Month: September 2010

Continental 816R2 FM transmitter

This is perhaps my favorite model FM transmitter, the Continental 816R2:

Continental 812R2A FM transmitter
Continental 812R2A transmitter, on the air

I have known this particular transmitter for almost twenty years.  It was installed new at WFLY 92.3 MHz in August of 1986.  I was reflecting on that today, as I replaced the bad 4CX250B driver tube which caused the output power to drop to 10 percent.  The power control is via SCRs on the HV power supply, not the more common PA screen voltage adjustment.  That means the transmitter comes on with zero PA voltage and ramps up to full power.  It makes the whole thing “smooth” like driving a Mercedes.

I have experienced a few overloads, which usually are accompanied by the room lights dimming slightly and the plate voltage turning off.  Again, no theatrics;  no big blue flashes, no loud arcs, etc.  Simply turns off the high voltage and light a LED on the overload board to tell the operator what happened.

Over the last 20 or so years, I think I have had three out-of-the-ordinary problems with this transmitter:

  1. The power supply pass transistor in the 802 exciters failed.  This is a TO-3 case mounted on a heat sink, something like a 2N2225 I think.  It runs hot.  Anyway, the exciter had no 20-volt supplies, which was pretty easy to diagnose.
  2. The SWR foldback did not work during an ice storm.  This transmitter feeds an ERI antenna without heaters or radomes.  About once every 2-3 years there is an ice buildup, which will cause the transmitter to fold back.  In this case, the transmitter overloaded and went off the air instead.  Traced back to a bad/dirty connector on the directional coupler.
  3. One of the SCRs exploded while running on the generator.  Figured out this was caused by harmonics from the generator exciter.  Replaced the exciter with a different version, no SCR problems were encountered after this fix.

I like the Continental tube-type transmitters, they are solid units that perform well and have years of reliable service if properly maintained.

History repeats itself?

Letter to the editor in the April 15, 1987, Radio World:

I’ve heard all the garbage I can stand about the AM stereo “issue.”  The problem?

1.  Motorola and Kahn each was a monopoly.

2.  The stations don’t want to waste $6,000 on the wrong system.

3.  The FCC is afraid it will be sued by the “sore loser” in a standard decision.

4. The receiver makers are afraid of wasting millions of the wrong system.

5.  The listeners don’t know anything about AM stereo.

Does any of this sound vaguely familiar regarding some AM “improvement” schemes currently being used?  Those that fail to learn from history are destined to repeat it, Bob.

AM stations to get interference Dollars

Oh, if only it were so, AM stations would be rich! Rich, I tell you.  This dates back to a 1987 article in radio world that details how AM stations receiving Cuban interference could submit to the USIA (US Information Agency), the State Department VOA oversight organization, detailed interference reports and requests for reimbursement of lost revenue.  In addition to that, the FCC would consider nighttime power increases and or pattern modifications, so long as no US or Canadian station was adversely affected by the changes.

Prior to about 1980 or so, Cuba adhered to the NARBA of 1950.  This allocated broadcast channels in the AM band, including clear channels for the US, Canada, Mexico, Cuba, and the Bahamas.  Later on, most of the countries in South America joined the treaty and most international interference was mitigated.

Then, for reasons only known to Fidel, Cuba began broadcasting high power on several frequencies.  The stations suffering the worst interference were all in Florida; WINZ, Miami, WVCG, Coral Gables, WNWS, South Miami, WEAT, West Palm Beach, WQBA, Miami, WKAT Miami Beach and WSUN, St. Petersburg.

In our neck of the woods, WICC suffered some pretty bad interference from CMKA on 600 kHz.  According to the treaty, CMKA was supposed to transmit with no greater than 2.5 KW.  Sometime in early 1981, they increased power to 150 KW.  The path between WICC and CMKA’s transmitter site is almost entirely over salt water.  Additionally, CMKA utilizes a fairly tall tower, 130 degrees according to the FCC database.

The interference was worst in the last fall and early spring.  Several local newspaper articles were written about the subject, noting that WICC developed a contest around the interference.  The station would drop its carrier for 10 seconds at 6:30 and 7:10 pm.  Listeners would then try to identify the Cuban songs playing on CMKA and drop a postcard to WICC with that information.  Winners were picked randomly from all the correct answers received (CMKA would also be heard on the studio air monitor).  Coincidentally, after several months of this, the Cuban station switched its programming to English.

In any case, I believe the USIA paid out a total of $500 K to the Florida stations.