Eddy -
Great info. Actually this confirms what I would expect to happen. What you do with the fan is
provide a constant temperature, whatever it is, much better for short-term stability.
The other oscillators involved in the output frequency are the Carrier Oscillator and the Band
Oscillator. Both are crystal controlled, and probably stay within 10 Hz or less over the limited
temperature range involved.
One more run on 40M?? Transmitter keyed, minimum GAIN control.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Eddy Swynar wrote:
Hi Again All,
At the specific suggestion of Garey, I went ahead& completed a 3rd stability
test on my Drake T-4X transmitter PTO...
I essentially repeated everything that I did in the very first test, i.e. I used the ICOM
751A as an umpire, both it and the T-4X were started at the same time (i.e.
"cold"), and the ambient basement temperature was 59F---but this time I LEFT
THE AIR EXTRACTING COOLING FAN ATOP THE FINAL AMPLIFIER OF THE T-4X CAGE TURNED OFF.
Again, I measured the frequency a total of SIX times, i.e. (1) immediately at
the start of the test, (2) 3 minutes into the test, (3) 15 minutes into the
test, (4) 30 minutes into the test, (5) 1 hour into the test, and finally (6)
two hours into the test. The actual PTO frequency itself was monitored with the
751A in general coverage mode...
The results with this third (and final---I promise!) test were most interesting
(NOTE: for comparative purposes, I have square-bracketed---[ ]---the changes
observed at the very same time intervals in the FIRST test, at the end of each
observation line). Here goes:
(1) 5455.0-KHz (cold start)---[0.0-Khz];
(2) 5455.0-KHz (unchanged)---[0.0-KHz];
(3) 5454.3-KHz (downward drift of 700-Hz from cold start)---[500-Hz];
(4) 5453.7 KHz (downward drift of 1.3-KHz from cold start)---[1.0-KHz];
(5) 5453.2-KHz (downward drift of 1.8-KHz from cold start)---[1.2-KHz], and,
(6) 5453.2-KHz (unchanged)---[0.0-KHz0
The end result in terms of PTO drift with the fan off was 1.8-KHz, versus
1.2-KHz with the fan on. Interesting! That's a difference of 600-Hz, or HALF of
the drift experienced with the fan running!
It's obvious that proponents of utilizing cooling fans for their Drake
transmitters are on to something, alright---but the benefits are NOT restricted
merely to prolonged tube life: PTO drift is reduced by one third, as well.
My guess is that the cooler environment afforded by the fan to the entire "works" of the
transmitter hastens the PTO in achieving its "ambient operating temperature." Because its
environment is more directly controlled with the fan, the PTO has less opportunity to diverge in as
great a frequency swing as it normally might, as may be evidenced by the results when no fan is
used.
Anyway, I'm still attempting to wrap my head around all of this, and I certainly do welcome any&
all feedback& comments from the readership. One final point that bears repeating, as was
mentioned to me by no less than two subscribers to the Reflector: the Drake manual specs for
stability are quoted for THE TRANSMITTER AS AN OPERATING WHOLE, and NOT just for the PTO alone. The
final transmitted frequency is a mix of many different frequencies, from self-excited
oscillators,& from crystal control. As the TV advertisements state, "RESULTS AT HOME MAY
VARY," Hi Hi.
This has all been a most interesting exercise, nonetheless---I only hope that
my ramblings herein have NOT inspired readers to unsubscribe from the list..!
~73~ de Eddy VE3CUI - VE3XZ
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