Hammarlund SP-600JX17 "Diversity" Receiver Page
The idea of the "diversity" receivers is the following: In the process of getting from the
transmitter to the receiver, the radio waves bounce around like crazy between the ionization
in the upper atmosphere and the ground. The upper atmosphere is rolling and boiling around and
is constantly in motion. This means that two otherwise identical antennas spaced a wavelength
or two apart may receive very different signal strengths or different signal polarizations. One
way to deal with this is to set up multiple antennas, pointed this way and that, possibly with
different directivities and different polarizations, then somehow pick the antenna that gives
you the best signal. Since this changes on a minute-by-minute basis, the process of picking the
antenna with the strongest signal needs to be automated somehow. This is the problem that
the diversity receivers were designed to solve.
The basic idea is that you have a different receiver on each antenna that are somehow tuned
to exactly the same frequency, then you somehow pick the strongest signal among them all
and listen to that. The JX17 synchronizes several receivers by setting one receiver to the
"master" and all the others to "slave." These receivers have extra RF connectors on the back.
You run cables from the master receiver to all the slave receivers. This distributes various
of the internal signals (VFO, 3.5 MHz, etc.) to the slave receivers. Unfortunately, that
is not quite sufficient: you still have to tune all the slaves manually to the same frequency to
align the RF stage tuning.
To pick the strongest signal, you then strap all the AGC voltages together. Thanks to the detector
diode in the AGC voltage generation, the result will be that the highest AGC voltage will
be automatically be the one that appears on the line. Consequently, the gain of the receivers that
are getting weak signals will be reduced and the one receiver that gets the strongest signal will
have the best audio level.
After all this, I conclude that all this is totally useless for most modern shortwave users, since
most of us have barely enough room for one antenna, much less five antennas or even two. Consequently,
the cute red knobs are just for show. Plus, given the design of the VFO distribution, it is likely
that unless they are receivers that I personally restored (more on this below), you won't be able
to make it work because the VFO signal level is much too low. I haven't seem any discussion of this,
but in my not-so-humble opinion, the transformer T35 was badly designed. Plus, in every JX17 I have
worked on (5 for 5 so far) the core of T35 was broken. When I opened the FCU up, bits and pieces
of T35 fell out. The original design has the signal take-off from only two turns of wire on
a torroidal core. The voltage generated is much too low to do anything useful. With a broken core,
T35 will neither send nor receive a signal in any case. I rewind these transformers onto a modern
ferrite core and I put a few more turns on the auto-transformer where the signal is picked off.
**WATCH THIS SPOT**MORE TO COME**