Dick and all,
A great pity that you've quit the project. I am not at all
experienced in electronics, and I went through many of what
you've written about.
I don't know whether you have an oscilloscope, I found that
extremely useful during making the TX. The one I have is a
simple, 20 years old Polish construction. It can hardly display
max. 15MHz signal (you have to be patient with adjusting
synchronization then :)
My first attempt with a signal source was a VXO with 10.000MHz
& 11.350MHz xtals that I simply found in my shack. I thought it
would be possible to pull them in a way that the difference
will be at 1.377MHz. This appeared to be impossible... Don't
ask how much time I spent on making the mixer stable and trying
to pull xtals a little more... Simple little thing and can make
you crazy. In the end, however, I managed to make a QSO
transmitting on 136.5kHz. The first PA had no more than 150W
output and I couldn't have got more due to strange oscillations
and instability. Additionally my output filter used to get
really hot (due to wrong coils?). At this stage I've lost some
nice FETs also.
So I started the whole thing from scratch. I built a beautiful
tiny VCO which ... appeared to be not stable enough for
anything. I really did all my best with it and... a little heat
from the PA stage and I am 50Hz away... Even with this sweeping
VCO I managed to make some nice contacts. A better-planned
output stage and change of output filter into air coils made it
possible to achieve 400W of output power (roughly, it takes 12A
at 39V and there is little loss, apart from the coil on power
supply line).
As for the power supply: the most expensive parts of the whole
TX was a couple of 100V-rated big capacitors: I first thought
that it would be better to use the PSU at 90V. I've been wrong:
I tried this once and this caused rather a nice explosion of
FETs (again). So now I prefer to stay at 45V but with twice as
much current...
Now I am using FT-990 as a signal source transmitting at
13.77MHz, attenuated and divided by 10 (74HC290). This is still
not perfect because the output 137.7kHz signal easily
interferes with the first divider stage. This causes strange
loops and might also lead to a big bang. So during the
transmission there are some additional grounding wires and
shielding, which appears to be just enough.
===
I am still thousands miles from the point where I could say to
anyone in the club: "Here is the TX, try it yourself". I am
afraid it will never be like that: this will never work safe
and solid. At least not the one made by me.
===
What I wrote about were just the main points in the TX
construction, I encountered many smaller troubles at each
stage, really. I think that when you get into problems it is
good to leave the project for a few days, after that you have a
fresh view on the problem and you find the solution quickly.
However I really had a lot of fun and learned a lot with it.
Thanks to members of this group you never stay alone!
73! Marek SQ5BPM
----------------------------------------------------
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