[AMRadio] More on the T-368 exciter

[D. Chester]

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 
permanently.

Don k4kyv

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