We have this guy that works for us who is atypical. We call him Pete because that is his name. The other day, he was slacking off on the job again, this time figuring out how to take a nap in a transmitter:
Pete working on a Harris FM25K
What are we going to do with him?
Actually, he is rebuilding the grid tuning section (AKA input tuning section), which is no small matter. Soon, we will have this 26 year old transmitter running good as new, or better than new. It already sounds much better on the air than it did before, the input tuning is broader and there is much less AM noise.
Currently, it is running about 70% power while we wait for a replacement amplifier from Silicon Valley Power Amps.
Man, this is taking longer than I though it would. We moved the Harris FM25K last week, all went well. The only hangup, as you can see, is the harmonic filter and the height of the racks next to the transmitter. The transmitter had to go on a 4×4 to get the filter up over the racks. The output from the transmitter to the harmonic filter cannot be changed in any way, shape or form (e.g. adding a little bit of line section to the top of the transmitter), else the transmitter will not run. So, up on 4×4′s it is.
WRKI WINE transmitter room
There we were, all ready to turn the transmitter on. Press the high voltage on button, lots of volts but no current and no power output. Seems something is wrong with the outboard IPA driver (over in the bottom of the rack, that thing pulled out with the manual on it).
The IPA is a Silicon Valley Power Amplifier 500 watt unit, which replaced the internal IPA driver about ten years ago. The tube in the Harris FM25K needs at least 390 watts to drive the transmitter to full power. Unfortunately, this particular amplifier was not in the best environments prior to the recent move. It was sitting in an unconditioned building on top of the backup transmitter in high heat and humidity. According to the manufacture, such abuse is bound to take it’s toll sooner or later. The later being, of course, the night we want to turn the thing back on and go home.
Time to drop back and punt. I found an old RVR 250 watt amp at a sister station nearby, which was also in pretty bad shape but repairable. That unit was pressed into service temporarily and with 200 watts drive, the old 25K put out about 11 KW. We need to affect permanent repairs to the RVR power amp before we place into temporary service. I don’t want any 2 am phone calls. The Silicon Valley Power Amp needs to have the amplifier module sent back to the manufacturer and rebuilt. They will refurbish the entire thing for something like $900.00 plus shipping. Considering what it does, that is worth it.
This is a little short cellphone video of the turn on at half power. This is a very loud transmitter, as such, I think the audio is a little distorted.
When this beast gets up to full power, I will update this, again.
Every good transmitter, tube transmitters in particular, require harmonic filtering. The last thing any good engineer or broadcaster wants is to cause interference, especially out of band interference to public safety or aviation frequencies. All modern transmitters are required to have spurious emissions attenuated by 80 dB or greater >75 Khz from carrier frequency. In reality, 80 dB is still quite high these days, especially in the VHF/UHF band, where receivers are much more sensitive than they used to be. A good receiver noise floor can be -110 dB depending on local conditions.
The principle behind a low pass filter is pretty easy to understand. The desired frequency is passed to the antenna, while anything above the cut off frequency is restricted and shunted to ground via a capacitor.
Low pass RC filter
In this case, the resistor is actually an inductor with high reactance above the cut off frequency. Often, these filters are lumped together to give better performance. This is a picture of an RVR three stage low pass filter:
RVR three stage low pass filter
RVR is an Italian transmitter maker that sells many transmitters and exciters in this country under names like Bext, Armstrong, etc. The inductors are obvious, the capacitors consist of a copper strip sandwiched between teflon insulators held down by the dividers in between the inductors.
Schematically, it looks like this:
Low pass filter schematic diagram
For the FM broadcast band, a good design cutoff frequency would be about 160 MHz. This will give the filter a steep skirt at the first possible harmonic frequency of 176 MHz (88.1 x 2 = 176.2).
Values for components:
Capacitors
Value
Inductors
Value
C1
20 pf
L1
74.7 nf
C2
54 pf
L2
75.1 nf
C3
54 pf
L3
73.9 nf
C4
20 pf
The inductors are wire, or in this case copper strap, with an air core. It is important to keep the transmitter power output in mind when designing and building these things. Higher carrier powers require greater spacing between coil windings and larger coil diameters. This particular filter is rated for 1 KW at 100 MHz.
