Today there will be a quiz. Ready? Look at this picture and see if you can spot the problem:
Problem with Harris SX2.5A transmitter
If you said “Hey, that green wire seems a little odd; it disappears behind the heat sink next to that screw then reappears again at the top,” you are correct. What really sucks is the green wire is the transmitter off connection to the remote control. So, when the PA board was secured to the heat sink, the wire was trapped between the board and the heat sink. Since the components were cold, it did not pinch through the insulation right away, no. Rather, after the transmitter ran for several hours at full power, it got hot enough to displace the wire insulation and cause a short. Doh! The transmitter is off and it won’t come back on!
This is a picture of the wire after it was removed:
Pinched wire
Haste makes waste. Unfortunately, it someone else’s haste that ruined my Saturday afternoon when I was supposed to be taking my son to little league practice. I am sure that some not so kind words will be exchanged very soon.
Radio Caroline went on the air forty nine years ago this weekend, broadcasting from the MV Caroline off the coast of England. Why is this important? Before off shore broadcasting was attempted, in Europe the only radio stations (and TV) were government owned. As such, they had a monopoly over the air waves and were very restrictive on which groups or types of music they allowed to be broadcast. Many of the so called “British Invasion” groups like The Beatles, Rolling Stones, The Who, The Kinks, etc got their first airplay on offshore radio stations like Radio Caroline or Radio London.
This video “Radio Caroline – A day in the life,” shows what it was like to be an offshore broadcaster:
By the haircuts and music, that appears to be sometime in the eighties.
Check out the Radio Caroline website for more information. From 1983 onward, Radio Caroline was broadcast from the MV Ross Revenge. This is an overview of the Ross Revenge transmitter hold. The movie “Pirate Radio” is loosely (very loosely, by most accounts) based on Radio Caroline/Radio London composite.
Radio London was the one of the other well known offshore radio ships.
I am sure that there are other tribute sites with lots of technical information on how they broadcast. Much of offshore radio was outlawed in the late 1960’s by several European countries. Radio London signed off August 14, 1967. Radio Caroline continued on in various iterations until about 1991 or so.
WBCQ is airing a radio ships special on Sunday March 31, 2013, 5,110 KHz starting at 6 pm eastern daylight time (2200 UTC).
I’ve been away working in Burlington, VT (WVMT, 620 KHz, Burlington) for the last coupla, installing this nifty Nautel transmitter:
Nautel XR6 transmitter, WVMT Burlington, VT
I like the Nautel units, both AM and FM; they are well designed, well built, rugged transmitters. I have lost track of how many of these units we service in the field, partly because they are becoming pretty much standard equipment at all of our installations.
Continental 315R-1 AM transmitter, WVMT, Burlington, VT
The transmitter it is replacing is a Continental 315R-1, which is based on the Collins Power Rock design. It is a PWM transmitter with a 15,000 volt power supply. In their day, these were not terrible transmitters, however, like their Harris MW-5/10/50 PDM brethren, frequent thorough cleaning is required to keep the dirt/dust from arcing over. Unfortunately, it is becoming more and more difficult to obtain parts for these units. This transmitter was installed in October of 1983, thus, almost thirty years of service is quite enough. This unit we did not cut up and scrap, rather, it is sitting by the back door, waiting for any takers.
Continental 315R-1 modulator/RF sections
The interior of the Continental 315-R1 transmitter. Modulator section is on the left, RF section is on the right.
The good news is, WVMT is another one of those “successful AM station” stories. You know, the kind of station that has local programming, local sports, news, community presence and most importantly, makes money. For all those diligently studying the “AM Problem” for the up and coming NAB conference this April, here is a clue: It’s the programming…
Nautel XR6 transmitter, WVMT Burlington, VT
This is the Nautel XR-6 on the air. Positive peaks, anyone?
AM modulation monitor
We turned that down a little bit. Also, the station does not run AM stereo, the AM stereo mod monitor is simply a usable relic of a bygone era.
WVMT is noted as the first radio station licensed to the state of Vermont, signing on on May 10, 1922. It has a three tower directional array located down in the swamp. For some idea of perspective, it is 1,150 feet (350 meters) from the transmitter building to the center tower, the towers are 411 feet (125 meters) tall spaced 405 feet (123 meters) apart.
WVMT three tower directional antenna array, Burlington, VT
WVMT antenna system from back of transmitter/studio building. That is a long walk over rough terrain in the middle of the night or anytime really, but especially in the middle of the night.
I found a 1981 Continental Electronics equipment catalog at an old transmitter site. These finds are great if one is interested in history and looking at the way things used to be done. This particular transmitter is a 2,000 KW (2,000,000 watt) medium wave unit:
Continental Electronics D323C, Circa 1981
I believe most units like this were destined for use by government broadcasters either the middle east or western Europe. I know there were several 1,000 KW medium wave stations in West Germany at one time. The Continental transmitter is basically two 1,000 KW units (323C) combined. They used a modified version of Doherty modulation, that is called “Screen and Impedance,” which accurately describes how it works. More information from the Continental Catalog can be found here: Continental D323C. The tubes (or valves depending where you are located) used in the D323C were 4CW25000A tetrodes as modulators and IPA the final was a pair of X2159, which is an impressive tube.
EIMAC X-2159 water cooled power tetrode
The tube sat anode up. The filament, grid and screen connections are underneath. Cooling water was pumped through the two connections on the top at about 130 gallons per minute depending on the plate dissipation. With a 30° C rise, that equals about 96,000 BTU per minute. The D323C had a dissipation of 400,000 watts for the carrier tube and 240,000 watts for the peak tube (640 KW total) under 100% modulation. That equals about 2 million BTU per hour. Notice the lifting hook, this tube weight in at 175 pounds. Tube date sheet here.
Continental no longer makes medium wave transmitters, their closest high powered broadcast product now is the 418/419 and 420 HF (shortwave) transmitters. The 420D does a wimpy 500 KW using a solid state modulator section.
I remember in the early 1990’s when I was at the Harris plant in Quincy, they were working on a 1,000 KW solid state DX series AM transmitter for Saudi Arabia. It had to be liquid cooled, which added another layer of complexity to an already complex system.
I don’t know if there is much call for 2 MW medium wave transmitters anymore as there are more efficient ways to reach remote populations and I can’t even imagine what the electric bill would be like.
