[TenTec] Solving PTO drift

[Jerry Haigwood]

Andrew,
      You don't need a scope but you will need a frequency counter.  You can
determine the drift is coming from the PTO by hooking a frequency counter up
to the output of the PTO and monitoring the frequency at turn on and then
every 10 minutes or so.  If you record the frequency, you will be able to
plot the drift over time and see where the drift is going.  Does it always
drift down or up in frequency?  Does it settle down after an amount of time?
After you have a plot of what the PTO is doing, then you will have a better
chance of compensating the drift.  Most VFO/PTO's drift upward in frequency.
This is caused by the wire in the coil expanding as the air temperature goes
up causing the distributed capacitance to increase.  To correct this
increase in capacitance, you will need some negative temperature coefficient
capacitors.  Typically N750 caps are used although you could use N330, etc.
You use the N750 cap to replace part of one of the frequency determining
caps in the circuit.  The other part of the cap is replaced with an NPO or
COG temperature coefficient cap.  You then cool the PTO off and then turn it
on and plot the frequency again noting the change.  If the drift is better
but not perfect, add some more N750 cap.  You continue to do this until you
have completely compensated the PTO.  It is an iterative process that is
time consuming.  You can find the process in some of the older handbooks.
Look for "temperature compensating a  VFO."  I have done this process
several times and although it is time consuming, it works pretty well.
Jerry W5JH


-----Original Message-----
From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]
On Behalf Of Andrew Moore
Sent: Sunday, September 04, 2011 8:02 AM
To: Discussion of Ten-Tec Equipment
Subject: [TenTec] Solving PTO drift

Before I start trying to isolate the source of my Corsair II's PTO drift, I
wonder if anyone has suggestions about possible causes.

I'm making a couple assumptions: 1) the drift is due to temperature
(probably a safe assumption), and 2) since the same amount of drift appears
on both TX and RX, the source is inside the PTO.  It's #2 that I'm not
entirely convinced of, since there are shared TX/RX paths elsewhere.  But
given the mechanical nature and sensitivity of the PTO, it seems to be a
good place to start.

Unfortunately I no longer have a scope here so troubleshooting this will be
a little tough.

I'm tempted to install a small quiet, brushless fan to the rig to stabilize
temperature, but I'd prefer to find the cause of the problem instead of a
workaround.

I don't want to go the DDS route yet on this rig. While I understand the
benefits, I'd like to keep this one as stock as possible.  I love the harsh
sidetone, sticky potentiometers and clunky relays.  I just don't love the
slight drift (and I've had stable Corsairs before, so I know it's possible
to get there).

Thanks,
--Andrew, NV1B
..
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