Troubleshooting an AM array

Today, there will be a quiz.

Recently, we had an AM antenna array go out of tolerance by a good margin.  This has been repaired, however, I though I’d post this information and see if anybody could identify the problem and the solution. Unfortunately, I don’t have a prizes to give away, however, you can show off your AM engineering prowess.

All of the information is pertinent:

  1. The station has two directional arrays (DA-2) using the same towers; the night time array is out of tolerance, the daytime array is not effected and is performing normally.
  2. There were no weather events connected with this event; no electrical storms, no major temperature changes, no rain events, no freezing or thawing, etc.
  3. The problem happened all at once, one day the array was performing normally, the next day it was not.
  4. Station management reports that some listeners were complaining that they could no longer hear the station.
  5. The ATU’s and phasor were inspected; all RF contactors were in the proper position, no damaged or burned finger stock, no evidence of damaged components (inductors or capacitors) was observed.  Several mouse nests were cleaned out of the ATU’s, however, this did not change the out of tolerance antenna readings.
  6. The towers are 1/4 wave (90 electrical degrees) tall.


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:

WDGJ overall RF schematic diagram
WGDJ overall RF schematic diagram, click for higher resolution

Aerial view of transmitter site, oriented north:

WGDJ aerial view showing towers as identified in schematic diagram
WGDJ aerial view showing towers as identified in schematic diagram

So, where would you begin?  Ask questions in the comments section.


Good troubleshooters are becoming rare these days.  To some, the idea of working through a problem, finding and then fixing an issue seems like a time consuming, wasteful evolution.  More often than not, it is easier to replace the entire assembly with a new one, throwing the old one away.  This is especially true with computer components.  The other option is to send a module or assembly back to the factory for repair.  Truth be told, often that is a good course of action when a fully equipped repair bench is not available.  Surface mount technology can be difficult to repair in the field, as can many RF components.

Being able to trouble shoot components and assemblies is still a valuable skill.  Finding and identifying trouble is a good skill no matter what it is used for.  I find analytical troubleshooting skills to be good life skill to have.  I think my in-laws are occasionally amazed when I walk into a situation and point to something and say: There it is, fix that.

Coil burned out on 40 amp RF contactor
Coil burned out on 40 amp RF contactor

Many times, however, there is no smoking gun. Those situations require a bit of investigative work. The first step in troubleshooting is developing a history:

  • Has this failed before
  • It there is history of failures
  • Has it been worked on recently
  • Is it new
  • Has it been installed properly
  • It it old
  • Has it been effected by some outside force like lightning or a power surge

This is where good maintenance records or maintenance logs come in handy.  Recently, I have found many places that lack any type of maintenance documents, which means the repair history is unknown.  This makes it difficult to find a good starting point and can greatly increase the amount of time required to troubleshoot a problem.

Once the pertinent history is gathered, it can be organized and analyzed for clues.  For example, if something has been worked on recently, that is a good place to start. If something has a past history of failures, that is a good place to start.  Newly installed equipment is subject early failures under warranty due to component failures.  Old equipment may just be plumb worn out.  Improperly installed equipment can exhibit all kinds of bizarre failure modes.   That information coupled with known symptoms would indicate a good starting point for troubleshooting the problem.

If no good starting point can be discerned, then the next step is to recreate the failure.  This usually means turning the thing back on to see what it does.  Chances are good that whatever the problem is, it will still be their.  Once a good set of symptoms have been identified, then it is time to start working at one end of the problem unit the fail component is isolated.

Often times, equipment manuals will have troubleshooting guides.  These can greatly speed up the process for large, complicated things like transmitters, generators, and so on.  There is also the tried and true troubleshooting chart:

Generic transmitter power supply trouble shooting chart
Generic transmitter power supply trouble shooting chart

This is an example of a troubleshooting chart for a transmitter power supply.  Many equipment manuals will have this type of information in the maintenance sections.

It is also important to note that when working on high voltage systems, it is necessary to have two persons on site at all times.

Good troubleshooting skills have many applications.

Secretly, I like it when things break, sort of

Not that I am a glutton for punishment or anything, but I enjoy troubleshooting. There is a certain satisfaction in the analytical aspect of tracking down a problem and fixing it, hopefully in a permanent fashion.  Figuring out where a problem is requires a good bit of detective work;

  • Examining the clues; what happened before the failure, what are the fault indications, are there any external factors
  • Round up the usual suspects; a good maintenance log is helpful here to track re-occurring failures.  If the failure cannot be attributed to an external source (such as power surge or lightning storm), what was the last thing that was changed or worked on.
  • Following the trail back to the origin; Often the first failed part found is a symptom, not the actual problem.  It takes some skill in reading schematics and making sense of a failure to trace it back to the real problem.

It can sometimes be exciting, like turning on the 25 KV high voltage supply and have big blue flashes issue forth from the top of the transmitter.  Sometimes it can be quite frustrating, like when the station owners refuse to spend money to fix a problem.  Sometimes it can be dull, like fixing the same problem over and over again because of the previously stated money problem.  It’s also disheartening when the problem was caused by the stupid DJ spilling soda in the console.  Not that all DJ’s are stupid, just the ones that spill things into consoles.

The challenge of finding the root cause can often be enlightening.  I have often discovered unrelated problems waiting in the wings while investigating the why of an outage.  It is great to fix those things before they burn the house down, but this approach often goes unnoticed by the ownership or management.  Lately, for some reason, an once of prevention goes un-noticed or unappreciated.

There is quite a bit of science to trouble shooting, but there is some combination of personal traits that make a good trouble shooter.   These are:

  • Inquiring or curious disposition.  It is fairly easy to get to the first failed module or part.  Discovering the reasons behind the failure and or getting down to the component level takes a good deal more effort.
  • Patience.  This goes with the second part above, it takes some stick-to-it-tive-ness to trace out the not readily apparent problem.
  • Good analytical skills.  Often failures generate a cause and effect scenario.  The effects can be  startlingly distractive and mask the causes and the under lying problem.
  • Ability to view the large picture.  This is critical to discover outside influences and other issues that are indirectly connected to the system or issue at hand.
  • Ability to analyse the system design.  This requires the background training and experience to look at a circuit diagram and discover non-error tolerant systems.  Sometimes these systems can be modified for better fault tolerance.

Poorly designed equipment is the bane of the broadcast engineer.  Equipment manufactures can sometimes fail to follow two key principles: KISS and maintainability.  KISS stands for Keep It Simple, Stupid.  There is no better design criteria than the KISS principle.  Adding layers of complexity increases the failure expectations.  Maintenance can be something as simple as cleaning or changing air filters.  Making maintenance tasks difficult almost ensures that they will not be done.

Bathtub design curve
Bathtub design curve

Eventually, all things wear out.  It also takes some large picture skills to know when it is time to replace equipment and that can vary greatly from situation to situation.