Dale -
The specification from Drake is
"Total drift is less than 100 Hz after warm up. Total frequency change
is less than 100 Hz for a ± 10% line voltage change."
I haven't done a check on the TR-4C, but the above IS typical of
separates. "How long is warm up"? Not specified, but I'd say 30
minutes, maximum. The TR-4C IS a lot of tubes packed in a small
cabinet. Keep in mind the original design was for mobile operation. so
it was made as compact as possible. This greatly increases the heat
density of the package.
The specification at least implies that worst case is less than 200 Hz
including a 10% change in line voltage. The Zener regulator has drawn a
LOT of criticism over the years, but it really IS a near ideal of a
"current source" to drive a Zener, which is the best case. Since you
didn't record line voltage, we don't know if this was a factor or not.
Certainly a fan helps to equalize the temperature throughout the radio,
but it MAY be exacerbating the problem. If nothing else, it increases
the "warm up" time, AND makes it more susceptible to room temperature
changes.
My questions would be
1. What happens if you turn the fan OFF and start from cold?
2. IS your line voltage changing? By how much?
3. Was the RIT turned OFF? If it was ON, varactors are not
particularly stable and the voltage source for the RIT pot becomes
another variable, even if the pot is not touched. Even if the RIT is
OFF, the output of the analog switches is still connected, and these are
known to become "noisy".
4. Does the frequency drift smoothly, or are there small "jumps"?
I would say this is NOT typical of a Drake PTO. However, the RIT PTO
is a "new" design (sounds funny 30 years later!) and was used ONLY in
the TR-4CwRIT and the TR-7. It does have more potentially "drifty"
circuitry than the "original", especially in the varactor and associated
analog switching IC for the RIT function. If you are so inclined, it
would be interesting to see what results were obtained with the RIT
disconnected (small terminal on the side of the PTO can, just behind the
drive mechanism). The varactor itself would still be connected, which
is a potential source of drift over temperature, but should be smooth
and probably in only one direction over room temperature changes.
There is at least one knowledgeable Drake person who claims there are
two classes of TR-7 PTOs, "those that drift, and those that don't". I
have four TR-7s and two R-7s and while I have not specifically measured
any of them, all seem to move about 100 -150 Hz (on their own digital
readout) from a cold start, and then stop. I am 95% CW, which is
certainly less demanding as far as short term stability is concerned
than SSB, but I haven't been conscious of a change in CW tone in fairly
long-term QSOs, and 150 Hz would certainly be detectable. I have also
used them on PSK-31, which is far more demanding in that regard,
although the tracking feature would probably take care of this type of
drift.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
DALE PARFITT wrote:
I have been plotting the PTO freq vs time (HP5385A counter) and notice
the PTO initially drifting lower 60Hz in the first 5 minutes then
moving up over the next hour 360Hz (from the starting point) then
lower again in the next hour- 130Hz from its high point. In the 3rd
hour it starts moving up again.
My question is this: is this normal, and secondly is some of this
being caused by Vcc fluctutations? I notice the regulator is a simple
zener- and they have their own TC curves.
I don't really feel this is too bad, but considerably wider drift than
my KWM-2A and Halllicrafters Cyclone.
I run a fan 100% of the time and this was under RX only condx.
73,
Dale W4OP
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