Updated 9/15/08
Collins 20V-Series Broadcast Transmitters Live Again On The Ham Bands!
The challenge of obtaining, transporting, cleaning, restoring and re-tuning an AM broadcast transmitter for use on the Amateur HF bands is daunting, BUT VERY REWARDING. The advent of these rigs has transformed and energized the world of HF/HF AM. That's no typo, but shorthand for Hi-Fi / High-Frequency AM!
Those who only see the stunning end-result of a painstaking restoration project might think that using a broadcast rig on Ham radio is 'cheating', but this process combines the muscle, courage, scavenging, machining, home-brewing and radio-electronic skills that go right to the foundation of what Ham radio used to be before rice-boxes made the service a sterile, plug-n'-play affair!
Collins 20V-3 and 20V-2
This is not to take anything away from the wonderful classic Johnson, Collins, Heathkit, B&W, Hallicrafters rigs and other Ham-oriented AM gear, but no sooner had WWII ended than the screws began to turn on the full bandwidth fidelity of manufactured AM rigs. Due to the gerrymandering of General Sarnoff defeating Major Armstrong and other responsible engineers, early television set up shop on frequencies that were entirely too low. This created a harmonic interference issue that caused manufacturers of Ham transmitters to pull in their horns on power and bandwidth.
The second blow came only a few years later when Single Side-Band started to gain traction in the mid-to-late 1950's. The AM transmitter manufacturers chose 'talk-power' over fidelity to 'compete' with SSB with disastrous results. These narrow bandwidth AM rigs lowered the bar so that SSB didn't seem so bad by comparison. There have been countless articles on 'opening these radios up', but out of the box, they literally surrendered their most important marketing advantage over SSB, FIDELITY!
In sharp contrast, broadcast gear built right up to the 1980's were musical, high-fidelity devices. Ironically, due to the advent of NRSC and IBOC, and their strangling effects on AM broadcast bandwidth, vintage AM broadcast rigs converted to Amateur use are the only reminders of just how good AM can sound! They have set a benchmark, attracting a whole new generation to AM operation.
Scenes From A Typical Broadcast Transmitter Restoration
These photos show some of the stages a typical broadcast transmitter goes through to be transformed from, what might look to some like a dirty old furnace or air conditioning unit, into a historical show-piece. Due to their superior construction, robust components and eagerness on the air, maybe these rigs should be called, GO-PIECES!
The interior of the rescued Collins 20V-3- Note the grunge and replacement industrial blowers that have no business in the Collins. The end-caps of the pressure boxes for modulator and finals have been butchered to accommodate these monstrosities:
One of several terminal strips
to be photographed, disconnected and cleaned:
Gutted but still dirty!
Washing is the next step but remember to remove water-sensitive meters:
What a difference some 'Formula
409' and a hose can make:
The front of the 20V-3 waiting to be scrubbed. Any washed parts that can't be sun-dried thoroughly after removing surface water with towels and Q-Tips must be exposed to intense fan-drying for 12-24 hours. This washing process is not just for aesthetics. Billions of mold-spores, bugs, pollen, deadly rat feces and DNA flowed through this rig's air-cooling system over the years. Having those nasties blown around the shack can't be good for anyone:
The tank circuit box, washed and drying in the sun. This module must be lightly modified for 160-meter operation and radically re-configured for 75-meters:
The drilling and tapping of the bottom of these rigs for heavy-duty wheels (casters) is usually done while the rig is lying on its side in the transport vehicle. This simple addition makes a two or three man job of moving the reassembled rig do-able by a crew of one:
Here are the guts reinstalled, looking factory fresh. Note the solid-state rectifiers. These are handy for the process of testing and tweaking. Once the rig is running as it should, it is preferable to restore the original 872B mercury-vapor tubes for that marvelous blue glow, seen clearly through the rig's large picture window. These rigs were built during an era when sponsors would be given a tour of the station as part of the advertising department's sales pitch:
Here is a good view of the push to talk relay system and glowing final tubes, utilized existing attachment points. Notice the end-cap plates fabricated to cover the butchered pressure boxes so that original-equipment fans could be re-installed:
The Collins 20V-3 Up and
Running on 3.870 MHz, ready for action on the weekly,
West Coast
AMI (AM International) roundtable!
This conversion and
preservation project is but one of a growing number.
Here are some useful links that might help you to do the same:
http://www.rwonline.com/reference-room/special-report/rwf-chromeJune20.shtml
http://www.collinsradio.org/html/manuals.html
http://www.k2pg.com/Gatesconv.htm
http://www.cafepress.com/amradio.2748671