Found in a pile of junk in the corner of an older transmitter site, this Gates BFE-50C or otherwise known as an M5675 Amplifier. This was used as an IPA in a Gates FM 1C transmitter installed around 1960 or so. The rest of the transmitter has long since departed, likely to the scrap yard, however, somebody though to remove this and set it aside.
Gates BFE-50C 50 Watt VHF amplifier
This unit is missing it’s grid tune knob. The grid tune capacitor is still there, however. There is also some evidence of heating on R403 and R407/408 likely due to a prolonged overdrive condition. Otherwise, it is in good shape.
Gate BFE-50C 50 Watt VHF amplifier back
The design is pretty simple, a pair of 6146′s in push pull, three watts in nets about 50-60 watts out, according to the manual, which can be found here (.pdf). The power supply voltages are fairly tame, 500 volts plate, 300 volts screen. The one thing that this design does not have is any type of harmonic filtering. When used with a larger transmitter, this makes sense because the transmitter output will have overall harmonic filters. If this was to be used on it’s own for any reason, a good harmonic filter would need to be designed and installed.
Gates BFE-50C or M5675 50 Watt VHF amplifier
The schematic is straight forward. Gates, the old Gates Radio of Parker Gates, designed good equipment. Click on image for higher resolution.
Gates BFE50-C input section
It is a bit hard to see in this picture; the input section consists of three turns of #14 gauge wire coupled to two 4 turn sections of 14 gauge wire on either side of it. This is matched to the grids Screen1 of the 6146′s with C401. L412, C411 and L413 form a low pass filter. L412 consist of one turn #14 gauge wire, L413 is five turns of #14 gauge wire. All coils are 3/4 inch in diameter.
Gates BFE-50C output section
The output section is even simpler, using just one loop of small diameter copper tubing. The plate tuning is accomplished by C407, loading is C406. Power output is adjusted by varying the screen voltage using R405.
Advantages of this design:
The 6146 tube is fairly rugged, at class AB the 50 to 60 watt output range is well within the plate dissipation for a push pull configuration.
No special parts needed, everything can be found or fabricated by hand
The 500 volt supply is fairly tame, maximum PA current should be less than 0.2 amps for 50 watt output and 50% PA efficiency.
Output tuning and load allow for tuning into less than ideal loads, if required.
If operated as a stand alone unit, some type of plate current meter should be used to aid tuning. A harmonic Filter would need to be designed and built for the output.
This is the main transmitter for WYJB in Albany, NY. The backup is the Harris FM20H3 on the right. I haven’t turned that unit on lately, but it normally makes quite a fuss the first time the Plate On button is pushed. The FM 20T on the other hand, is mellow and even tempered.
WYJB 95.5 Mhz, class B, transmitter Albany, NY
One other thing of note; The FM20T is still on its original tube. I looked up the maintenance records for this transmitter, it was installed in December of 2000. Eleven years later, the 4CX15000A is still cranking out 15 KW TPO, which is impressive. I found that high power ceramic vacuum tubes actually seem to last longer when run closer to their limits than those that are running at half power.
After the tube is in the transmitter, run it a full filament voltage for at least an hour or so before turning on the plate voltage. This allows the getter to absorb any stray gases in the tube.
Once the plate voltage is applied, proper tuning should be completed a quickly as possible. Tuning procedures vary from transmitter to transmitter, however, the general idea is to obtain the maximum power output for the least amount of plate current while keeping the PA bandwidth within acceptable limits. Some transmitters can get narrow banded at high efficiencies, which manifests itself as higher AM noise.
After the tube has been in use for 90-100 hours, the filament voltage should be reduced gradually until a drop in the transmitter output power is noticed, then increased by 0.1 volts.
This maximizes the filament life for that particular transmitter and power output. Once the filament can no longer boil off enough electrons, the tube power output drops and it is time to replace it.
