FM transmitters – Page 8 – Engineering Radio

Modifying the old Broadcast Electronics Transmitter

In the previous post, the issue with the WVOS-FM transmitter was detailed: The PA feed through/bypass capacitor had arced to the PA cavity causing lots of unwanted off-air time. When I went to order the replacement parts, of course, they were not available. It seems that Broadcast Electronics changed the design of its transmitters in the late 1980s to use a different feed-through arrangement.

They were nice enough to send us a nifty retrofit kit; BE part number 959-0272 which replaces BE part number 959-0115. If interested, the six pages of installation instructions are available here, for your reading pleasure.

The retrofitting itself was quite the job; drilling six mounting holes and one one-inch feed through hole in the PA cavity, mounting the new feed through housing, rewiring the high voltage connection to the tube and back to the HV bleeder assembly, etc. What with all of the drilling, sawing, filing, deburring, and whatnot, I began to wonder if the transmitter would ever run again. This is the transmitter before the modification:

This is the old high voltage feed through hole, arc mark clearly evident.

Broadcast Electronics FM3.5A old high voltage feed though

This is the modified feed through/bypass configuration.

Broadcast Electronics FM3.5A new PA feed through capacitor — Broadcast Electronics FM3.5A new PA feed through/bypass capacitor

While doing this work, I removed the tube and put a plastic sheet in the bottom of the PA cavity and around the HV parts at the bottom of the transmitter. Somehow, getting aluminum filings in the tube socket seemed like a bad idea. I also thoroughly vacuumed out the entire transmitter once all of the metal work was done.

I removed the Kapton capacitor plates from the old feed-through arrangement and reinstalled the Teflon insulating plates to keep the air flow out of the tube cavity going in the correct direction. The new capacitor looks very beefy, perhaps it will never fail again.

Once the installation work was done, I brought up the transmitter first with no screen and no connection to the tube anode. Then with the tube connected, and finally with the screen supply turned on. The tuning needed a brief touch up but all in all, the transmitter came up and ran well with the new feed-through arrangement.

The old Broadcast Electronics Transmitter

Alternate title: More blown-up stuff

This Broadcast Electronics Fm 3.5A will be thirty years old in April. We should have a party!

Broadcast Electronics FM 3.5A, WVOS-FM, LIberty NY — Broadcast Electronics FM 3.5A, WVOS-FM, Liberty NY

Unfortunately, this transmitter is not doing too well these days. The PA high voltage feed through capacitor has arced over to the PA cavity, causing the station to be off the air.

BE FM3.5A HV feedthru capacitor — BE FM3.5A HV feed thru capacitor

Naturally, this happened over the weekend, parts will not arrive until Tuesday at the earliest, and the station is without a backup transmitter.

Obviously, trouble shooting this was a two-person job. Never work alone on HV equipment. The symptom was the main circuit breaker was tripping after the HV on command. Starting from the transformer end of the HV power supply circuit and working toward the anode of the PA tube, all of the components were tested by isolating each component then turning the HV on. Special care was taken to discharge all components after each test. The capacitors and bleeder resistors were reconnected at the same time. There is too much risk involved with charged 8 KV capacitors and no way to bleed that charge to ground. Everything worked up until the PA cavity was reconnected (without the tube), then the breaker tripped again. Thus, the above feed-through capacitor was removed and disassembled, revealing the damage.

The question is, how long should transmitting equipment last? After all, if one were running a freight delivery company, you would not be driving around in thirty-year-old trucks, would you? No, not if you wanted to stay in business. Like all electro-mechanical equipment; transmitters, consoles, STLs, antennas, computers, etc wear out. A smart plan would be to have a replacement schedule and be putting money into a capital equipment replacement fund. Equipment life varies with the type. Getting twenty years out of a main transmitter is a pretty good service life, going beyond that is pressing one’s luck. Ten years on any one computer is a very long time. Then there are certain transmitter manufactures that drop support on older units, which makes it difficult to keep them operating. Owners and managers need to be cognizant of the age and condition of critical infrastructure. As field engineers, how much time do we devote to keeping antiquated equipment running, or should we even be servicing it at all? As independent contractors, we incur a liability whenever we touch something. Where does the ownership’s responsibility lay in providing safe, functional equipment for their stations? All interesting questions.

