I have been tasked with fixing one of these glorious contraptions. Aside from the usual Energy Onix quirks; design changes not reflected in the schematic diagram and a company that no longer exists, it seems to fairly simply machine. Unfortunately, it has spent its life in less than ideal operating conditions.
Upon arrival, it was dead in the water. Found copious mouse droppings, dirt and other detritus within and without of the transmitter. Repaired the broken start/stop switches, fixed the RF drive detector, replaced the power supply capacitors and now at least the unit runs. The problem now is the power control is unstable. The unit comes up at full power when it first switched on, then it drops back to 40 watts, then after it warms up more goes to about 400 watts and the audio sounds distorted. This all points towards some type of thermal issue with one of the power control op amps or other composite device.
After studying the not always accurate schematic diagrams, the source of the problem seems to be carrier level control circuit. This is based around a Fairchild RC4200AN (U10 on the Audio/PDM driver board) which is an analog multiplier chip. That chip sets the level of the PDM audio output which is fed into the PDM integrator circuit. Of course, that chip is no longer manufactured. I can order one from China on eBay and perhaps that will work out okay. This all brings to mind the life cycle of solid state components. One problem with the new technology; most solid state components have a short production life, especially things like multiplier chips. Transmitters are generally expected to last 15-20 years in primary service. Thus, transmitter manufactures need to use chips that will not become obsolete (good luck with that), or purchase and maintain a large stock of spare parts.
In the mean time, the chip is on its way from China. Truth be told, this fellow would be better off with a new transmitter.
Second post in the series, “things to do with a truck body tool box.”
We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping. The SX2.5 transmitter being of an age when, apparently, VSWR fold back circuits were just a gleam in Hilmer Swanson’s eye. The correct description of the sound made over the air during this event would be “motor boating,” because that is what it sounds like. Obviously, very undesirable.
Thus, the isocoupler was removed from the tower, dried out, water proofed and replaced. That lasted about six months.
Once again, the isocoupler was removed from the tower, a capacitor was remounted, drain holes and a small vent added to the top of the unit and it was replaced. That lasted about a year.
I am getting a little tired of this and so is the client. Time to rethink the entire set up.
We had several left over parts from various AM decommissionings over the last few years which included these nifty sample loop isolation coils:
Why not repurpose one of these to make an isocoupler for the translator?
Enter; the truck body tool box. This one is slightly smaller than the last one, measuring 23.5 x 18 x 16 inches (60 x 45 x 40.5 cm).
The isolation coil consists of 35 turns of 3/8 coax on an 11.5 inch diameter form. The coil length is 15 inches. I calculate the length of the coax on the coil to be out to be right around 100 feet using the π x D x (turns) formula. I measured the inductance with my analyser, which came out to 200 μH. Not to shabby.
The coax is Cablewave FCC38-50J which has a velocity factor of .81 and the TDR shows it to be 100 feet also.
At 860 KHz, the isolation coil presents 1,200 impedance. I don’t think that will be good enough for that cranky old SX2.5. I decided to make a parallel LC circuit (AKA a tank circuit) to bring up the impedance some.
Tank circuit formula:
FR = Resonance frequency in Hertz L = Inductance in Henrys C = Capacitance in Farads
Given that I have two left over capacitors, one is a .001 μF and the other is a .0012 μF, those values determine where the coil needs to be tapped. I also wanted to have a good bit of coil in the circuit on the tower side before the capacitor tap to dampen any lightning strikes on the tower. Thus the inductance needs to be about 28 μH.
Using Wheeler’s coil inductance formula:
L= (d2 x n2)/(18d+40l)
L = inductance in micro Henrys
d = coil diameter in inches
l = is coil length in inches
n = is number of turns
I removed a small portion of the outer jacket on the coil at approximately the 28 μH point (12 turns) then installed a .0012 μF capacitor. I used a small variable capacitor to tune for resonance on the carrier frequency. With this set up, at 860 KHz, there is >47,500 impedance. That goes down to about 16,000 ohms +/- 10 KHz.
