February 2019
M T W T F S S
« Jan    
 123
45678910
11121314151617
18192021222324
25262728  

Archives

Categories

Module swap guys

Nautel V-10 Power Supply

Nautel V-10 Power Supply

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.

Underground Electrical feeder burn out

In the never ending saga of things I have not yet seen, we had a 350 kcml 3 phase underground feeder burn out yesterday at our AM transmitter site.

Actually, it happened the day before, around noon.  I received a call from the remote control that we were on the backup generator.  Upon arriving at the site, I found several trees down on the three phase primary down the street.  I figured that was the cause.  After checking the generator fuel supply, oil pressure, temperature, phase volts/amps, I decided that everything was okay and the power company would be along shortly to restore power.  I then continued up the road to our FM site to do weekly maintenance.

Upon returning to the office several hours later, I looked at the utility company website.  They have a pretty cool interactive map application that shows all outages and give restoration times.  The area around our transmitter site showed no outages, therefore I figured it had been cleared.

I committed two errors here:

  1. Not calling the utility company myself to ensure that the outage was reported.  I assumed that the tree across the three phase primary was the cause, it was not.
  2. Not calling the transmitter site remote control to check the generator status after I checked the website.

To be honest, I don’t know if I am coming or going these days.  With seven radio stations, each with it’s own transmitter site, three of them Directional AM stations, and three studio locations spread out over a 75 mile stretch, it is difficult to keep up with the small details.  Did I mention that I am solo, the engineering assistant position was cut two years ago.  But, I am not here to make excuses…

The net result is the generator ran all night long.  The next day, when I checked the transmitter in the morning, I was surprised to find the generator still running.  Unfortunately, I had an FM station on low power (see post below) that needed to be taken care of first.  When I finished replacing the RF module in the FM transmitter, I made my way to the AM site.

I called the power company then checked the generator fuel, the propane tank was down to 10%.  Yikes, better get this taken care of fast.  I will say the power company showed up pretty quickly.  After some measurements with a hand held meter, it was determined that the under ground feeder was open between the pole and the transmitter building.

The line man was not at all surprized, in fact, he called it before he even went up in the bucket truck.  After some back and forth with his supervisor, who came out in a pick up truck, it was decided that they would run a temporary overhead feed to the meter can.

temporary overhead electrical feeder

temporary overhead electrical feeder

They also did some research in there records and discovered that they (the utility company) own the underground cable and therefore they would dig it up and fix it.  That’s nice because otherwise it seems like it would be an expensive repair job.  On a station that makes not a lot of money.

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.

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 wieghing it’s 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.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

Free counters!