This site also has two other radio stations, WZMR, 104.9 and WAJZ 96.3 , both class A using solid state transmitters of less than 1,000 watts:
WAJZ and WZMR Energy Onix solid state transmitters
Not the prettiest sight in the world, but it does stay on the air. There is no money to go back and neaten up this work, unfortunately.
The tower supports all three antennas. There was some discussion of a common antenna for all three stations, however, WZMR is a directional station, thus it would require it’s own antenna. Doing a common antenna for the other two stations was cost prohibitive, so the tower supports three two bay antennas.
WYJB, WZMR, WAJZ FM antennas, New Scotland, NY
The stations are all located in the New Scotland, NY tower farm. WYJB is licensed to Albany, WZMR is licensed to Altamont and WAJZ is licensed to Voorheesville.
I had the opportunity to work on one of these recently, thought I’d post a few observations. The transmitter itself comes in three parts, the FM100 which serves as the exciter and driver, the PA2000, which holds the RF amplifiers and combiner and the PS2000 with supplies the DC voltages to run the PA.
Crown FM2000A transmitter running at half power
That configuration has some advantages and disadvantages. First, it takes up much more rack space than the comparably powered Nautel VS2.5. Second, because the unit does not come with slide out rack rails, each part needs to be removed from the rack for servicing, which makes things a little difficult when working alone as the PS2000 weights quite a bit. As far as the rest of the design, the PA2000 is very modular, all of the PA modules, controller card, fuse board and RF combiner easily come out of the chassis for service.
Crown FM2000A top cover removed
This unit had been in service at WBEC in Pittsfield, MA for an undetermined amount of time. As such, there was quite a bit of dirt and bugs inside the PA chassis. I used a air blower to clean everything out. Checked the fans for bad bearings, checked all RF connections for signs of overheating, etc. I also cleaned out the power supply and rinsed all of the air filters.
Crown FM2000A front cover off
My other minor complaint is the power adjust pot is under the front cover. When making adjustments and such, the LED display indicates operating constants based on a little LED light next to the display. The legend is on the cover, which has been removed to adjust the power. Minor thing, but slightly annoying, none the less.
There are four RF modules in the PA2000, each one generating 500 watts. This particular transmitter has a bad device in PA3. When the transmitter is running the DC fault LED flashes and the PA3 reading shows no current. The device is a BLF278, which is a fairly common, inexpensive RF MOSFET. According to the factory tech, they can be replaced in the field provided one can solder. After replacement, there is no special tune up or anything needed as the module is wide band.
Crown PA2000 500 watt RF module
The four modules are combined then sent to the RF output filter which has the low pass harmonic filter and directional coupler.
Crown PS2000 output filter
It is a pretty simple transmitter, no bells or whistles or fancy things like IP connectivity. Overall, it seems to be well made, robust, modular, efficient. The remote control interface is via DB-25 connector on the back of the PA2000.
I did not get a chance to hear it on the air, I was just cleaning and testing the RF sections. The exciter is an FM100 transmitter, which I had to change frequencies on. I found that to be self explanatory.
It would be fun to compare this to some of the other broadbanded FM amplifiers like PTEK and Armstrong.
This is cute. A small (VS allegedly stands for “Very Small”) integrated 2,500 watt FM transmitter. This one we just finished installing as a backup transmitter for WSPK, on Mount Beacon, New York.
Nautel VS-2.5 FM Transmitter
This site has a Nautel V-7.5 as the main transmitter. That unit is very reliable, however, this transmitter site is non-accessible 4-5 months out of the year due to ice and snow. The last time we had an off air emergency due to a crippling ice storm, it took an entire week to clear away all the downed trees so we could gain access to the site via snowmobile. As such, every system needs dual or even triple redundancy. Lack of said redundancy has lead to several prolonged outages in the past.
