In message Thu, 26 Jun 1997 18:07:55 -0700,
Clark Savage Turner WA3JPG <turner@safety.ICS.UCI.EDU> writes:
>
>> Have you recieved any info on how to solve the PTO drift problem?
> Looked into this a little bit, and no one seems to have the answer.
> My Corsair PTO drifts about 200 Hz in 40 min, then back 100 Hz and then
> stable for a while unless I open the window. A good breeze will
> take me 50 Hz in a half hour it seems.
Ok guys lets look at this in a rational manner. The above amount of drift is
really very good for a PTO from cold to hot. The older rigs usually did much
worse, even the Collins. My Corsair will do about 400 Hz in the first 30
minutes and then is stable. I would think that 50 hz should be all but
un-noticeable.
To Temp. compensate a LC type PTO is a very time consuming and sometimes
difficult task. Most settle for +/- 200 Hz as good enough. There are two
ways to do it. Keep the temp. constant (use an oven-must be on all the time)
or use temp. compensating caps. If you go for the caps, and most do because
of the cost of the oven, then from a cold start with a frequency counter
connected you must plot the frequency change and direction over time. If the
frequency drifts higher then you must change some of the C to a + temp.
coefficient. That is if the freq. goes higher with time in the normal state
then we have to change some of the C to go higher in C with an increase in
temp. Do This by reducing the C value of any N750 or N1000 cap and increase
the value of the NPO or change a N1000 to a N750 or N250. You could also go
to a cap with a P rating. But the fact is that it takes a long time to do
this as for each run you have to allow all the components in the PTO to
return to room temp. after you change C and do the run again and again until
you get it right.
I suspect that TT in design did this on the first few PTO's and
determined the proper combination of N750 and NPO values to get a +/- X
value and then used these values in the production units. However if their
suppliers change the specs. on the caps. then over time the drift specs.
could change. If the rig you have was ok when new but now drifts then
something has changed. Aging of the caps and inductors can cause this, even
aging of the metal parts and wear in the moveable parts can do it. Doing the
rebuild of the PTO may clear up a lot of drift and I would do this first
before playing around with the caps.
73 Jim K4CGY
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