Andrew:
That sounds more like frequency jumps, not drift. In that case I would
look for a cold solder joint or get a PTO rebuild kit from Ten Tec.
73,
Bob WB2VUF
On 9/4/2011 9:40 AM, Andrew Moore 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@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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