AM – Page 12 – Engineering Radio

Accidents, mistakes, mishaps and other tales

Let’s get started:

Results of a deer vs automobile accident

It does not look like much, however, that is about $5,500.00 worth of damage. What you don’t see is the mashed oil cooler and radiator. This happened on my way from one place to another during the early morning hours. I was traveling at about 55 MPH when a deer bolted from the woods and entered the roadway from the right. I did not have time to a break.

A momentary lapse of attention causes a loss of $80.00. I think I was adjusting the defroster as I was driving down the road when suddenly, I felt the car tilt over to an alarming degree. You can see the tow truck getting ready to pull it out. Fortunately, there was no damage to the vehicle.

This is on the access road to one of our transmitter sites. The station has a legal right of way through this property, however, the neighbor seems to object. I spoke with him and showed him a copy of our deed, he has since changed plans.

One side of a balanced audio connection disconnected

This is the downside of using category cable to make audio connections. The wires are not as rugged as say Belden 8451. This was causing problems because it is at an AM studio/transmitter site.

Burned 30 amp three phase contactor — Burned 30 amp three-phase contactor

Three-phase, 30 amp, 240-volt contactor installed in a 480-volt system. Lasted a few years, anyway.

White face hornets nest — White face hornet’s nest

New tenants on one of our towers. This is a white-faced (or bald-faced) hornet’s nest. They are really paper wasps, but that difference aside, these beasts are nasty, aggressive, and have a painful sting. Normally, I am a live-and-let-live kind of person, but in this case, they gotta go.

This is at one of our AM client’s site. Somebody, quite some time ago it seems, made this test load for a 1 KW AM transmitter. It is very nice, with carbon ceramic resistors, 50 ohms, and surprisingly little reactance. Then they attached it to this piece of plywood. As one can surmise, the load gets quite hot under full power and full modulation conditions. We remounted this in a cage-type enclosure and bolted it to the cinder block wall.

Scala PR-950U cross polarized — Scala PR-950U cross-polarized

The client at this station is complaining of intermittent STL dropouts and low signal strength at the receiving end. Found this Scala PR-950U antenna mounted for vertical polarization, but the antenna element is horizontally mounted. We’ll call it “vorizontal.”

Ribbon cable from a Cummins 135 KW generator

This was discovered during routine maintenance and thankfully not during a power outage. Mice got into the control box of a newish Cummins 135 KW generator and chewed through what looked like a data buss cable. The generator would not run and the cable and control board needed to be replaced.

There are more bulging capacitors removed from flat panel monitors.

And so on…

Whatever happened to the CFA?

Remember way back when, perhaps in high school or college, you met this really cool person who seemed to be wonderful in every way? Yeah, then you got to know them a little better, and, well, those first impressions changed a little bit.

Crossed Field Antenna, Courtesy of Wikipedia

The Crossed Field Antenna (CFA) sort of reminds me of my first prom date. There was a lot of promise there, but plans fell through.

From a 1999 Radio World article:

Imagine an AM antenna one–fiftieth of a wavelength long, that needs no radial ground system, occupies a small parcel of land, produces little or no RFI (Radio Frequency Interference), has great bandwidth and performs better than a full–sized vertical radiator.

This potential new antenna was all the rage during the early 00s or whatever you call that decade. I remember thinking to myself; I will believe it when I see the test results. At one point, there was a battery of tests run in the installation in Egypt and China. The test results are spotty at best, however, none of these installations performed up to expectations. While it looks like a cool idea, and it would have been great to see it succeed, it seems that sheer willpower alone will not make a particular system work outside of the laws of physics. There are a few of these still in operation out in the wild, mostly in Egypt.

Fixing small problems

This happened recently at an AM station we were doing work for. It seems the modulation monitor was not working when connected to the backup transmitter. A quick check of the RG-58 coax showed that I had the correct cable plugged into the monitor selector relay. Another check with an ohm meter showed the cable was okay. Then I looked at the connector on the monitor port of the transmitter and saw this:

Looks like the pin is too far back in the connector. This is an old-style BNC connector with solder in center pin:

BNC connector center pin — BNC connector solder type center pin

The center pin has a blob of solder on it, preventing it from seating properly in the connector body. I could have lopped it off and applied a new crimp on connector, but my crimp tool was in the car. I didn’t feel like walking all the way through the studio building, out into the parking lot and getting it. Therefore, I used a file and filed off the solder blob then reassembled the connector:

The transmitter was installed in 1986, I think the connector had been like that for a long time.

It may seem like a small detail to have the modulation monitor working on the backup transmitter, however, the modulation monitor is also the air monitor for the studio. Switching to the backup transmitter but not having a working air monitor would likely have caused confusion and the staff might think they are still off the air. I know in this day and age, a lot of station do not even have backup transmitters, but when something is available, it should work correctly.

I like my cool network analyzer and all that, but sometimes it is the Mark 1, Mod 0 eyeball that gets the job done.

Transmitter repair

Sometimes it is obvious and relatively easy, other times not so much. This summer we have had wave after wave of afternoon thunderstorms. It is almost like living in Florida; almost, but not quite. Anyway, with the storms occasionally comes some lightning damage. At most of the transmitter sites we service, every step has been taken to ensure good grounding and adequate surge suppression. This is especially true of sites that have been under our care for a few years. Even so, occasionally, something gets through. After all, those five-hundred-foot steel towers do attract lightning.

Broadcast Electronics AM5E output tuning section

This is the output section of the BE AM5E transmitter at WROW. The transmitter got pretty trashed; a bad PA module and power supply and this capacitor in the output section. This particular transmitter is 14 years old and this is the first major repair work we’ve had to do it.

Broadcast Electronics AM5E output tuning capacitor

The capacitor was fairly easy to change out. As a general precaution, both capacitors were changed. There was a spare PA module and power supply on the shelf, thus the transmitter was returned to full power relatively quickly.

Broadcast Electronics AM5E output forward and reflected power meters

The rest of the antenna system and phasor were inspected for damage, a set of common point impedance measurements taken, which showed that no other damage was sustained.

Next, the 30 year old Harris SX2.5 A transmitter at WSBS. This failure was slightly more exotic; the transmitter started randomly turning itself off. The culprit, in that case, was this:

Harris SX2.5 remote control interface bypass capacitor

Literally, a two cent part. The transmitter remote control uses optoisolators. The inputs to these opto-isolators are RF bypassed to ground on the back of the “customer interface board.” After determining that the remote control was not malfunctioning, it was down to either a bad opto-isolator or something really silly like a bypass capacitor. This capacitor was on the ground side of the remote off terminal. It shows short on the capacitance meter and 4.1 K on the ohm meter, just enough to randomly turn the opto-isolator on and shut down the transmitter. Being a Harris transmitter, removing and replacing the “customer interface board” was no easy matter. Overall, it took about three hours to find and repair this problem.