The Horror-Mitters
This page commemorates (for me at least) a project I completed in 1978.
A
working tuneable FM transmitter for the two meter amatuer using only
found junk and components scavanged from old television sets
by VK3ZZC Oct 2016
This
transmitter project was started in the summer holidays of 1977 when I
was still in high school to use with my new Ham licence that
was
issued in 1976. It produced its first signal in March 1978
when the Melbourne VK3RML repeater's squelch opened in
response to me crossing two bits of wire together to activate the TX
circuits. To say that I was astonished as "my abomination"
came to life is not to understate the emotion. It went
through a number of revisions and was last used sometime in 1984 when I
left home to establish my own life. With this device I spoke to
people that I would never otherwise have spoken to, and spoke to people
that resulted in doors being opened , the consequence of that was
the launch of my career in High Technology. My late father kept
this device for me, even though most of my stuff had been discarded, I
think he appreciated the significance of this device to me.
There
are no surviving schematics, the circuit is its own "self documenting"
circuit. Valve circuits are simple and repetitive enough
to not really require a circuit, allthough now I wish I had made
one. The circuit is remarkably advanced and very ambitious to the then
18 year old something me way back then. The rationale for this
transmitter was that back in 1978 I had the so called AOLCP
licence which only permitted operation above 52Mhz, as I was one of the
lesser men who could not learn Morse Code ! I also had no money
to purchase a ready made 2M rig, back then they were expensive !
My only source of components back then was discarded valve television
sets of which there was a steady supply on municipal hard rubbish
collections . The other source was the significant number of
second hand electronic junk retailers of which Melbourne had
about five. Even most junk transmitters were still out of reach
to me financially at that time.
The images show the 2nd
version of the horror-mitters, the revision being required after one of
the mains power transformers caught fire while I was on air. The
circuit consists of a solid state vfo that tuned from 5 to 9 Mhz.
This was mixed with the output of a multiplier chain that
multiplied a random crystal thought I had purchased for forty
cents to about 157Mhz. This was mixed down to
144-148Mhz in a balanced mixer using two sharp cutoff
pentodes followed by two stages of amplification using originally a
12BY7 to drive an RCA 832 tube. This tube was replaced with
a pair of 2E26s. The FM modulation was achieved by direct audio
drive of a varicap diode in the VFO. There was no
pre-emphasis, compression or clipping. A if strip from
another discarded television set was used as my off-air monitor.
I measured the frequency of my VFO using my prized ex-army
C13 set. Thus , after doing bit of arithmetic I could reliably
put myself on chanel at 2 Meters using only my HF set as the main
frequency reference.
Initial alignement and tuning was done
with a long gone grid dip oscillator that had been
callibrated with lecher lines ! Many other amatuers, wealthier
than me, came to visit with their digital frquency counters and
were allways astonished that with even this primitive setup
it was possible to be on frequency with free running oscillators
on 2 Meters. My main problem was drift. At that age I did
not yet understand how to mitigate thermal drift or the need for
regulated power supplys for my vfo. I had to use the
"armstrong" method of frequency lock ! The other significant
problem was the lack of sheilding in the main chassis.
My transmitter was only just barely stable as a result and a
number of QSOs had to be terminated when others informed me that not
only was I triggering VK3RML but Geelong and Ballarat and
others all at the same time ! The other highlight was exhibiting
my complete setup at an 1978 open day at Monash University in front of
an incredulous public. Golly, that was a long time ago.
The
reciever part of the horror-mitters was somewhat remarkable in its own
right. The Army C13 wireless set that had served so well as a
frequency reference was used as the reciever, The RX signal
was mixed down with pre-drive transmitter signal
to produce an IF signal on the same chanel tuned on the
C13. Thus I had automatic off air monitoring and a fairly
sensitive reciever as well. The original down converter had used
a rebiult televsion set tuner with a twin triode cascode
preamp and a pentode mixer. The C13 set had a phase demodulator
that was really too narrow for amatuer FM, so I used AM
slope detection most of the time, but could change to Phase Mod
when the signal fm deviation was small. It was a
hard slog. The pinacle for me was having a two way contact on the
distant Ballarat repeater with this setup. That was rare DX , at least
for me !
epilogue.
finally
its the year 2016. This object had served its creator with a
dividend that I am not capable of computing. I removed a
few souvenirs from the chassis, took these pictures and put it outside
on a hard rubbish collection. There was someone in my neibourhood
that scavanges electronic parts , just like I did 40 years ago.
The chassis vanished in a few hours as I bade a fond farewell to the
past and a prayer of thanks and praise.
