Hi Evan -
We've discussed this before, but something just (so I'm slow...)
occurred to me.
Setting the TR-7 to a dial frequency, as measured by the internal
counter. I've gotta think about this, or perhaps someone can explain it
more clearly, BUT ...
The reference for the signal 'generating' oscillators are the SAME
reference oscillators that control the counter, including the counter
reference and gate timing circuits. So there are all kinds of
inter-relationships between these signals that determine what you see on
the dial.
I think a better test would be to tune in a known stable source, such as
WWV, (or a stable, crystal controlled local oscillator,) and use an
external counter to measure the audio beat note over time. Every 7 Line
unit I've ever 'looked' at, if there was any drift at all, was UP in
frequency. So if you set the dial for a 400 Hz beat note, you'd have at
least a 2 kHz range for measurement. IF you're not sure, or have one
that drifts down you could start with a 1 kHz beat note. Using this
method, you'd have a minuscule error from the ionospheric variations on
WWV, and a negligible error from the BFO stability, but both are
insignificant for these measurements.
Some of the audio programs such as Spectrogram, Wavosaur, etc. have
recording capabilities that would make it even simpler.
I also wonder what takes so long to stabilize in a TR-7 with a fan on
it. The oscillators are all pretty much 'out in the open', so I would
expect it to settle down within an hour.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-B, C-Line&
TR-4/C Service Supplement CDs
<www.k4oah.com>
K9sqg wrote:
Fellow Drake Enthusiasts,
The discussion of the VFO stabilizers prompted me to describe what I
have observed about 7 line PTOs over the past 25 years or so.
At the risk of offending, I would like to express some observations I
have made through tests of my own equipment, TR-7s (over a dozen or
so), TR-7As (three), R7s (two), and R7A (one). My experiences have
indicated two groups or classes, if you will, of drift
characteristics. One set of equipment appears to drift well under 50
hz per hour, while the other set can drift 100-200 hz per hour.
Stabilization varies from a few hours to almost 24 hours. Note that
I can't be precise since (1) I wasn't using an external counter with
known accuracy, (2) the resolution/display of the DR7 is 100 hz, and
(3) I didn't make detailed records of all the equipment for archive
purposes, from my initial tests. Having said that, let me present one
record I did make recently of one of my TR-7As. Frequency was set via
the PTO dial approximately midway between the next highest and next
lowest 100 hz readouts. The times are Zulu and the frequency was on
40 meters, normal ambient room temperature, from a cold start. No
transmit cycles were used. A cooling fan was used to exhaust air from
the rear of the rig.
1340 hrs 7258.0 kc
1348 7258.1
1410 7258.2
1425 7258.3
1458 7258.4
1541 7258.5
1551 7258.5
1627 7258.5
1726 7258.6
1838 7258.6
2352 7258.7
0733 7258.9
1208 7258.9
1343 7258.9
1421 7258.9
1528 7258.9
1824 7258.9
From the above observational data, you can see that the PTO did
eventually stabilize after about 18 hours or so, for the given TR-7A,
in receive only, no transmit cycles. You can see that the initial
drift was about 200 hz per hour for the first three hours. The other
"class" of PTOs drifts substantially less and stabilizes sooner. I
have not done any investigation to see if this observational data
correlates with serial numbers on the rigs. In comparison, I have
observed that the RV-75 external VFO keeps drift well under the 100 hz
resolution of the DR-7 for days at a time, how much I don't know.
This email is for informational purposes only and is not meant to
offend anybody. I realize your experiences are likely to be different
and I present mine for what they are worth.
Have a great week and enjoy those Drakes.
73,
Evan, K9SQG
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