[TenTec] PTO Drift Problem

Clark Savage Turner WA3JPG turner@safety.ICS.UCI.EDU
Fri, 27 Jun 1997 09:57:35 -0700

> > 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.

> 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

I understand what you are saying, but most other rigs I have seen do
better from this time period.  My Kenwood TS 130V vfo, for instance, 
drifts less and stabilizes sooner and sticks there.  My old Drake
C line PTO drifted less than this on a very bad day.  They all move
100 Hz or so in the first 15 minutes or so, of course that is acceptable.
I would just like to be able to start my Corsair cold, let it warm up
for a half hour, set in on our local swap net (SSB) and lie back in
my chair and nap a bit.  What always happens is that within the hour,
I can no longer understand the SSB since the PTO has moved and it
sounds really awful.  I can relax with my Kenwood vfo or my Drake
pto, is this really the luck of the draw that I got good ones?

> minutes and then is stable. I would think that 50 hz should be all but
> un-noticeable.

That is true, it is of no great concern.  I think my rough measurements 
are not quite right for every day.  Like I said, after a half hour
warm up, if I set in on 7240 for the swap net, close my eyes, then
within the hour I will have to get up and reset the PTO to hear the
SSB clearly.  Not a problem on my other rigs.

> 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

Good point, I understand this.  I may just be technically impaired, but
was wondering if there were caps that didn't move that much and 
alleviated the compensation problem.  Is it the coil that changes L
requiring the C to change?  Maybe I should know the answer, but I 
don't.  I just know that my little NorCal 40 transceiver, costing me
about $60 to buy the kit, has a vfo that is so stable it is amazing.
Why can't I work my Ten Tec PTO to do this?  If it is the matter of
temp changes to the L in the circuit, I see that I am limited by
design.  I may relax and learn  to live with it, or to go through the
craziness of working the C coefficients if I am so inclined.

Or, I may try putting a small bulb in the PTO to bring the temp up
a bit and see if I can find a cheap way to simulate an oven!  Anyone
try this?

>    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

I am sure you are right, and I respect their efforts at the time of
design and building of these things.  I am just hoping that there is a
way, today, of improving the situation.  

> 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.

I am going to do this next week, just got the kit.  Good suggestion.

Thanks for the thoughtful reply, Jim.


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