Proper drive levels for a Harris SX series transmitters

I was working on a Harris SX5 the other day and snapped a picture of the scope while measuring RF drive levels.  There are still quite a few of these units out in the field, judging from my search engine results.  I thought it would be helpful to post something about it.  The RF power amp boards for the Harris Gates solid-state AM series transmitters are the same design, I believe.

In order to fully drive the RF MOSFETs in this particular series of transmitters (Harris SX1, 2.5, and 5 including A models) at least 26.5 volts peak to peak is required.  Less than that and the device will turn full-on and internally short.  To measure RF drive, the transmitter must be in local with control voltage on, with the rear door interlock defeated (this can be safely done if the transmitter is wired with separate AC feeds for control and RF power supply). Make sure the RF power supply is defeated and will not turn on.  Measure across the input of each of the toroids that feed the gates of the RF devices.

Harris SX series transmitter drive level test
Harris SX series transmitter drive level test, 27.45 volts, 1,110,000 Hz

It should measure between 26.5 and 29.5 volts. This one measures 27.45 volts peak to peak. Each input toroid on every PA board should be measured as the toroids themselves have strange failure modes and may pass resistance and continuity tests, yet still not provide proper drive voltage to the attached devices. This has to do with core permeability.  Each toroid feeds two RF MOSFETs, replace part is IRF-350.

As always when dealing with an SX transmitter, good luck.

Oh damn, La cuarta parte

Today there will be a quiz.  Ready?  Look at this picture and see if you can spot the problem:

Problem with Harris SX2.5A transmitter
Problem with Harris SX2.5A transmitter

If you said “Hey, that green wire seems a little odd; it disappears behind the heat sink next to that screw then reappears again at the top,” you are correct.  What really sucks is the green wire is the transmitter off connection to the remote control.  So, when the PA board was secured to the heat sink, the wire was trapped between the board and the heat sink.  Since the components were cold, they did not pinch through the insulation right away, no.  Rather, after the transmitter ran for several hours at full power, it got hot enough to displace the wire insulation and cause a short.  Doh!  The transmitter is off and it won’t come back on!

This is a picture of the wire after it was removed:

Pinched wire
Pinched wire

Haste makes waste.  Unfortunately, it was someone else’s haste that ruined my Saturday afternoon when I was supposed to be taking my son to little league practice.  I am sure that some not-so-kind words will be exchanged very soon.

Always double-check your work.

Radio Caroline, 49 years after

Radio Caroline went on the air forty-nine years ago this weekend, broadcasting from the MV Caroline off the coast of England.  Why is this important?  Before offshore broadcasting was attempted, in Europe the only radio stations (and TV) were government owned.  As such, they had a monopoly over the airwaves and were very restrictive on which groups or types of music they allowed to be broadcast.  Many of the so-called “British Invasion” groups like The Beatles, Rolling Stones, The Who, The Kinks, etc got their first airplay on offshore radio stations like Radio Caroline or Radio London.

This video “Radio Caroline – A Day in the Life,” shows what it was like to be an offshore broadcaster:

By the haircuts and music, that appears to be sometime in the eighties.

Check out the Radio Caroline website for more information.  From 1983 onward, Radio Caroline was broadcast from the MV Ross Revenge. This is an overview of the Ross Revenge transmitter hold.  The movie “Pirate Radio” is loosely (very loosely, by most accounts) based on Radio Caroline/Radio London composite.

Radio London was one of the other well-known offshore radio ships.

I am sure that there are other tribute sites with lots of technical information on how they broadcast. Much of offshore radio was outlawed in the late 1960s by several European countries. Radio London signed off on August 14, 1967. Radio Caroline continued on in various iterations until about 1991 or so.

WBCQ is airing a radio ships special on Sunday, March 31, 2013, at 5,110 KHz starting at 6 pm Eastern daylight time (2200 UTC).

The Nautel XR6 AM transmitter

I’ve been away working in Burlington, VT (WVMT, 620 KHz, Burlington)  for the last coupla, installing this nifty Nautel transmitter:

Nautel XL6 transmitter, WVMT Burlington, VT
Nautel XR6 transmitter, WVMT Burlington, VT

I like the Nautel units, both AM and FM;  they are well-designed, well-built, rugged transmitters.  I have lost track of how many of these units we service in the field, partly because they are becoming pretty much standard equipment at all of our installations.

Continental 315R-1 AM transmitter, WVMT, Burlington, VT
Continental 315R-1 AM transmitter, WVMT, Burlington, VT

The transmitter it is replacing is a Continental 315R-1, which is based on the Collins Power Rock design.  It is a PWM transmitter with a 15,000 volt power supply.  In their day, these were not terrible transmitters, however, like their Harris MW-5/10/50 PDM brethren, frequent thorough cleaning is required to keep the dirt/dust from arcing over.  Unfortunately, it is becoming more and more difficult to obtain parts for these units. This transmitter was installed in October of 1983, thus, almost thirty years of service is quite enough.  This unit we did not cut up and scrap, rather, it is sitting by the back door, waiting for any takers.

Continental 315R1 modulator/RF sections
Continental 315R-1 modulator/RF sections

The interior of the Continental 315-R1 transmitter.  Modulator section is on the left, RF section is on the right.

The good news is, WVMT is another one of those “successful AM station” stories.  You know, the kind of station that has local programming, local sports, news, community presence and most importantly, makes money.  For all those diligently studying the “AM Problem” for the up and coming NAB conference this April, here is a clue: It’s the programming…

Nautel XR6 transmitter, wvmt Burlington, VT
Nautel XR6 transmitter, WVMT Burlington, VT

This is the Nautel XR-6 on the air.  Positive peaks, anyone?

AM modulation monitor
AM modulation monitor

We turned that down a little bit.  Also, the station does not run AM stereo, the AM stereo mod monitor is simply a usable relic of a bygone era.

WVMT is noted as the first radio station licensed to the state of Vermont, signing on on May 10, 1922.  It has a three tower directional array located down in the swamp.  For some idea of perspective, it is 1,150 feet (350 meters) from the transmitter building to the center tower, the towers are 411 feet (125 meters) tall spaced 405 feet (123 meters) apart.

WVMT three tower directional antenna array, Burlington, VT
WVMT three-tower directional antenna array, Burlington, VT

WVMT antenna system from back of transmitter/studio building.  That is a long walk over rough terrain in the middle of the night or anytime really, but especially in the middle of the night.