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@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@contesting.com [mailto:tentec-bounces@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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