AM Transmitters – Page 3 – Engineering Radio

Well designed circuit boards

Sometimes it is the little things that catch the eye. When I was installing a Nautel transmitter recently, I was admiring the circuit boards used for the transmitter controller. I have seen a few circuit boards that are functional, but leave a little to be desired in the form department. Does it really matter? Perhaps not, but often times those tiny, almost insignificant details come back to bite you. Little things like having the voltage regulator pins correctly placed or putting a toggle switch on the correct side of the board. I have seen both mistakes from another, well known transmitter manufacturer.

Anyway, these are a few photographs of some well designed, well laid out circuit boards.

This is the main controller board.

Surface mount components.

Logic chips.

Nautel XR harmonic filter, part back part is the circuit board

Part of the harmonic trap for the XR series transmitters.

It really is the little things that make big differences. A circuit board under a cover that few people will ever see may seem like a very small and insignificant detail, but I notice and admire these things…

Proper drive levels for a Harris SX serries transmitters

I was working on a Harris SX5 the other day and snapped a picture of the scope while measure RF drive levels. There are still quite a few of these units out the 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 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 the each of the toriods that feed the gates of the RF devices.

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 a 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:

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, it 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:

Haste makes waste. Unfortunately, it 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 off shore broadcasting was attempted, in Europe the only radio stations (and TV) were government owned. As such, they had a monopoly over the air waves 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 the 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 1960’s by several European countries. Radio London signed off 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, 5,110 KHz starting at 6 pm eastern daylight time (2200 UTC).