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.

EAS

Emergency warning siren station
Emergency warning siren station

EAS, or more properly, the Emergency Alert System, is a government mandated system of encoders and decoders designed by the federal government to alert the public in case of war or other emergency.  It, and it’s predecessor, EBS (Emergency Broadcast System) have never been activated by the federal government.  Both systems, however, are used extensively by local and state governments for things like weather alerts, amber alerts, etc.

Back in the mid-90’s the FCC had a chance to redo the EBS and produce something that was a streamlined and effective tool for public warning.  Unfortunately, the EAS system is neither.  Rather, it is a cumbersome system of weekly and monthly tests scheduled around pre-conceived notions that how the system is tested every week will be how the system works in an emergency.  In practice, this is generally a good theory of system design, but it has failed miserably with EAS.  The reasons why are thus:

  • Most all emergencies are local or at most state wide events.  To this day, very few state and or local government emergency managers would be able to activate EAS for their area.  The reason is there is minimal if any interface with the LP-1 EAS stations or station personnel.  Ignorance and apathy on behalf of both radio station personnel and government officials is the main culprit.
  • Most stations are un-maned for large portions of the day.  Even if government official could/did call the station, chances are, nobody would be there.  If by chance, arrangements were made to contact station employees at home, they would have to interface with the EAS equipment remotely, which ads complexity to an already complex system.
  • EAS messages are still mainly relayed from radio station to radio station, the so called daisy chain net work that has been shown numerous times to be unreliable.
  • The system of SAME codes, FIPS identifiers is not necessarily bad, the application in this case leaves something to be desired.  The FCC had a chance to update EAS before the HDTV rollout.  One would assume that any improvements could have been built into the new TV sets that are now being sold, but again, that opportunity was missed.  For example, I suggested that each TV have a set up screen option where the owner could input their zip code.  They could also choose what types of alerts they would want to know about and even base the alerts types on the time of day.  Live in a flood zone, the FFW (Flash Flood Warning) 24/7.  Live in tornado alley, TOR (Tornado Warning), etc.  The cable companies then pipe in the local NOAA all hazards radio station.  All the sudden there is a real national alert system in place using mostly non-broadcast wireless systems.  Add to that the ability to sign up for emergency e-mails and text messages for specific areas (many places are currently doing this) and there is multiple message paths.

The system as is is not reliable and sooner or later that will be shown with a large scale failure.  Recently, the FCC held a summit with the Department of Homeland Security.  The cliff notes version of this event is: Yes, the system can be made better.  Let’s keep throwing the same ideas at the wall and see if anything sticks this time.  Excuse me if I don’t do back flips, this is the same information that was discussed during the last “lets revamp EAS” discussion back in 2005 (04-296).

In the mean time, the EAS continues to be a good fund raiser for the FCC enforcement bureau.  Which, you know, it is easier to go to a licensed radio station and bust them for not re-transmitting the RMT (Required Monthly Test) than it is to go out and bust some of the numerous pirate radio operators, some of whom are operating in the same city/metropolitan area as a field office.

The shame of it is, it could work without a great deal of cost, very well.

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.

Stuff that program directors like

If you work at a radio station that still has a local program director instead of one at the corporate programming lair (I know, sooooo old school), then you might be interested in this.  I compiled a list of things that radio station program directors like:

  1. Good ratings.  A good rating book means that they are great program directors and they really know their stuff.  Bad ratings means that engineering dropped the ball (again) when the station went off the air for 30 seconds during afternoon drive.
  2. Taking credit for anything good.  Sort of goes with the good ratings above, but this extends out to all other aspects of a radio station, promotions, sales, news, and even engineering.
  3. New Processing.  Any new gizmo or gadget that changes the sound of the microphone or entire station, for better or worse, is good.  The more flashing lights the better.  The more knobs to adjust the better.  Things that can be plugged into computers and remotely controlled are the ultimate.
  4. More.  More of anything is better, more compression, more expansion, more highs, more mid-range, more lows, more gain, more de-essing, more loudness, more power, more punch, more reverb, more crack, more more more.  If they could just have a little more, the station would be number one.
  5. Any other new piece of equipment.  Watching a program director look at a new studio is like watching a two year old open presents on Christmas morning.  I know, I have a two year old.  Unfortunately, the studios don’t stay new looking for long.
  6. Taping notes up in the studio.  I have one studio where every stationary piece of equipment has a note taped to it.  Mind you, the notes have nothing to do with the equipment they are covering up, they are more like general directions, phone numbers, and other miscellaneous pieces of information.
  7. Free stuff.  Used to be called payola  or plugola, now it is a free lap top, or a trip to Disney paid for by the record rep.  I’ve even seen some mysterious mike processors show up (see number 3).
  8. Rigging up lights to alert operators.  This is a great one, the studio operator does not know if the Marti (or Matrix or ISDN) is active, so they want a light to indicate there is someone there.  Or a light on the phone hotline, or a light for the EAS machine, the back door, the coffee machine, the silence sensor (never mind they are in the studio, they still need a silence sensor light)
  9. Blaming other people when things go wrong.  The program director is infallible.  If something goes wrong, it is somebody else’s fault.  Always. And forever.  Amen.

Some one suggested that I put up the video “More, more, more” by Andrea True Connection to go along #4.  Well, okay, I guess.  It is not a terrible song but the video kinda suxor.  From what I can tell, Andrea True is a former p0r n star that turned signer for just this one hit. Looks like it was filmed on a p0r n set too.

Feel free to add anything else that I may have forgotten.  Of course, this is all in good fun.  I’ll to a “stuff radio engineers like” post as soon as I figure out what that is.