Apparently, this coaxial cable has a hot spot:
7/8 inch air dielectric coax with jacket melting off
The back story:
I received a text this morning that one of our clients station “had a lot of static on it, it might be off the air.” Upon arrival, I found the Nautel VS2.5 transmitter with 0 watts forward power and an output network fault. Reset the transmitter and the forward power and reflected power increased together, triggering another output network fault. I was able to turn the transmitter power down to 100 watts, at which point it stayed on, with 50 watts reflected power. I also noted the dehydrator running continuously and 0 PSI line pressure.
I wandered around the back of the building where the coax goes out to the tower and discovered the dripping plastic from the melted jacket. I reached up and first checked the cable to see if it was warm (it was not). Then I shook it and heard what I thought was water sloshing around inside. This is the original Andrew 7/8 inch cable from when the station signed on in 1972 or so. Very likely that further up the tower, something has chaffed through the outer jacket and shield, allowing water into the cable.
I drilled a small 5/32 inch hole at the lowest point in the cable before it enters the building. The result was a steady stream of water, which was aided by some additional pressure from a spare N2 tank. I let it drain while I ran down to town and got some lunch. I came back a half an hour later, turned the transmitter on and was satisfied to see 100 watts forward power with 1 watt reflected. I ran the transmitter up to full power for a while, then deciding discretion is the better part of valour, turned it down to half power; 820 watts which nets 8 watts reflected power.
Needless to say, the transmission line needs to be replaced as soon as possible.
AM radio stations are rough customers. They frequently operate on the margins, both in terms of ratings and revenue. Their transmitter plants are complex and very often have been on a reduced maintenance schedule for years, sometimes decades. Those of us that understand the operation of AM transmitter plants and all their quirky behaviours are getting older. I myself, feel less inclined to drop everything and run off to the AM transmitter site when things go awry. Seldom are such efforts rewarded, much less acknowledged. Station owners are also finding that their previous demands are unrealistic. For example, time was that any work that takes the station off the air had to be done after midnight. These days, I can tell you, I will not be working at your radio station after midnight. You can find somebody else to do that work.
Thus, today, we took this particular AM station off the air from Noon until 3 pm to diagnose and repair a problem with the four tower daytime array. Once again, this involved a shift in common point impedance and a drastic change in one tower’s current ratios.
Antenna Tuning Unit, mice have made a mess
In all fairness to the current owner, this ATU reflects years of neglect. At some point, mice made a home in here and created a mess. The ATU smells of mouse shit, piss and mothballs. It is full of mouse droppings, grass seeds and fur. All of the ATUs in this array are in similar condition.
Paper wasp, inside ATU
It was warm enough that the wasps were active, if not a little bit lethargic.
Broken stand off insulators in ATU
This coil is being held up by the tubing that connects it to other components. When the ATU was built, no nylon or cork bushings were used between the insulators and the wall of the ATU they were mounted on. Heat cycling eventually did all of the insulators in.
Catwalk to the other towers
Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.
The tower bases are all elevated above the theoretical maximum water level. The ATUs are also up on stands with platforms build for maintenance access.
ATU Work “platform”
I cannot even blame the current owner, who has spend considerable money to make repairs and upgrades to this site. It is very difficult and very expensive to catch up with deferred maintenance. Sadly, most AM stations we encounter have similar or worse problems.
I think it is too late to save many of these AM stations. The technical issues, lack of revenue, perceived poor quality, lack of good programming are all taking their toll. At this point, the hole is so deep there is no hope of ever getting out. The FCC’s faux interest in “revitalization” followed by two years of stony indifference seems to be a final, cruel joke.
I wrote a little Haiku about Thanksgiving dinner:
Old transmitter beckons
Dishes get cold
Not exactly the 5/7/5 of a traditional haiku, but close enough. This year, it was the nearly 30 year old Broadcast Electronics FM35A at WEBE. A set of readings from the remote control reveal; zero forward power, zero plate current and 12.8 KV plate voltage. My first assumption was some sort of drive issue; a failed exciter or IPA driver. After starting the backup transmitter and making sure that it was running stably, I spoke with the program director and told him we would be out next morning.
