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.
I began fooling around with radios when I was 10 years old or so. First, I built one of those shortwave radio kits from Radio Shack, which was back when they still sold radios.
Then I bought a small tube type AM transmitter at a garage sale. The woman there said her son built it several years ago from a kit and it had the instruction manual. I don’t even know who made the kit. After some experimentation and changing out some tubes, I got the thing to transmit on about 1600 kHz, although it was a little hard to nail down as it drifted quite a bit until everything heated up. I don’t know what power that thing put out, but it was certainly less than a watt.
All of this lead to a brief stint in the military as a radioman. That was an interesting field, albeit different from what I thought it would be when I signed up. It was during this time that I did some part time work at an AM/FM/TV station assisting the Chief Engineer. Once it was established that I actually knew something, my responsibilities grew until I was assigned the AM/FM part of the deal.
After a year of that, I moved to a different city for family reasons and took the Chief Engineer job at a local AM/FM station. The AM station was a 50,000 watt directional in the high end of the band which had a Harris MW-50B transmitter. My previous station had a Bauer 10,000 D AM transmitter. What could be so different? Plenty I learned, on my second day.
We were subjected to a wicked lightning storm, which, Murphy being present, took out the main transmitter. The backup was a GE BTA25 which was running at half power because of the age of the 5891 final tubes.
The symptoms of the MW-50 where as follows: It would run along fine then there would be a big blue flash and a cannon shot boom, followed by the step start relays cycling and it would come back on the air. There were no overload lights nor any other symptoms leading up to the overload or subsequent to it.
I began by killing the power and shorting out all the high voltage parts with a shorting stick. I noticed that things inside this transmitter where a little unusual, so I got the manual out and started reading. The most unusual aspect of this transmitter is the 25 KV isolated box that the PA stage occupies. 25,000 volts DC is a great big potential and what I found over the years is that this transmitter needs to be kept very clean. Of course, this unit had not been, and that was a part of the problem.
The other unique aspect of this transmitter is the damper diode, which is required by PDM transmitters to conduct voltage during the negative modulation peaks. If the damper diode breaks down for any reason, the PA supply voltage tries to go to infinity, which is a good deal larger than 25KV and all sorts of problems begin.
To make a long story somewhat shorter, this is the problem I had. The solid state damper diode had one bad section, which was causing all sorts of corona problems during heavy negative modulation peaks. It took a call the Harris factory to determine this. The entire diode assembly needed to be replaced because every section is matched. That cost a couple of thousand dollars as I recall.
While I was working on the MW-50B transmitter, I was not impressed. It seemed a little cheap and flimsy. Later, when I voiced my concerns with the station management, the Harris transmitter salesman stopped by and said I needed to get with the program if I wanted to work in that market. This was a Harris town you see, if you start bad mouthing our products, you’ll be the one to suffer. Well, he retired, I kept looking around for other AM transmitters. Three years later I went to work for the competitor across town. Today that station has a Nautel ND-50.
The MW50 went off the air once every 6 months for the entire time I worked at this station. It was always something different, power supply rectifier, bad PDM board, bad directional coupler, arcing insulator on the isolated box, etc. I began to feel it didn’t like me, and I know I didn’t like it. In fact, you could say I have never really liked Harris transmitter products ever since.
Update: Okay, I left a few things out of the narrative:
The 50 KW air cooled power supply was the light weight version. Most MW-50 transmitters had 100 KW oil cooled supplies. The problem with the 50 KW power supply was it was designed with a zero safety factor. All of the rectifier were running at or near maximum current and voltage. It only took one of 144 diodes to go bad, either short or open, and the whole transmitter would crash. Again, no overload lights or other indications of problems. We later installed air flushing fans in the power supply cabinet to keep things cool and that helped out quite a bit.
The other thing was a DC feedback sample to the PDM card. It seems that if the filaments were turned off before the bleeder resistors took the 25 KV supply to zero, the remaining voltage would be routed to the PDM card via the DC feedback sample, blowing the foil off of the circuit card. We fixed this by installing a gas discharge tube with a series resistor at the connection point for the DC feedback sample.
Then there are the infamous 1N914 diodes in the directional coupler that Dave points out below.
I am sure I am forgetting something else, but you get the idea.
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:
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.
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.
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.
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:
Then it goes up the hill:
Some sections are worse than others:
Along the way there are some nice views:
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 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.