For about 4 years, I lived in a house next to a four hundred foot radio tower. Although I never actually saw lightning strike the tower, I heard it several times. Like everything else, after a while, you get used to it.
This is a video of lightning hitting the WSIX STL tower in Nashville, TN. The camera work is a little unsteady, the strike occurs around the 1:36 mark:
Gone is the day when the radio station engineer had to trouble shoot down to the component level, often crawling in and out of transmitters to get at the suspected part. I for one, spent many a long night at a transmitter site chasing some weird combination of symptoms down to the $0.34 1N914 diode in the directional coupler (see previous post about the MW-50).
It is a skill set now mostly confined to manufactures’ repair departments, for which they charge a pretty penny. Nowadays, the technician simply slides out one module or circuit card and slides in another. If that doesn’t fix it, panic ensues. I know of several class C FM radio stations that are now relying on the computer guy to fix transmitters, because, you know, it’s cheaper.
To be fair, most engineers are contractors and many of those simply do not have the time to trouble shoot to the component level. So, they ship everything back to the factory then pass the cost on to their client.
Then of course, most circuit boards these days are surface mount systems, which are hard to work on if you don’t have the right tools. Normally an expensive temperature controlled soldering station is required, as well as a magnifying glass.
All of these things combine make circuit board work something to be outsourced. Unfortunately, a night spent trouble shooting was often a great learning experience. I have done some of my best work when my back was up against a wall and I was out of options.
I make the attempt to fix things locally, unless the transmitter or other item is under warranty or not having a spare/attempting to trouble shoot will take the station off the air. I think it is important to keep abreast of technology and keep my trouble shooting skills up to par. Besides, I find it gratifying that at least I can still fix things.
Engineers are funny. We all have our likes and dislikes and our reasons for both. I don’t really like Harris products. Even when I was in the military, their stuff seemed a little “light.” I suppose having to deal with an MW-50B transmitter at my first full time chief engineer gig didn’t help that impression. The MW-50 would “blow up” every six months or so. I say blow up because that is the only way I can describe it, no overload lights or any other indication of trouble until the blue lightning flashes and thunder from the PA section. What a POS.
Other Harris transmitters, such as the SUX-1, FM20H, Gates-1 etc have also left me less than impressed.
In order of preference, my choice of AM transmitters would be:
Any Nautel solid state unit. Nautel makes good equipment that is well supported.
Any BE solid state transmitters. I favor the A model over the E model, but both are good. One condition, they must absolutely be well grounded and all of the toroid filters provided by the manufacture must be used when installing.
Any tube type Continental transmitter. There are older units, I believe 816R but they work well and sound good on the air.
We have a Harris Z5-CD transmitter for one of our FM stations. Brand H is not my preferred make, however, it was already installed when we bought the station, so I have to live with it.
This particular site gets hit by lightning strikes often. Normally, it does not affect anything until the transmitter gets turned off for maintenance. Then, almost invariably, when turning the transmitter back on one of the modules will fail. Most often this is manifest when one of the two power supplies shut down causing the transmitter to run no more than 20% power.
The way this is trouble shot is to slide each module out and turn the transmitter back on. When the power supply stays on, the bad module has been located. A confirmation test is to check the MOSFET for a short circuit between Drain and Source. This short circuit condition puts a direct short on the power supply causing it to crow bar and turn off.
So, once the bad module has been located, and the spare module is installed in the transmitter, then what? Most engineers call Harris and ship the module back for repair. Most engineers don’t want to mess with unsoldering a surface mount MOSFET and soldering a new one in. I find it moderately entertaining to fix things myself, so I do not do what most engineers do.
The MOSFET in this particular module is the BLF177, made by NXP. Harris will sell you one for quite a bit of money. You can also buy one from Mouser for about half the cost.
Once the parts are obtained, the worst part of the entire job is unsoldering the old MOSFET. This takes some patience and skill. What I found works best is to melt some solder on the foil leads and get them good and hot. Since this MOSFET is already destroyed, we don’t have to worry about heat etc. The one thing you do not want to do it actually break the MOSFET open. That is because it contains beryllium oxide, a known carcinogen. Once all the solder is liquid, carefully pry the foil up with a small screw driver. There are several components that have to be moved to work on this.
After the old MOSFET is removed, clean up the solder pad with a solder pump and solder wick. I like to use a little liquid flux on the solder wick, it makes things go faster.
Once all the old solder is cleaned off the solder pads, I brush a light coat of liquid flux in the pad. Again, this makes things go faster.
The new MOSFETS are very sensitive to static discharge, so I always use a static drain wrist band when handling. I place both MOSFETs on to the circuit board. I then solder them on using as little heat as possible from the soldering iron. Again, the MOSFETs are sensitive to heat and one can easily be destroyed if it gets too hot.
This is the module with the new MOSFETs soldered in. I use defluxing compound to remove all the extra flux. Once it cools off, I test the new module with a DVM:
If the MOSFETS are good, they will have an internal resistance of around 3.3 MΩ. If the module is bad the MOSFETS will read only a few ohms if shorted:
That is how you do it. I think Harris charges $775.00 per module to repair. I fixed this one for $240.00, but that is not the reason I did it. I did it for the fun that was in it.