WSPK signal flow diagram
Last year, we were finally able to install a backup antenna after 63 years without one. This year, it is time to upgrade the rest of the backup equipment. The new auxiliary transmitter is connected directly to the auxiliary antenna via a five port coax switch. This allows for use of the dummy load for testing when we are present, but removes a potential failure point in the coax switch. There have been at least two incidences of the disk jockey accidentally transferring the transmitter into the dummy load when taking transmitter readings. Hopefully this configuration will be fairly idiot proof. I am making an interlock panel that will prevent both transmitters from being on the air at the same time.
Nautel VS 2.5 connections
This site is a work in progress.
The backup processor is at the transmitter site, the main processor is in the rack room at the studio. This works well because the main processor occasionally looses its mind and needs to be rebooted. It would be a significant pain to drive all the way up to the transmitter site just to reboot the processor. It might not happen at all during the winter. The back up processor has no mind so it is not an issue.
The VS transmitter is attractive because it has a built in exciter that accepts composite, AES or IP audio. The exciter also has a built in Orban processor as an option. Thus, if it really hit the fan, we could use the LAN extender to get the audio to the site. Further, it could be addressed by any studio in the company WAN. Which is cool, when you think about it.
Nautel continues to crank out innovative, dependable products and there is nothing wrong with that.
I thought it would be interesting to do a comparison between the two types of transmitters, both AM and FM. I have been doing this thing for 25 years and have quite a bit of experience working on all types of transmitters. Some of the broadcast transmitters I have personally worked on over the years include:
CSI/CCA, Visual, Energy Onix, Bauer, McMartin, QEI, some Italian something or other, etc. Various makes and models.
I think I have a fair amount of transmitter experience under my belt. What I have found is that certain brands of transmitters are better than others, regardless of whether they are tube or solid state. There are several differences in each type, obviously. As to some blanket statement about which is better, solid state or tube, I don’t have one. My statement would be “It depends.”
Tube transmitters are more rugged and will take more abuse than a solid state unit. Things like heat, lightning, EMP, mismatched antenna won’t phase a well designed, well manufactured tube transmitter. On the other hand, they are less efficient AC to RF, have higher B+ voltages, have hard failure modes, and are more difficult to linearize, if that is required for some reason.
Solid state transmitters are more broadbanded, easier to change frequency, they have soft failure mode due to redundant amplifiers and power supplies. The voltages are lower, thus they are safer to work on.
Here is a complete list of advantages and disadvantages of each type:
Attribute
Tube
Solid State
Comment
Ruggedness
Very rugged, able to take heat, EMP, lightning, mistuned antenna, poor operating environment, etc
Not heat tolerant, lightning and EMP can damage MOSFETS, switching power supplies sensitive to AC mains issues
Advantage: Tube
Electrical Efficiency
Less efficient
More efficient
Advantage: Solid State, however efficiency gain can be wiped out due to larger air conditioning requirement
Failure mode
Hard, most often
Soft, most often
Advantage: Solid State, failure of a single module or power supply generally will not take unit off the air
Frequency agility
Difficult
Easy
Advantage: FM Solid state transmitters can easily be moved. AM transmitters still require extensive retuning.
Re-occurring cost
More
Less
Advantage: Solid State, as tube changes are required every two to three years
Maintenance
Same
Same
Advantage: neither
Servicing
Requires skilled engineers to service and trouble shoot
Modules and power supplies are often hot swappable and returned to manufacture for repair
Advantage: Solid State, however either type requires occasional measurements with specialized test equipment
Servicing safety
High voltages, contact will be fatal
Lower voltages, but can still be fatal
Advantage: Solid State
Redundancy
Low
High
Advantage: Solid State, multiple power amps and power supplies give solid state units more redundancy
Cost
Less
More
Advantage: Dependant on TPO, Higher powered solid state transmitters are much more expensive than there tube type counterparts
Availability
Good used market, some new FM transmitters still being built
Good new and used
Advantage: Tube
Reliability
Dependent on brand
Dependent on brand
Advantage: neither
For some reason, the latest Broadcast Electronics tube type transmitters seem to have very long tube life. I installed an FM20T at WYJB in Albany, New York, in early 2001 and it is still on the original tube, some ten years later. Same can be said for the 2005 FM20T and FM30T installation at WHHZ/WKZY, Gainesville, Florida. Those tubes shows no sign of giving up anytime soon. I don’t know if that is an unusual trait of the transmitter or that particular tube.
