|[AMRadio] More on the T-368 exciter|
k4kyv at charter.net
Fri Mar 27 02:07:38 EDT 2009
It is my understanding that the heating element that envelops the PTO in all
these rigs is designed for operation under extreme temperature conditions,
and it is not meant for improving stability at normal ambient temperatures.
The drift problem is caused by turning the oscillator stage off during
receive periods and starting it back up at every transmission. It takes a
few minutes for the components inside the can to come to constant operating
temperature. Running the tube filaments all the time won't cure the
problem; you need to run the oscillator plate voltage all the time too. My
75A-4 does the same thing when I first turn it on from a cold start.
Isolating the oscillator stage well enough that I can keep it running all
the time completely cured the problem with mine.
Another cause of drift in both my exciter PTO and the 75A-4 is slight
variations in oscillator tube filament voltage due to line voltage changes.
I found that even a change of only one or two volts from the nominal 115
volt mains voltage was enough to cause noticeable drift. So I run my T-360
PTO unit tube filaments off a regulated 6-volt DC supply that I happened
to have on hand. For the 75A-4 I run the whole receiver off a miniature
Sola constant voltage transformer that I found at a hamfest. It was
probably manufactured sometime in the mid 1980's, and was designed to run
a personal computer in an office setting, so it is very quiet, without the
loud buzzing noise that most of these transformers are notorious for. It is
rated for 75 watts, just about exactly what the 75A-4 draws.
The T-368 drift shouldn't be enough to hurt anything when running AM, but it
can be a problem with CW on the higher frequency bands, since the oscillator
frequency is multiplied for the upper ranges.
I once tried connecting up the heater in one of my R-390A's. It actually
worsened the frequency wandering as the thermostat tripped on and off. It
takes a substantial temperature variation to activate the thermostat.
One possibility might be to disconnect the thermostat and run the heater at
reduced voltage during stand-by periods, using a relay powered by the
station's transmit-receive switching system. Feed the heating element from a
variable voltage power supply, perhaps a 24v transformer running off a
variac, and experiment to see if there is a voltage setting that exactly
compensates for the cool-down while the oscillator is on stand-by. Then a
small transformer with the proper output voltage could be wired in
This message was typed using the DVORAK keyboard layout.
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