If one considers paradise an FM35A. Going through another iteration of blown transmitter fuses for WEBE, Bridgeport, CT. Yesterday, I spent the afternoon examining the transmitter and found several interesting things:
Fresh arc tracks on the PA cavity and PA loading capacitor
The shoes and bars in the high-voltage contactor were severely pitted
One of the mains phases (middle) in the high voltage supply appears to be heating up, likely due to a loose connection.
Discolored wire on buss bar
I checked and re-tightened all of the mains connections. Apparently, this is an old problem, as the Allen screw was tight. Interestingly, the fuse that was blown was on the red phase, which is different from what it was last time.
I spent the afternoon filing and sanding off the arc track marks in the PA cavity. It is very important to file flat all sharp points that were the result of arcing. Any sharp points will induce corona. I also filed down all of the contacts in a high voltage contactor, which took a fair amount of time. These are soft copper shoes and bars that had so much pitting and carbon I wonder how they didn’t catch on fire. I filed them flat. We were back on the 35A transmitter at full power by 4:30 pm.
If this happens again, I will bring my megger out and check the insulation on the wire between the disconnect switch and the HV power supply.
When I left the site at 5:30, I felt like we did some good work.
It’s the middle of the night and the phone is ringing. That is never good. The transmitter is off the air. You call the remote control and try to put the main transmitter back on the air. No good. The backup comes up, no problem. Shaking off the sluggishness, you get dressed and head out the door. The transmitter is about 30 miles away, but it’s in the middle of the night, so there is no traffic. While driving, you are thinking of all the things that could be wrong. The blower motor was sounding a little loud last trip. The exciter has some reflected power. The PA tube is two and a half years old.
Upon arrival, there are several overload lights lit, including the driver’s plate. An investigation is in order. You turn everything off and open the doors. The trouble seems to be a bad IPA power supply. There are spares on the parts shelf, so you put one in. Put the transmitter into the dummy load. You turn on the filament and the transmitter comes to life again. Reset the overloads.
Broadcast Electronics FM35A transmitter ready to be turned on
Now you are standing there looking at the plate-on button. Was it really only the IPA or was that just a symptom? Was there something else that took out the IPA power supply? What will happen when I press the plate-on button? Will it come on normally or go BANG! I hate BANG! By the way, my tradition in a situation like this, if on a mountain top somewhere, I go outside and pee. I give the situation one more run through the mental checklist, then come back inside and press the button.
Broadcast Electronics FM35A transmitter high voltage on button
Please excuse the blurry picture, it is hard to take a picture of yourself turning on a transmitter…
This is perhaps my favorite model FM transmitter, the Continental 816R2:
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:
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.
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.
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.
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.
Broadcast Electronics FM35A transmitter
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:
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:
Blown 200 Amp fuse
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