Upon arriving at the transmitter site, I found the BE transmitter had no filament voltage. An obvious clue, I began working backwards from the tube socket until I found this:
Broadcast Electronics FM35A filament voltage regulating transformer
This is the auto-transformer that regulates the filament voltage. Schematically, it is noted as T204 and it is in series with one side of the filament transformer. This one is burned open. The bad news; Broadcast Electronics does not stock this part, it is a special order item, the replacement part costs $2,800 dollars and it will take a few weeks to get here. The good news, after digging through our stock of old transmitter parts, I found an exact replacement:
Replacement part, T204, BE FM35A
Replacement part name plate, T204, BE FM35A
We will be installing it on Monday morning.
This is a rule that I always find difficult to enforce. Since switching into contracting mode, I am often at any particular studio once per week or less. It seems to me, no matter what signs are posted or what words are spoken, the DJ seems to hear; “It is okay to eat and drink in the studio.”
Of course, with that attitude, the inevitable is bound to happen:
RS-18 Millenium console on/off button membrane
To make things worse, this was spilled on the main mic on/off buttons. These button membranes come in groups of six and are not inexpensive. The complaint was “The main mic will not turn off.” Ah well, I am paid to fix things after all. The DJ’s are only inconveniencing themselves at this point.
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 a 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:
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.
The newish Nautel VS2.5 transmitter installed at WJJR had an RF module failure. This particular model transmitter does not have slide in RF modules as other Nautel transmitters do. To fix this transmitter, it has to be pulled out of the rack, flipped over and opened from the bottom. The module replacement is very straight forward, there are five solder pads that connect to wires carrying the input, output, power supply and bias voltages.
Nautel VS2.5 transmitter RF modules and combiner
The troubleshooting guide gives good instructions on how to check the PA MOSFETS with a DVM. I found that 1/2 of the device in PA1 was bad:
Schematic Diagram, NAPA31
All in all, not a very hard repair. This was under warranty, so a replacement RF pallet was sent to the station without charge. The problem is more about where the transmitter is located:
Killington Mountain, Killington, VT
Killington Peak is the second tallest mountain in Vermont, topping out at 4,235 feet (1,291 meters). In the winter, one can take the chair lift to the top. In the summer, the road is drivable with a four wheel drive. In those in between months, access to the top can be very tricky at best. We had a pretty wet spring this year, so the roads up the mountain are just now becoming passable for vehicles.
Even after reaching the parking lot, there is still a 10 minute walk to the peak, another 200 or so feet up a steep, rocky trail.
Further complicating things, this transmitter is wedged into this little shack, which holds; a BE FM3.5A transmitter (defunct WJJR), a Harris HT3 transmitter (WZRT), an ERI combiner, two racks of equipment (STL’s, Exciters, remote controls, etc) a backup QEI transmitter, an Onan generator transfer switch:
Killington Peak fire tower, WJJR WZRT transmitter building
Both stations run into this ERI half wave spaced antenna:
WJJR WZRT ERI antenna
It is very tight in this transmitter room. There is a new tower on Killington Peak, which is still under construction. At some point, the plan is to move into the larger building next to the new tower.
Killington Peak tower
On a clear day, the view from the top is spectacular. On this day, the peak was in the clouds, so not so much:
Killington Peak view
It is a great site, the HAAT is 2590 feet (790 meters) and the stations carry forever on relatively low power outputs.
I have seen many a Dell LCD computer monitor go south for want of a $0.50 part. Dell must have gotten a hold of a bad batch of capacitors, because almost invariably, the problem is with the power supply capacitors for the back light. The symptoms are; the monitor goes very dim and can only be read when shining a light on it, or the power button flashes green.
A new Dell 19 inch (E1914H) monitors runs about $90.00 – 110.00. I can repair a defective unit in about 20-30 minutes or so, which makes it worth while for the client. When repairing equipment, the cost of labor and parts balanced across the cost of new equipment should be a prime consideration. Sometimes, it is simply not worth the time to repair something. Others, like this instance, it makes sense as long as the repair is simple.
This is a Dell E198FPf LCD monitor. After the initial diagnosis:
Dell E198FPf LCD monitor back lighting problem
First step is to remove the stand and the four screws behind the stand bracket.
