EEEK! IT’S SAFETY CODE SIX!

Industry Canada used Canada Post to drop a small bomb on AM stations since our last column… a belated Christmas present, as it were. Safety Code Six, enacted by Health Canada a number of years ago to protect the public from excessive levels of RF radiation, has finally come home to roost at AM transmitter sites. For most of us, the main impact up to now from Safety Code Six has been more strict guidelines from our tower riggers when tower work has been needed. Now Industry Canada has informed us that AM transmitter sites must be made to comply with Safety Code Six right now, which is a little bit ironic, since all AM transmitter sites break the Code inherently to a certain extent. Even a 1 kW transmitter site will have areas near the tower(s) where one can get overexposed, or come in contact with currents exceeding the Code limits.

Fortunately, Industry Canada is looking for some fairly practical matters for compliance. The intent is to prevent the general public from getting too large a dose of radiation, or excessive contact current. The first line of defense at an AM site is the fencing around each of the towers in the array. It should be at least two meters tall and locked. There should be red danger signs around the towers—more about that in a minute.

Since there are if not hot, then perhaps warm areas near the towers, we’re now required to place amber warning signs at the site’s perimeter, which may be a new concept for most of us. Very few AM transmitter sites have perimeter signage already in place, so this will be something new. Part of the Safety Code Six bulletin contains a description of the signage necessary. An important comment from Industry Canada adds that the signage should be bilingual. CAB has come up with some fine examples for your local sign-maker. They must be at least 9” x 12” in size. Send me an e-mail if you need a copy!

TRANSMITTER MAINTENANCE TIPS YOUR TX MANUFACTURER NEVER TOLD YOU!

I have a small pet peeve with the Nautel transmitter company. They seem to hate to inform the transmitter-using public (you and me) when there’s something that needs saying about one of their products. I’ve tried to tell them over and over that we love to hear about these things, especially if they help us to improve their transmitter’s reliability. Maybe one day this stuff will appear on their web site, but until then…

(A) Those who have been maintaining Nautel AMPFET 50 and the ND series of Nautel transmitters may already know this, but those I canvassed did not, so I include the information here: there is a tuning coil on the back of each PA amplifier cube. The manual tells you that you need only worry about adjusting the coil if you change the transmitter operating frequency. The manual is incorrect! You may also need to adjust the coil if you have changed more than “a few” of the RF power transistors, P/N IRF 140.

When International Rectifier upgraded the IRF 140 transistor, they tripled its gate capacitance. As you change more and more transistors in your transmitter with the new type, the resulting detuning causes more and more current to be drawn from the exciter RF output. Eventually, the transmitter will either shut down from low RF drive, or blow the fuse powering the RF amplifier inside the exciter.

Nautel has a test jig available for inserting in the feed to each cube, which produces a sample voltage proportionate to the current drawn by the cube. The coil is then adjusted for minimum sample voltage.

(B) Next time you are repairing a PA assembly in an elderly Nautel transmitter, check those three huge electrolytic capacitors in the cube assembly. I have been double-checking lately, and have been surprised to find quite a number of them have dried out and opened up. They are the main filters that regulate B- voltage for the transmitter, and I imagine the transmitter operates better when they’re doing their job (!) I don’t know why I was surprised by this—how many electronic devices do you know of that don’t require their electrolytic capacitors to be replaced after 10-15 years? It just never occurred to me… but make sure it occurs to you, next time.