[R-390] COSMOS PTO Calibration

[Jon Schlegel]

GM Pete and all,

Point well taken.  After spending time considering what was done to T 
comp these oscillators, simply removing one of those delicate caps 
does seem a bit provocative.  I don't know what temp specs were 
required of the PTO but the needed temp range had to be greater than 
the ~20C over which you tested.  A 10 Hz shift is certainly minimal 
and would be especially so if linearity was not hurt and the end 
points over temp did not significantly shift relative to each 
other.  Operationally, the change over temp would then be just a 
small offset of dial calibration over temp; easily correctable by the 
calibrator.  Perhaps swap the NPO for the N750 and run it over temp 
again to see how it affects T comp.  Bottom line - looks like you 
have a viable spare.

If you would like better temp performance, observe the necessary 
change in C and then come up with a new combo of T comp caps to 
maintain the expected original change in C over temp.  This assumes 
that although the C value of the 370 pF stud mount cap may have 
changed, it also assumes that the temp coefficient of that cap did 
not change.  At this point though we are rather deep in an area of 
diminishing returns...

Another thought about T comp needs is that it's my understanding that 
many of these receivers were used for RTTY and temp performance may 
have been driven totally by those requirements.  Basic PTO temp 
performance AND the oven may have been needed for long term "set and 
forget" tuning stability.

Best regards,
Jon Schlegel
WA3MVM


At 04:52 PM 5/20/2008 +1000, you wrote:

>List.. g'day.
>Calibration /adjustment of the COSMOS pto has been covered fairly
>extensively and removal of one turn from the endpoint coil appears in most
>cases to allow correct 10 turn spread.
>However , having decided to check out one of my spares , I found the
>situation where one turn off the  coil and adjustment of the slug did not
>bring the  end points within spec--- about 5 kHz low.
>Reasoning that one or more of the capacitor components had  gone high, I
>snipped out one of the miniature 10 pf across the whole inductance .
>Result is that the PTO endpoints, after processing, now accurately set and
>linearity spot on.
>No doubt the purists will take me to task on this as the 2 miniature caps in
>parallel: one is  N750 and the other NPO.... I could not identify which one
>I removed . The posibility too ,is that the large  screw mounted 370 pf  has
>changed value cannot be  discounted, but the effort on checking this  and
>even getting a replacement made the  previous option a more viable
>proposition.
>Stability  before  and  after appears not to have  changed, and heat
>treatment from  room temp. at 50 F to 90F only changed the freq. by a max of
>10 Hz.
>The testing was done with a maximum of  convenience...on the bench using a 2
>voltage regulated variable supply giving the 150 and 250V required and  the
>output loading to a frequency counter.
>
>Any comment appreciated together with caveats on the empirical procedures.
>No doubt there is the possibility that if unstable the   cap or whatever
>might revert to normality or worse--- but them's the breaks !
>
>Cheers
>Pete
>
>
>
>
>  Pete D.Williams
>jupete at bigpond.net.au
>METUNG 3904 AUSTRALIA
>
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