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 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.

NXP BLF177 MOSFETS
NXP BLF177 MOSFETS

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

Harris Z series PA module with MOSFETS removed
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
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 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.

Harris Z FM series PA module repaired
Harris Z FM series PA module repaired

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
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
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 mountain top 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 tool box.  I can say this with all honesty; not me.  In the past they have also rented a helicopter, used a snow cat, snowmobiles 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:

Gate to transmitter road
Gate to transmitter road

Then it goes up the hill:

Transmitter site access road
Transmitter site access road

Some sections are worse than others:

washed out road
washed out road

Along the way there are some nice views:

City reservior near transmitter site
City reservior near transmitter site

Finally, the gate to the tower farm:

Access gate to transmitter site
Access gate to transmitter site

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:

view looking north
view looking north

The reason why we came:

Transmitter room
Transmitter room

That is a 24 year old BE FM5B transmitter.  The back up 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 be 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.

Why we like Nautel Transmitters

Because they work.  The old adage is, you get what you pay for.  There are many transmitter manufactures.  There are plenty of transmitters out there that are less expensive.  Those less expensive transmitters sound fine on the air, their AC/RF efficiencies are great, they look snazzy in the sales brochure.  I am sure the RF sales guy can spout out ten reasons why they are this or they are that.

And that is great.

Their parts count is intentionally kept low, so gone are the redundant power supplies, fans, RF amps and controller cards.  Gone are the extra heavy output capacitors, combiners, LC connectors on the RF stages.  Gone is the heavy grounding buss, the shielded covers on the controller, etc.

So, ask the slick RF sales guy if he is going to be available to answer the phone after the 2 am lightning strike.  Of course, he’ll lie… whisper sweet nothings in your ear…

To avoid all that, behold:  The Nautel V40 FM transmitter.  This is four V-10 transmitters into a magic T combiner.  The V-10 already has good redundancies.  Four of these things ganged together should be nearly bullet proof (and over the last three years, it has been).

WHUD Nautel V-40 Transmitters
WHUD Nautel V-40 Transmitters

This site has been fraught with power problems because it is at the end of a very long utility company feeder line. We also installed a LEA series surge suppressor.  We like the LEA unit, it has saved our bacon many times over.

Inside view of LEA surge supressor
Inside view of LEA surge suppressor

These transmitters normally run at about 7 KW each.  I can turn any one transmitter completely off and the others will automatically adjust their output powers keeping the station at full power.   That means daytime maintenance!  We like not having to drive to the transmitter site at night to vacuum.   It is really cool.

Therefore, to recap (in case you missed the major points of the story), we like the Nautel transmitter because:

  1. It does not go off the air
  2. If something breaks, I can turn off an individual transmitter and fix it
  3. I can clean them and do everything I need to during normal working hours
  4. They sound great on the air
  5. Nautel has excellent customer service
  6. They look cool

There you have it.