dial.
> I think these are obviously the linearization screws. I do not want to
> screw up the linearization of this PTO, I am very happy with it as is.
>
> Oh, and I pulled the metal rectangular cover from the box, and found
> some resistors.  Hmm, that 2.2k resistor looks a little suspiciously
> brown, better check it out.... actually 8K!!! This makes the score on
> this radio for a total of seven 2.2K resistors that were too high
> in value by hundreds of percent! (Almost all of them in the plate lines).
> So I fixed that.
>
> So how do I adjust the endpoint on this PTO? No maker's label on the 
> outside,
> but there is a refurb sticker from Raytheon, indicating a service date
> of Feb 7, 1976. At one point I brought it up and I think the suggestion
> was that it was a Motorola? I could be recalling that incorrectly.
>
> Tim KA0BTD
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