WDST, Woodstock, New York

WDST is a well-known radio station in Woodstock, NY. Formatically, I would call it Adult Album Alternative (AAA) and it is one of my favorite stations to listen to. We also do the engineering work for this station. While I was there last week, I snapped a few pictures of the studios:

All of the studio use Audioarts R-60 consoles, which are in good condition considering their age. Lots of guest microphones and the windows look out into a performance venue.

WDST music library, located in hallway outside of studio — WDST music library, located in the hallway outside of the studio

The music library is extensive.

The production room, another R-60 console. I don’t know where the microphone disappeared to, perhaps it was borrowed by the morning show.

Technical Operation Center (TOC). WDST uses NextGen from RCS for music storage, playback and automation. Other equipment includes ISDN, POTS phone, Distribution Amps, Limiters, streaming computer, STL, etc.

WDST transmitter, Broadcast Electronics FM5C

The transmitter site is on Hallihan hill, across the street from the old ATT long lines site. The station uses a Broadcast Electronics FM5C transmitter.

Forward power, almost five whole kilowatts of flame throwing power.

WDST antenna, Hallihan Hill, Kingston, NY

The antenna is a Shively 6810 2-bay half wave spaced.

The curious case of the WKZE Notice Of Violation

On June 19th, WKZE received a notice of violation from the FCC’s New York Field office. The crux of the issue seems to be interference being generated on 784.8 MHz (WKZE 8th harmonic) to a new Verizon Wireless installation located nearby:

47 C.F.R. §73.317(a): “FM broadcast stations employing transmitters authorized after January 1, 1960, must maintain the bandwidth occupied by their emissions in accordance with the specification detailed below. FM broadcast stations employing transmitters installed or type accepted before January 1, 1960, must achieve the highest degree of compliance with these specifications practicable with their existing equipment. In either case, should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation.” The eighth harmonic of Station WKZE-FM (784.8 MHz) was causing interference to the Verizon Wireless transmitter located approximately 500 feet away.

First off, we note that the WKZE transmitter is not allegedly causing interference to a Verizon Wireless transmitter, but rather to a Verizon Wireless receiver. That may be splitting hairs, however, since the FCC is quoting a technical rules violation, they can at least get the technical language right.

A brief examination of the rest of FCC part 73.317 is in order to find the specification cited in section (a). Section (d) states:

(d) Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Since 784.8 MHz – 98.1 MHz is greater than 600 KHz, this is the section that applies to the WKZE situation. Thus, the interfering signal must be greater than -80 dBc to trigger the Notice Of Violation (NOV) from the FCC. The station ERP is 1,800 watts or +62 dBm. Measurements were made with an Agilent N992A spectrum analyzer using an LPA-1000 log periodic antenna. At a 12-foot distance away from the WKZE transmitter cabinet, the signal on 784.8 MHz was found to be -94 dBc or 0.000063 watts. At the base of the Verizon Wireless tower, the measurement was -124 dBc, or 0.000000025 watt, which is barely perceptible above the -130 dBm noise floor. There does not appear to be any violation of 47 CFR 73.317. Rather, the issue seems to be Verizon Wireless’s deployment of the 700 MHz LTE band and the use of high-gain antennas coupled with high-gain preamplifiers on frequencies that are harmonically related to broadcast stations nearby. In this particular installation, the antenna has 16 dB of gain, minus a 4.5 dB of transmission line loss into a 21 dB preamplifier before the receiver. At the output of the Verizon preamplifier, the signal on 784.8 MHz was measured at -89 dBc, which is still in compliance.

By these measurements, clearly, WKZE is not in violation of any FCC regulation. It makes one wonder, does the FCC understand its own rules? Or, is this a matter of favoritism towards a huge corporation over a small independent radio broadcaster? Is it a matter of “broadband at the expense of all others?” There are several of these broadcast to 700 MHZ LTE interference cases pending throughout the country. This could set a dangerous precedent for broadcasters and other RF spectrum users as wireless giants like Verizon throw their weight around and eye even more spectrum to press into broadband service.

Commlaw blog has a good post on this subject: Harmonic Convergence?

Update: The response from the WKZE attorney can be found here, including the above-mentioned actual measurements.