FM transmitters – Page 15 – Engineering Radio

FM antenna mounted on the side of a smokestack

If a person were to drive south down I-95 through Bridgeport, CT and look off to the left, they would see a 500-foot smokestack for a coal-fired power plant. Side mounted on that smokestack is a 6-bay Shively FM antenna. The antenna is more visible when driving south. That would be the antenna for WEBE 107.9 Mhz. This is right downtown, therefore, I would imagine this station has no problems with reception.

Bridgeport Power Plant smokestack, viewed from the west

WEBE is a class B FM with a full 50 KW ERP. Most FM’s around here take advantage of a nearby mountain to gain some altitude and thus reduce the TPO a bit. There are several class B stations that run less than 5 KW into a relatively small antenna, but they are way up in the 900 to 1000-foot HAAT range. In this case, the power plant is located right on the Pequonnock River Bay, so the AMSL at the base of the smokestack is only 10 feet. This means lots of watts out and a fairly large antenna.

They are using Broadcast Electronics FM35A for the main and backup transmitters. They were installed in late 1986 and are a little long in the tooth.

They run near a 12 KV plate supply, about 3.8 amps making 34 KW TPO. That goes into a six bay Shively 6 bay 6813 antenna centered at 475 feet, which makes the HAAT 117 meters.

One of the problems encountered at the site is the smokestack emissions. It seems that a fair amount of mercury comes out to the top of that thing. In the past, this has caused major problems with the antenna shorting itself out and burning up transmission line. Because of this, the entire antenna system, radomes, and transmission line is supplied with Nitrogen from this liquid nitrogen tank:

The antenna then intentionally bleeds N2 into the radomes continuously, overpressurizing them, to keep the smokestack emissions out. This type of tank is needed because a conventional N2 tank would last about a day, whereas the liquid tank lasts about 20 days.

The BE FM35A decided to blow a 200 Amp fuse on Friday afternoon:

I had a BE FM30A that would randomly trip the 200 amp main breaker every once in a while. I could never find anything wrong with the transmitter, it would just come back on and run normally again after the breaker was reset. I even replaced the breaker thinking breaker fatigue. Still happened. In the end, we replaced that transmitter. In this case, I don’t see that happening anytime soon.

BE FM35A heavy iron:

Broadcast Electronics FM35A plate transformer

I would not want to replace this thing, it must easily weight 1,000 pounds.

And rectifier stacks:

Broadcast Electronics FM35A rectifier stacks

12,000 volts DC. That will light up any dirt, dust, piece of fuzz, etc. in the transmitter.

It is one of the more unique FM transmitter sites I’ve ever been to. Every time I see it, I am reminded of that song, Smokestack Lightning. My favorite version of that song is the live recording by the Yardbirds

Rebuilt tubes

As broadcasters, we don’t really hear that much about ceramic power vacuum tubes these days, as more and more broadcast transmitters migrate to solid-state devices. Once upon a time, however, power tubes were the engine that drove the entire operation. Tubes had to be budgeted for, stocked, rotated, and replaced on a regular schedule. Some of those dern things were expensive too.

Take the 4CX35,000A which was used in the Harris MW50 transmitters. The transmitter used two of these tubes, one in the RF section and one in the modulator. As I recall, new tubes cost somewhere north of $8,000.00 each from EIMAC. Plus, in the A models, there were two 4CX1500A driver tubes. All of which could add up to an expensive maintenance cost every two years or so.

The next best option was to buy rebuilt tubes. Rebuilt tubes were about half the cost of brand-new ones. Some people complain that rebuilds don’t last as long, or only last half as long as the new tubes. I never found that to be the case. I often found other factors that affected tube life far greater, such as filament voltage management, cooling, and by extension, cleanliness.

I can say I never had a warranty issue with ECONCO tubes. I cannot say that about EIMAC, as during the late 90s and early ’00s (or whatever you call that decade), I had several brand new 4CX3500 tubes that were bad right out of the box. These days, ECONCO and EIMAC are both owned by CPI.

I spoke with John Canevari from ECONCO who had a lot of information. For example, as the tube ages, the filament gets more flexible, not less. Most ceramic power tubes use a carbonized tungsten filament containing some small amount of thorium. As the tube ages, the filament can no longer boil off enough electrons and the emission begins to drop off. That is the normal end of life for a power tube. Occasionally, some catastrophic failure will occur.

