[R-390] Adjusting Linearity Stack Collins PTO

[Jim M.]

I like this spreadsheet idea!  I already have an Excel spreadsheet from
when I did my Cosmos PTO last year.  It shows the expected frequency
output at each quarter revolution.  Think I will do this for the
Collins, once I muster enough umph to get started.  I also have a wooden
test jig I made where the PTO mounts and a large calibrated wheel (a
circular scale from a PDF file I downloaded from another site).  This is
how I know how much it is off.   I will try to take some digital pics.
and post them as I go.  Thanks to all the replies here I think I can get
started.  We'll see how it goes.  Stay tuned.   73 Jim N4BE

Bill Smith wrote:
> 
> 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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> > R-390@mailman.qth.net
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> 
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