Maintenance – Page 7 – Engineering Radio

Repairing a solid state FM transmitter module

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 troubleshot 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 crowbar 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.

Harris FM Z series transmitter PA module with cover removed

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 screwdriver. There are several components that have to be moved to work on this.

Harris Z series PA module with MOSFETS removed

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.

Harris Z series FM transmitter module new MOSFETs waiting to be soldered

The new MOSFETS are very sensitive to static discharge, so I always use a static drain wristband when handling them. I place both MOSFETs onto 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:

Harris Z series FM PA circuit board under test, resistance is 3.3 Mohm

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:

Harris Z series FM PA module under test, DVM reads 1.6 ohms

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.

A trip to the FM transmitter site

A few pictures from my last trip to one of our FM transmitter sites. This is a mountaintop site, in as much as a medium-sized hill is a mountain around here. This site has a 2.3-mile road through the woods that is almost impassable 3-4 months out of the year.

Previous engineers have walked up the hill with a toolbox. I can say this with all honesty; not me. In the past, they have also rented a helicopter, and used a snowcat, snowmobile, or an ATV with snow tracks. I’d do those things provided they are safe and insured. As I get older (and wiser), I realize that the only person who going to look after my well-being is me.

Anyway, the trip starts here, at the gate:

Then it goes up the hill:

Some sections are worse than others:

Along the way there are some nice views:

City reservior near transmitter site — City reservoir near the transmitter site

Finally, the gate to the tower farm:

There are two digital TV stations, several cell phone carriers, some government two-way gear, some FM translators, Media Flow, and us at this site. There are also some Ham radio repeaters off to the side in another building. All in all, a pretty RF-intense site.

The view from the top:

The reason why we came:

That is a 24-year-old BE FM5B transmitter. The backup is a Gates FM5G, which aren’t we glad we have a solid reliable transmitter selection for such a remote site. Actually, we were supposed to put in a new Nautel V-10 here last year, but the money was spent on computers instead. Oh well, good thing there will be no computer crashes when we go off the air.

A standard maintenance trip consists of meter readings, comparing the reading to the last set of readings, changing the air filters, checking the remote control and calibrating it to the transmitter, checking the tower light sensor, etc.

Normally, the backup transmitter would run into the dummy load, but the backup transmitter no longer works. Parts are not available to fix it, so we operate without a net. One of the previous general managers asked if that keeps me awake at night, to which my answer was no, not at all.

Tower down

You know it is going to be a bad day when:

a farmer mowing grass took a wrong turn on June 16, KFEQ AM lost a guy wire and eventually one of its four towers came toppling down

From Above Ground Level magazine.

Is hiring the farmer down the road to come and mow the AM field a smart thing to do? It depends, I suppose, on whether or not your towers will be standing afterward. Hopefully, the guy had some insurance, if not then the station is basically screwed. The article did not mention that, although it did state that “The station is weighing its options,” Which does not sound good.

The good news is at least they were doing the maintenance. Most AM stations these days don’t even bother to mow the fields. Look at this picture:

Tall grass at an AM transmitter site. Owner says don't cut it. — Tall grass at an AM transmitter site. Owner says don’t cut it.

It is not that I don’t want the grass cut, I do. However, I am not going to pay for it out of my own pocket, that is ridiculous. So, it grows.