There are many steps in the rebuilding process:

Dud is received from the field, the serial number is recorded and the tube is tested in.
The tube is prepped by sand-blasting the sealing rings
It is opened
The filament is replaced. In 60-70% of the cases, the grid is replaced. In those tubes that have a screen assembly, 20-60% of those will be replaced.
The Interior of the tube is cleaned
The tube is resealed and tested for leaks with a gas spectrometer
The tube is placed on the vacuum machine. Tubes are evacuated hot, smaller tubes take 12 to 24 hours, and very large tubes can take up to one week.
The tube is nipped off of the vacuum while still hot. When the tube is fully cooled the vacuum scale is normally around 10^-12
The exterior of the tube is cleaned and replated. Silver for tubes that are socketed and Nickel for tubes that have leads.
The tube is retested to the manufacturer’s original specification or greater.

After that, the tube is sent back to its owner or returned to stock. John mentioned that they are very proud of their vacuum tube processing machines, so I asked if he could send along a picture. They certainly look impressive to me, too:

Not exactly sure which tube type these are, but they sure look like 4CX15,000:

vacuum pump on rebuilt ceramic power tubes — Vacuum pump working on rebuilt ceramic power tubes, photo courtesy ECONCO

Econco has been in business since 1968 and rebuilds about 600-1,000 tubes per month. In the past, broadcasters used most of the larger tube types. However, with the majority of broadcast transmitters shifting to solid state, other markets have opened up such as industrial heating, military, research and medical equipment.

RCA BTF-20E

Does anyone need any parts for one? Long-time reader and commenter John has one that looks to be in good shape that he is willing to part with or part out. I had three of these units in Harrisburg and my recollection is they were pretty solid units. When tuned properly, they were low noise and sounded good on the air.

The one issue I had was with the small 100 pf pass-through/by-pass capacitors in the IPA. Several went bad and were no longer functioning as bypass caps. The result was the transmitter would self-oscillate. I think there were seven or eight of them and I replaced them all at once. The exact model of that particular transmitter was a BTF-20ES1, which was one of the last FM transmitters off of the factory floor before the broadcast division went under.

John says:

If anyone needs parts out there, I will probably cannibalize this unit as I have just too many. The separate power supply is inside the garage along with the latest version of the harmonic filter entirely made of copper.

I don’t know if the entire unit actually runs or if it is parts only. It certainly looks like a clean unit. As I recall, Comark bought out all the RCA broadcast parts and service. Comark was then sold to Thomcast, which was sold to Thales which I think spun off its transmitter division to Grass Valley Group. Grass Valley started by making TV master control switchers, routers, and other video equipment.

Anyway, if you are looking for RCA parts for FM transmitters, contact me, and I’ll put you in touch with John.

Harris FM25-K

Old blue, I like to call them, the Harris 1980s model transmitters with black faces, white cabinets, and blue trim. I have yet to find one that I really like, the FM 25-K is, well okay. Sort of like that 200,000-mile jeep that works, most of the time, and it’s paid for.
This particular FM-25K transmitter is located at WIZN in Charlotte, VT.

This transmitter was new in 1987. It had a bad day yesterday, deciding to throw a temper tantrum and trip the HV power supply breaker. Fortunately, the station has a backup transmitter. When we arrived, we found the HV power supply feed through the insulator at E1 arced over and broken. Again, fortunately, this station’s management believes in stocking spare parts and a replacement was on hand.

Harris FM25K HV power supply feed through insulator

This is part of the RF filter for the HV power supply. This happened once before, about two months ago. The replacement insulator then was used, so that might be a factor. Two months ago, both capacitors in the Pi filter and the HV power supply cable (RG-8 coax) were replaced all the way back to the rectifier stacks.

The FM25-K can produce spontaneous high-frequency oscillations if not tuned properly. We looked at the transmitter output with a Rode Schwartz spectrum analyzer and found it to be clean. Exactly why it blew out another feed-through insulator is a bit of a mystery. Since the first replacement was a used part, we surmise that it may have been cracked. If this replacement insulator arcs, there needs to be a full investigation.

As I said in the beginning, I have found these transmitters to be okay, not the best, not the worst. Most of the problems I have encountered with the K series FM transmitters had to do with the controller cards. There are two, one analog and one digital. That’s what Harris calls them anyway. Like the SX transmitter, and the MW transmitter to a certain extent, the control circuits are way over complicated and full of +/- 5 volt CMOS logic. Having that type of control logic connected to a radio tower (e.g. lightning rod) is asking for trouble.