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:
BNC connector pin improperly located
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 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:
BNC 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.
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.
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.
Recently, we had an AM antenna array go out of tolerance by a good margin. This has been repaired, however, I thought I’d post this information and see if anybody could identify the problem and the solution. Unfortunately, I don’t have prizes to give away, however, you can show off your AM engineering prowess.
All of the information is pertinent:
The station has two directional arrays (DA-2) using the same towers; the nighttime array is out of tolerance, and the daytime array is not affected and is performing normally.
There were no weather events connected with this event; no electrical storms, no major temperature changes, no rain events, no freezing or thawing, etc.
The problem happened all at once, one day the array was performing normally, and the next day it was not.
Station management reports that some listeners were complaining that they could no longer hear the station.
The ATUs and phasor were inspected; all RF contactors were in the proper position, no damaged or burned finger stock and no evidence of damaged components (inductors or capacitors) was observed. Several mouse nests were cleaned out of the ATUs, however, this did not change the out-of-tolerance antenna readings.
The towers are 1/4 wave (90 electrical degrees) tall.
Readings:
Tower
Phase angle as licensed
Current ratio as licensed
Phase angle as read
Current ratio as read
1
147.2
0.583
149.5
0.396
2 (reference)
0
1.00
0
1.00
3
-137
0.493
-125.8
0.798
4
107.5
0.481
92.7
0.355
5
-38.1
0.737
-60.2
0.623
6
-178.7
0.382
142.8
0.305
Licensed values for common point current is 13 amps, impedance is 50 ohms j0 and there is normally no reflected power on the transmitter. On this day, the common point current readings were 8.9 amps, impedance 38.5 ohms +j5 the transmitter had 340 watts of reflected power.
This is the overall schematic of the phasor and ATU:
WGDJ overall RF schematic diagram, click for higher resolution
Aerial view of the transmitter site, oriented north:
WGDJ aerial view showing towers as identified in the schematic diagram
So, where would you begin? Ask questions in the comments section.
The technical problems with AM broadcasting can be broken down into three broad categories:
Interference from other AM stations
Interference from unintentional radiators (AKA electrical noise)
Poor receivers
Much of the poor fidelity issues with AM broadcast audio come from the narrow IF bandwidth of the typical AM receiver. Older AM receivers had much wider IF bandwidths, sometimes as much as 15 KHz +/- carrier. As the AM band was overfilled with stations starting in the late 1940s, this became a big problem. The tube-type front ends with great sensitivity but not very much selectivity was unable to cope with adjacent channel interference, leading to what was known as “monkey chatter.” This type of interference can be technically described as the higher audio frequency peaks from adjacent channel stations being demodulated. Those hearing this type of interference found it very annoying and rightly so. Thus, receiver manufacturers were deluged with complaints about the poor quality of their units. The solution was simple; narrow the bandwidth until the “monkey chatter” disappeared. This new de facto standard IF bandwidth turned out to be +/- 3 KHz carrier.
It does not take a rocket scientist to see that 3 KHz audio is slightly better than telephone quality. This was the beginning of the perceived AM low fidelity problem. In the meantime, FM broadcasting, after years of lagging behind in spite of its superior audio, made great strides into mainstream acceptance.
NRSC-1 was supposed to reduce this type of interference by limiting AM broadcasting stations’ audio bandwidth to 10 KHz. The idea was to attempt to keep the modulation index somewhat within the allotted channel. This standard was mandated by the FCC in 1989, after which receiver manufacturers were to change their design to allow for broader IF bandwidths, thus improving AM fidelity. There was even an AMAX standard adopted by some receiver manufacturers. Unfortunately, by this time, the majority of AM stations were transitioning from music to talk radio. The new standards were too little much too late.
A quick scan with a quality AM receiver shows that many stations are transmitting high-quality audio, which, with a properly adjusted IF bandwidth can sound remarkably good:
Screen shot – WEOK True Oldies Channel
This is a screenshot from an SDR (Software Defined Radio) showing WEOK, Poughkeepsie, NY broadcasting the True Oldies Channel. The signal strength is slightly low, but this is a rural area and the noise floor is also low. I limited the bandwidth to +/- 7.5 KHz carrier because of the pre-emphasis used on most AM stations makes the high-end sound strident. Looking at the spectral display, there is more audio available beyond what I am listening to. This brings me to this; AM fidelity is not inherently inferior, it can sound quite good. There is no reason why AM receiver manufacturers cannot improve their products to include some advanced features;
Variable IF bandwidth based on signal strength
Variable user selected IF bandwidth
Sharp selectivity – adjacent channel rejection
Selectable sideband demodulation (carrier plus upper or carrier plus lower sideband)
While this will never sound as good as FM stereo, it still can sound pretty good, especially with older music recorded before say 1975 or so.
Manufacturers would have to have some impetus to include these features in their chipsets, such as multiple requests by listeners who are looking for better AM quality, which leads us back to programming…
The other issues with AM electrical noise reception and interference from other radio stations are surmountable, so long as there is a reason to. This, leads us back to… programming.