That should make things better.
Then I mounted the coil and capacitor in the truck body tool box. There is a fair amount of stray capacitance from the box itself, which raised the resonant frequency by 5 KHz.
Resonance is slightly above the carrier frequency with the permanent fixed .0012 μF capacitor. I think this will change once the unit is connected to the station ground plane. The network analyzer indicated there is too much capacitance in the circuit. Unfortunately, this may be as good as it gets, however, the analyzer shows the impedances are still pretty high:
Deviation from Carrier (KHz)
The base impedance of this tower is 34 ohms on the carrier frequency, so the isocoupler should be invisible to the transmitter across the 20 KHz occupied bandwidth of the station.
The FCC38-50J cable has a loss of 1.04 dB per 100 feet at 100 MHz, which is the figure I will use to calculate the insertion loss on the FM translator antenna system.
The old isocoupler is made with RG-214, but likely a somewhat shorter length. RG-214 cable has a loss of 1.9 dB per 100 feet at 100 MHz.
Before and after measurements with the network analyzer show a very slight change in the reactance at the tower base. Nothing major and easy enough to tune out with the series output inductor of the ATU.
If I where to do this again, I would simply tap the coil at ten turns from the bottom, measure the inductance and install the proper value capacitor. Since this had to be constructed with the parts on hand, less the truck body tool box, it because a bit cumbersome to get close to the resonant frequency.
All this got me thinking; there are other possible uses for such a design. Crossing a base insulator with Ethernet cable always presents some unique problems. I know the WISP forum that I read, they are always talking about how difficult it is to mount an antenna on an AM tower. What if… armoured Cat5e or Cat6 cable was used with water proof RJ-45 jacks? Something like that could carry Ethernet data and DC voltage past the base insulator to a three or four around sectorized access point and an edge switch or router mounted on the tower.
Anyway, it would not be hard to make coils and install capacitors for the right frequency
This information is from an occasional reader who wished to remain anonymous.
Another AM station surrenders its license, this time from north of the border. CKSL, London, Ontario, Canada is gone for good. Current owner, Bell Media, has determined that it would cost more to repair the deficiencies with the antenna system than economically feasible, especially considering it’s low ratings. Here is their filing with the CRTC:
Bell Media is the licensee of CKSL-AM 1410, assuming stewardship of the station in 2013 as part of the Astral Media acquisition.
A technical review of the transmitter site was recently completed both by Bell Media and contractors, which has resulted in the determination that the AM array poses an unacceptable risk from a health and safety perspective. The five towers are experiencing serious structural degradation and also require repairs to the aviation safety lighting system. In addition, the building which houses the transmitter has shifted off its foundation (as have several of the individual tower sheds).
Given these problems, Bell Media would need to make a significant financial investment to bring CKSL-AM’s transmitter up to compliance with Human Resources Development Canada, Industry Canada and NavCanada operational codes and standards, all of which is estimated to exceed $3 million dollars.
From a market perspective, CKSL-AM has consistently ranked last out of all ten commercial stations in the London market, both in audience share and revenue generation, over the last several years. In fact, since 2013 the London market has seen radio revenues drop 4% and CKSL-AM generates the least amount of revenue of the stations in the market. Even with a significant investment in programming, this trend is unlikely to be reversed.
In light of the significant capital costs coupled with the absence of revenue and audience share, Bell Media is respectfully requesting the revocation of the CKSL licence.
Well, 24/7 comedy will do that to you. Somebody in the business said to me recently “The listeners are abandoning radio!” No, it is the broadcast station owners who are abandoning their listeners and their cities of license. I have a news flash for all current broadcast station owners; as surprising and radical as this might sound, bland, boring, canned, completely irrelevant, dismal, uninformative, unimaginative, unentertaining, dreary, stale, unenjoyable programming will drive away even the most loyal listeners. People really want to listen to radio, it is an easy habit and readily accessible. Radios are ubiquitous; they are in our kitchens, bedrooms, cars, hotel rooms, offices, restaurants, barber shops, etc. That, however, may not always be the case, as more and more people move Spotify, Pandora, or Apple radio when they are tired of the disappointment. I was listening to a certain sports radio format the other day and I kept waiting for something interesting to happen. I waited and waited. I would say to myself; okay, this will be the segment when I will learn something or be entertained. This upcoming guest will say something interesting. Sadly, those expectations were never met and I will never tune into that station again. Elevator music would have been better. Worse than sports radio, 24/7 comedy is the absolute death knell. This is like saying; we are out of ideas and we do not care.
