Don’t throw away a TV transmitter; it may
become your next 6m beacon...
by Peter, HB9RUZ
In the
autumn of 2007 in several parts of Europe, Switzerland being among them, many
analogue TV band 1 transmitters stopped operating.
On the one
hand, the hope for 6m operations unhindered by TV birdies and the hope that the
severe restrictions on power and polarisation would be lifted in Switzerland at
that very date were only partially fulfilled. On the other hand, during a
search for parts to be used for my future projects, I had the occasion to dig
into the relics of one of the out-phased TV transmitters. The material I found
there was older than 20 years but of good quality and prevailingly
silver-plated. While raiding for useable parts, I focused on material suited
for the construction of a 50 MHz. p.a. That turned out to be a blind alley
because the parts of the 10 kW tube P.A. of the TV transmitter weighted several
hundred kilograms and even the transistorized driver stage needed a high
amperage power supply. This was kindly offered to me, but how should I move
this around in my flat and the considerable cooling requirements for these
parts, which up to the tube p.a. stage were working class A.
So, I
narrowed my search to smaller modules and that had at least a faint affinity to
HF, e.g. protruding N sockets. The fact that the search had to be performed in
an open trash container on a dark, cold, and wet morning of January neither did
encourage me to analyze in detail what I crammed into my knapsack and what I
had to leave in the wet.
When at
home again and analyzing the objects of my treasure hunt it gradually became
clear, that I had shuffled together almost all of the modules of a complete TV
Band 1 exciter. Admittedly, these modules were designed 20-30 years ago. But at the same time this meant that these
parts had successfully passed a test phase of several decades.
After having
disposed of most of the the modules which I had evaluated to be video modules,
it became clear to me, that not only the modules I thought to be HF, but also
the video IF modules at 38.9 MHz, contained parts suitable for later service on
50MHz.
Later on, I started studying the block diagrams found on the back of the modules,
which I was scanning for parts to be handy for projects on 50 MHz. Suddenly,
these diagrams, at first nearly meaningless, when looked at separately, started
to grow together. What was forming was the method the Band 1 TV video carrier
had been put together. My subconscious mind told me then, that what I had
circumscribed hitherto with the misty term ‘projects on 50MHz’ in reality
meant ‘6m beacon’. Disassembling those
well behaved i.e. 50 Ohm terminated modules to use their parts for the new
beacon would be a sheer waste of time.
Isn’t it that 94.15 - 38.9 is 55.25 and isn’t it that 55.25 is only
separated by a shabby 10 percent from 50 MHz?
It was all
here and it was free of charge: 38.9 MHz oscillator, 94 MHz oscillator, mixer
and pre-driver modules together with a backplane of generous layout and a linear
power supply with enough spare capacity. Even a DC and a RF monitoring module
as well as two 7 pole band pass filters were present. Slowly but inescapably I
couldn’t get rid of the suspicion, that the designer of these well shielded
modules primarily had intended to build a 6m beacon, but ceding to his basic
instincts of pure monetary nature, had diverted it from its intended use and
tendered it to industry as a TV transmitter. To cover this downfall, he had
cleaned up all traces leading to its ham radio origin and had even shifted the
treacherous 50MHz QRG to 55MHz.
Would it be
easy to bring this object back to its original destination? Well, it was more
than just ordering a new crystal from QT
. Bruno, HB9PNR, one of the members of our
HB9SIX operator group, during weeks got very monotone answers from me at his
daily ‘what are you doing’ questions. These went from the pontificating
‘studying a TV transmitters function’ to some less auspicious comments as
‘trying to find out why nothing blew, when I accidentally shorted the +5 to the
+24 V rail’. I would like to stress however, that the functional descriptions
of these modules and the schematic diagrams were given to me by an insightful
soul but several weeks later. It would have saved me much time and trouble if I
had been in their possession from the beginning. Only intending to disassemble
the TV stuff, I initially had not gone for the schematics.
Each of the
oscillators running on a frequency different of the wanted signal makes it
simple to avoid even the slightest trace of a residual carrier. Edi, HB9DRW,
advised me to find the spot most suitable to gate one of the oscillator signals
with the signal from the keyer (model FB1). In addition he
helped me finding out the rough values of resistors and chokes I had to replace
during the paperless modification phase. This was when I found out the hard way
the advantage of combining thermically tightly dimensioned passive elements
with an indestructible power supply. It
unambiguously
showed up the spot, where
something had suffered under my modification attempts. Without a wiring diagram it was not easy
though to find out what the ‘something’ had been before, because the markings
of the element involved in the accident mostly had been discoloured by heat.
After having gained some experience, I proudly explained to my wife, that I was
able now to tell a former resistor from a former choke by the smell of their
burnt remains. Instead of the expected admiration I only got a comment about
better venting the shack.
The
retuning of the intermediate and antenna output filter put my patience to the
test.
Since a specific strategy for tuning of multipole filters was not known to me,
I had to do it by trial and error by continuously watching the effect with a
spectrum analyzer. The availability of the 7 pole residual band video filter
was no luxury, since it enabled to separate the wanted signal at 50.058 MHz from
the nearby oscillator signals, these still being present at the mixers output
port.
As you can see on the photo, the
centre of the panel is momentarily carrying an operating hours counter. This empty space behind it could be used for
a so-called precision offset unit, allowing the wanted signal to be locked to a
DCF77 or GPS derived frequency standard.
What the
manufacturer once claimed to be a TV transmitter could now replace the present
HB9SIX beacon. It weighs 35kg in its protecting flight case from Bullight in
Emsdetten, Germany. Admittedly this
reflects a rather bad output to weight ratio. The TV transmitters pre-driver
module serving now as the p.a. is delivering a meagre 4 Watts of output
power. By shifting its bias point
towards class AB operation, I had been able to increase somewhat its
efficiency. The total mains power consumption still is at about 80 watts,
mainly due to the crystal ovens and the heat dissipating stabilization circuits
of the power supply. It seems that the designing engineer wanted the crystals
to work at 68 degrees centigrade. Again I suspect this to be one more of his
futile attempts to proof that this object had been designed as a TV transmitter
and not a beacon.
The
creation of this spare beacon does late justice to Paul, HB9MFD, who proposed
to modify a commercial set to a beacon when discussing the way to go seven
years ago (see SIX NEWS issue 74). Is a
TV transmitter not the most luxurious of all commercial sets in a sense?
The present
HB9SIX beacon has, with the exception of three interruptions not due to
failures, flawlessly been running for more than 5 years and nobody of us wants to get
rid of it. But just in case of emergency, a spare beacon is now at hand. It is heavy, power hungry, and its nameplate
makes you believe that you are looking at a TV transmitter. But we know better
now…
Band 1 TV
transmitter 3/3W-BD1 by messrs Rohde &
Schwarz, Munich, Germany
Photo by Karl Meier, http://www.meipicture.ch/
back to the summit of the Säntis mountain