New Nautel NV-40 at WVPS, Burlington, VT

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 spent just five days at home. Some of 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 transmitters 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 remaining 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, are 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 dock, 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. Manchester transmitter site loading dock
Nautel NV-40, Mt. Mansfield transmitter site loading dock

Unpacked:

Nautel NV-40 uncrated and read to move down the hallway
Nautel NV-40 uncrated and read to move down the hallway

Moving into the final position in the WVPS transmitter room.

Movers putting transmitter into final location and removing pallet jack
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
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
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.

All is not well in Paradise

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:

  1. Fresh arc tracks on the PA cavity and PA loading capacitor
  2. The shoes and bars in the high-voltage contactor were severely pitted
  3. 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
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.

What happens next

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
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
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…

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.