LCD monitor stand removed
The hardest thing about this repair is getting the bezel off. Dell uses a bezel around the monitor face that uses little plastic clips to hold it in place. To get the bezel off, one needs to press the clips toward the center of the monitor while lifting up. It requires the careful application of force.
Dell E198FPf monitor bezel
I start on the bottom and use a small screw driver in one of the slots to get it started. I start on the bottom because if the plastic gets a little marred, no one will see it when the repair is finished. Once the first clip is released, then the others and be released by twisting the bezel carefully toward the center of the monitor while lifting.
LCD monitor bezel removal
Once the bezel is removed, the wiring needs to be disconnected. This consists of the back light, the data buss and sometimes the on/off switches, which are mounted on the bezel.
LCD monitor backlight connector
LCD monitor data buss connector
After all the wiring is removed, there are either two or four screws that hold the power supply to the monitor screen.
LCD monitor power supply bracket screws
Finally, the power supply board is exposed. Depending on the model of the monitor, the hex head screws that hold the VGA connector may need to be taken off. Sometimes not.
LCD monitor power supply
Removing the screws on the back of the power supply board exposes the capacitors and other components.
LCD monitor bulging capacitors
And the culprit is discovered. These two bulging capacitors are causing the LCD monitor backlight power supply shut down making the monitor unusable. The larger one is a 1000 uF 25 volt and the smaller is 680 uF 25 volt. I replaced both with in kind 35 volt units. I also took the liberty of replacing the rest of the electrolytics on the power supply board (total of five additional capacitors). While the unit is disassembled, it is far easier to replace all the $0.50 components than to do it one at a time over the next few years as each fail. This monitor should be good for another 5 years of service at least. These values vary somewhat from monitor to monitor. Also, if only repairing one or two monitors, the parts can be obtained at Radio Shack for $1.99 each.
It is a good way to regenerate equipment, even if they are set aside as spares.
Today there will be a quiz. Ready? Look at this picture and see if you can spot the problem:
Problem with Harris SX2.5A transmitter
If you said “Hey, that green wire seems a little odd; it disappears behind the heat sink next to that screw then reappears again at the top,” you are correct. What really sucks is the green wire is the transmitter off connection to the remote control. So, when the PA board was secured to the heat sink, the wire was trapped between the board and the heat sink. Since the components were cold, it did not pinch through the insulation right away, no. Rather, after the transmitter ran for several hours at full power, it got hot enough to displace the wire insulation and cause a short. Doh! The transmitter is off and it won’t come back on!
This is a picture of the wire after it was removed:
Haste makes waste. Unfortunately, it someone else’s haste that ruined my Saturday afternoon when I was supposed to be taking my son to little league practice. I am sure that some not so kind words will be exchanged very soon.
Always double check your work.
Almost every broadcast engineer has to do some type of bench work. While I enjoy a certain amount of bench work, it is not my strong suit. I suppose if I had to do it more often, I would become more proficient. Truth be told, I would rather be at a transmitter site than sitting work chair studying schematic diagrams. It is becoming increasingly difficult to make repairs in the field due to surface mount components. The company I work for has a repair and rework shop where almost anything can be repaired. There is one bench tech, who is pretty proficient with power supplies and RF amplifiers among other things. There is a complete set of test equipment including several Tektronix spectrum analyzers and oscilloscopes.
Likely the most versatile piece of equipment is the IFR 1500 service monitor.
IFR 1500 communications service monitor
The bench itself is fairly large:
Shop work bench
There is also a good stock of spare equipment that can be rented out while repairs are being made:
Shop spare equipment
Repair work includes by is not limited to:
- RF repairs; Moseley STL systems, Marti STL and RPU systems, TFT STL systems, most exciters, IPA modules, etc
- Transmitter repairs and retuning
- Mechanical devices like transmission line dehydrators, transfer switches, etc
- Switching and linear power supplies
- Uninteruptable power supplies
- Remote control equipment; Gentner VRC-2000, Burk ARC16, Moseley MRC-1600
- Audio Processing; All Orban equipment, Symetrix, Valley, DBX
- Audio equipment; Amplifiers, consoles, reel to reel machines, cassette decks, CD players, DAT machines, etc
I am sure there are many other things that I am leaving out.
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
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
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.