Here are a few pictures of the former CKSL-AM transmitter site:
Actually does not look too bad, at least the field is mowed. I have seen much, much worse. Those bolt together towers, though. I would bet that they are the real problem, bolts are deteriorating faster than the tower steel. Very likely all the towers need to be replaced and that is why the license is being surrendered.
If you are a radio geek, get out there and take some pictures of your favorite radio station. If the current trends continue, eventually they will all be gone.
Alternate title: Building and ATU in a truck body tool box.
Alternate title II: I should get paid extra for this shit.
There is an AM radio station that is near death but the owners do not want it to go away. Nor to they want to spend very much money to keep it around, thus the dilemma. At the transmitter site, there are a multitude of problems; leaking roof, very old rusty ATU, rotting support posts and transmission line bridge, equipment racks rusting out, nothing is grounded properly, the building is full of junk, snakes and mice have moved in. To further complicate things, the tower and transmitter building serve as an STL relay point for two of the market’s FM stations. There is also two translators with antennas on the tower. The ATU and tower light choke box are rusting through, which is causing arcing and broadband RF noise that is interfering with the FM station’s STL receiver. There was a home made isocoupler for one of the translators that was allowing AM RF back into the building which was creating havoc with everything. Because of this, the AM station is currently silent. In short, it is a mess.
The red box on the bottom is the ATU, the plywood box on the top with the peeling yellow paint is the home made isocoupler, the tower light choke box is behind the isocoupler.
This was the capacitor that was feeding the antenna, .0041uf, 10KV 8 amps.
We started remediation on this last February, which is not optimum time for replacing rotting wooden posts. However, we were able to clean out the building. The leaking roof has been repaired. I was able to find a few old racks from a Schafer Automation system to replace the rusted out original racks. I began the process of grounding the equipment racks, the incoming transmission lines for the STL, etc.
We will have to find out how they are getting in, the plug up those holes.
Then there was the ATU and tower light choke enclosures. Original to the 1952 sign on, they were past their serviceable days. Since this is all being done on a budget and nobody wants to spend money on an AM station that has little or no listeners and even less revenue, we had a problem.
Then somebody suggested building an ATU in a truck body tool box. Well… This isn’t the Meadowlands, so if there are no other alternatives then okay, I guess. Off to Amazon to order a tool box. This particular unit seems fine, my only comment is on the gauge aluminum (or aluminium if you prefer), which is slightly thin for holding up all those parts.
Still, the box itself is nice enough and certainly better than the old one. I was able to reuse the inductor and the Delta current meter but the old Sangamo capacitors crumbled in my hands when I removed them. I also saved the feed through bowls, J-plugs and other parts. I used some copper strap to run a good RF ground from the input to the ground connection. Overall, I am pretty pleased with the finished product. It is a little bit tight in there, but this station only runs 1 KW, so it should be fine.
So, new pressure treated posts installed, the box was mounted and the transmission line connected.
The reused racks are old, but serviceable and a big improvement over the old, rusting out racks. I was able to bond each rack to the ground strap that used to connect to the RCA BTA-1 transmitter. There is one more rack to install to the right of these two. That should give us more than enough rack space for this site.
The station is back on at full power and not interfering with the FM STLs or the translators. You can actually touch the rack and not get an RF burn!
We are also working on an air conditioner.
Other work at this site; cleaning out the building, replacing the tower light photocell, installing a ground buss bar, some STL lightning protectors, dress the transmission lines, etc. It is a work in progress.