[TenTec] Solving PTO drift

[Michael Wilke]

Andrew,

I have had the same problem with my Omni-A. I`ve come to suspect that the
problem is more mechanical than electrical and may have to do with leaving
the PTO sitting on one portion of the band. The PTO was rebuilt by Tentec
about 10 years ago, but the rig sat unused for several years. It is now in
regular use, but will occasionally exhibit the freq shift you describe.

After investigation, I have found what helps more than anything else is to
exercise the PTO from on end of the band to the other and when not in use
leave the PTO somewhere other than where you normally operate.

This after various electrical checks and calls to Tentec.

73,

Mike  WB4AQL

On Sun, Sep 4, 2011 at 9:40 AM, Andrew Moore <andrew.nv1b at gmail.com> wrote:

> Thanks Jerry - very helpful.  The drift isn't so much a steady increase or
> decrease over extended time, but rather, both up and down frequency
> variation by +/- 100 Hz or so, within a span of several seconds.  This
> continues intermittently, even after the rig has been on for half an hour
> or
> more.
>
> I'm going to see if I can find a donor PTO that I can take some time to
> thoroughly rebuild, so I can continue to enjoy the Corsair II in its
> current
> state in the meantime.
>
> Thanks & 73,
> --Andrew, NV1B
> ..
>
>
> On Sun, Sep 4, 2011 at 9:30 AM, Jerry Haigwood <jerry at w5jh.net> wrote:
>
> > 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
> > ..
> > _______________________________________________
> > TenTec mailing list
> > TenTec at contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
> > _______________________________________________
> > TenTec mailing list
> > TenTec at contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>