Rebuild That Relic of an AM Transmitter
Many people think old
AM transmitters are worthless.
If a high-level, plate-modulated tube transmitter
is replaced by a shiny new solid-state transmitter, why should the old one be kept?
Heck, you might get $500 for the relic.
Well, I am here to tell you that the
new whiz-bang transmitter should be backed up by something that runs when
One station owner told me the worst
management decision he ever made was to get rid of his old transmitter after
the new one arrived. He paid for that decision by being off the air for three
Solid-state transmitters need to see a
50-ohm load with little reactance in order to run properly. Old tube
transmitters can run into a “coat hanger,” as many veteran engineers would say.
If the AM antenna is misbehaving, a tube transmitter can be your savior while
antenna repairs are made.
Most transmitters sold today are touted
as being very reliable because a power amplifier module can fail and the
transmitter will stay on the air at reduced power. That is fine until there is
a logic problem, a power supply problem, a driver problem, an output network
problem or some other show-stopper.
100% Modulation. Tubes with good
emission give low-distortion results.
Having grown up in a tube transmitter
era, I really appreciate the simplicity of transmitters from the 1960s. Probably
the best example in my mind is the Gates, now Harris, BC-1G. It is a 1,000-watt
transmitter using 833A tubes that really do glow in the dark. One of my clients
bought two of these for use as backups for his newly purchased solid-state
transmitters at two new AM stations he constructed in recent years.
Warning! If you decide to take
on a project like this, and if the transmitter you are going to work on has
PCB-filled capacitors or PCB-filled transformers, you really want to replace
them before turning the power on. The same goes for mercury vapor rectifier
tubes. Anything else in an old transmitter is harmless except for the 2,500 or
more volts when the high voltage is turned on.
The first part of a transmitter rebuild
project is to test the transmitter and make repairs necessary to get it running
to factory specifications on the original frequency into a dummy load. Be sure
to spend some time cleaning and replace any questionable components that you
can spot with your eyes.
125% Modulation. The audio is turned up another
Most transmitters from the 1950s through
the 1970s came with a tuning chart in the instruction book. Capacitors values are
listed for every frequency on the dial. Also shown is the number of turns
necessary on each of the tuning coils for each frequency, or group of
If your project involves changing
frequency, you will need a crystal for the RF oscillator. Back then, they were
known as “fundamental” crystals that oscillated at the frequency at which the
transmitter was intended to run. Later transmitters used less expensive crystals
that were two or four times the operating frequency of the transmitter. That
higher frequency was divided down to the final operating frequency. Fundamental
crystals are available from International Crystal www.icmfg.com for operation at 1 MHz and above, or from JAN crystal
www.jancrystals.com for any channel
on the AM band. You might even find what you want at commercialradiocompany.us.
I start a tune-up at the crystal
oscillator before turning on the high voltage at the lowest transmitter power
setting. Once things look right at low power, I go to high power and really
make it sing.
On AM transmitters, there is more to
making them run than getting power at carrier. I use an oscilloscope on an RF
sample port at the output of the transmitter to watch the modulated envelope
when an audio oscillator at 1 kHz is connected to the audio input of the
Tubes with good emission give
low-distortion results as shown on the “100 percent modulated photo.” Running
regular programming audio tells me almost nothing about the audio performance
of a transmitter. Another photo shows the audio turned up an additional 2 dB. You
see the carrier go down to zero for an extended period of time and the positive
peaks go up with the additional audio. That is +125 percent positive
If the transmitter design is
less-than-great or the tubes are a bit low on emission, you might have to be
satisfied with 100 percent positive modulation. I write this assuming the
transmitter will be used on the air only in emergencies.
Best to run the old transmitter at
least once every three months. You will really appreciate the value of a backup
transmitter the first time you use it on the air. Sure, it is not as energy
efficient, but it will reduce the sweat on your brow by a factor of 100 when it
is in on the air.
Mark Persons, WØMH, is certified by the Society of
Broadcast Engineers as a Professional Broadcast Engineer and has more than 30
years experience. His website is www.mwpersons.com.