[R-390] Adjusting Linearity Stack Collins PTO

[Bill Smith]

Has anyone tried this?

1.  Set up a spread sheet with a list of frequency entries every 50 kHz.
(nominal) from 3.455-2.455 mHz

2.  Rotate the PTO from one frequency end to the other.  At mechanical 50
kHz intervals, using a counter, document the measured PTO frequency.

3.  Remove the cover.

4.  Repeat step 2, above with the cover off.

5.  Subtract the frequency differences steps 2 and 4 for each measurement in
the list.  The result should be the error due to capacitance contributed by
the cover if this scheme is to have any merit.

6.  Add (subtract) each result obtained in step 5 to each frequency in step
1.   This list becomes the calibration standard for the PTO.

7.  Calibrate (adjust) the PTO at each mechanical 50kHz stop to the
frequencies obtained in step 6.  This is a predicted error + frequency so
that when the cover is replaced, the error will be compensated by the cover.

8.  Replace the cover.  If the calculated error values are correct, the PTO
should now be linear.  Hopefully you won't have to go through a second
calibration cycle.

73 de Bill, AB6MT
billsmith@ispwest.com


----- Original Message -----
From: "Dave or Debbie Metz" <metzd@intelos.net>
To: <r-390@mailman.qth.net>
Sent: Friday, December 27, 2002 3:00 PM
Subject: Re: [R-390] Adjusting Linearity Stack Collins PTO


> At 09:04 AM 12/27/2002 -0500, Jim M. wrote:
> >Thanks...I found this link, and also some references to Electric Radio
> >articles, doing a Google.com search (an amazing thig).  Note:  The PTO
> >alignment discussions on the r390a.com page appear to discuss only the
> >Cosmos manufactured PTO.  The linearity adjustment for Collins mfgd PTOs
> >is considerabily different.
> >
> >http://groups.yahoo.com/group/r-390/message/4718
>
> Jim,
>
> Right you are on the difference between collins and Cosmos.
>
> A long time ago, I tried it and had limited success on a PTO that was out
> 6-8 kc with a procedure that went something like this:
>
> Take a junker PTO and cut a slit in the inner can above the camstack about
> a 1/4" wide to allow you to insert a piece of insulated something to push
> on the cam stack just above the roller.  Then, adjust the tightening screw
> so that you can move the stack. First, you need to find the worst
> offset  from end to end such that you are only going to push the stack
down
> to either increase or decrease the freq. ( could never quite figure out
the
> way to pull the stack up to go the opposite direction in frequency.)
Then,
> mount the pto into the receiver with only the inner slotted can and push
> down at each 25khz increment to either raise (or lower, I cannot
remember).
> the freq to the desired freq. I only used the inner can with the slit.
This
> creates a bit of a problem because you have no rear mount so you have to
> brace it somehow. You should "T" in a counter to measure the freq with a
> counter. Remembering the PTO goes from 2.455--3.455 inversely as the freq
> is acending you need to set up a chart to know what is the desired PTO
freq
> @ each 25khz indicated.
>
> I am sorry for the lack of detail but it's been a few years and I forgot
> the details.  I am sure there must be a better way. However, I was  not
> able to figure a way to adjust the camstack short of pulling the PTO
> hundreds of times with a trial and error method. I seriously doubt that
> Collins did anything trial and error.  I am so in awe of the engineering
of
> this setup. What a sophisticated way to linearize a very delicate
oscillator.
>
> I know that there has been a lot of talk about the PTO manufacturers here
> on the list but a long time ago I had a conversation with Paul Zechinno
at
> Mil Spec Communications, (Not Rick Mish) and he was less than
complimentary
> on Cosmos compared to the Collins PTO's. He had years of experience with
> these and felt that the Cosmos was probably fine when it left the factory
> but would not weather the years like the Collins units in the non A 390's.
>
> However, bottom line: Don't do it!  If you  have a PTO  within one or two
> KC, you have NO problem. It never was perfect and I doubt seriously that
> you can achieve anything better.
>
> 73's
> dave
>
>
>
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