WKZY, Gainesville, Florida
The above comparison seems to heavily favor a solid state transmitter. As a general rule, brand new solid state transmitters both AM and FM have advantages in almost every category except high power FM transmitters, where tube types still make sense. From a used transmitter standpoint, there is nothing wrong with a tube type transmitter, provided it has a solid state IPA. I have noticed the 4CX250B driver tubes most often used in FM IPA stages have markedly reduced reliability of late. I would also tend away from transmitter makes and models where the manufacture is no longer in business or no longer supports the product.
Sorry for the prolonged absence. I have been, quite literally, out of reach for the last two weeks. In fact, for the entire month of July, I have spent just five days at home. Some if the travel was for work and some for pleasure.
On the work side of the equation, WVPS in Burlington, Vermont has a new Nautel NV-40 transmitter. WVPS is the NPR affiliate for Vermont Public Radio and it’s transmitter site is located on top of Mt. Mansfield, in Stowe, VT. I will do a separate article about the Mt. Mansfield transmitter site because it is an interesting place. WVPS is a Class C FM on 107.9 Mhz. They have one HD subchannel for the VPR classical music format.
The Nautel NV-40 transmitter is greatly updated from the V-40, which was installed at WHUD. Basically, the V-40 is four ten kilowatt transmitter combined. It is a novel approach and offers quite a bit of redundancy as entire transmitters can be switched off and worked on with the other three remain on the air at full power.
The NV-40 is a single large chassis with internal combining networks. It uses different RF modules but the same power supplies. The entire thing is controlled by a fancy GUI on the front of the transmitter, but also has the ability for manual control if the GUI fails. That is a key feature not seen in other transmitters which simply won’t work without the fancy computer. Other things that I like, the ability to control all of the biasing and other options via the GUI and things like a spectrum analyzer and lissajous display. The ability to look at several graphic displays at once makes it easy to configure and monitor.
The transmitter arrived at the top of the mountain via a local moving company. After unloading it on the loading doc, it took some amount of doing to get it down the hall into the transmitter room. The thing weighs in at 1,600 pounds after being uncrated.
Nautel NV-40, Mt. Mansfield transmitter site loading dock
Unpacked:
Nautel NV-40 uncrated and read to move down the hallway
Moving into final position in the WVPS transmitter room.
Movers putting transmitter into final location and removing pallet jack
The connections were made to the transmitter, including connecting grounding strap to the back, 200 amp electrical service and the RF output connection via 3 inch rigid coax.
Nautel NV-40 installed
The remote control consists of basic transmitter functions going to a dial up Gentner remote control and a Network connection going to the GUI. The network connection allows persons on the network to use a web browser to look at the GUI. The HD radio connections are made via a HD radio importer and exporter, located at the studio, which also uses the network, via a connection on the exciters, to send the HD subchannel. The analog main channel is via an AES/EBU connection from the STL.
All connections go through large toroids to help isolate the transmitter from any lightning related surges.
Before I left, we tested it at full TPO into the dummy load. All worked well, the only outstanding issue was getting the HD radio importer/exporters to work over the network, which was out of my jurisdiction.
Author and Nautel NV-40
Here is a rather blurry picture of your author standing next to the NV-40 with the exciter and GUI turned on. There are to IEC power connectors at the top of the transmitter that go to the GUI and exciters. This allows those part of the transmitter to run on UPS’s, which is nice, being that the GUI takes about a minute to boot up after power failure.
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 where the result of arcing. Any sharp points will induce corona. I also filed down all of the contacts in high voltage contactor, which took a fair amount of time. These are soft copper shoes and bars which 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 4:30pm.
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.
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~Benjamin Franklin
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~Alan Weiner
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