Working with RF can produce some head scratchers. Most transmitter manufactures tend to use the same type of connector for things like exciter RF outputs and composite inputs. Over the years, I have become well stocked with all sorts of BNC and Type N connectors. Satellite equipment uses Type F connectors, Analyzers use Type N, Oscilloscope uses BNC, GPS equipment uses SMA and so on. Except when they don’t. As any good engineer will tell you, when they don’t will be in the middle of the night at some mountain top location while the station is off the air.
After one such incident, I invested in a TPI-3000A adaptor kit. This kit has both the male and female versions of Type N, F, SMA, BNC, TNC, UHF, UHF mini and RCA. They can be mated in any combination using the Universal interface.
TPI 3000A adaptor kit
On more than one occasion, this little kit has meant the difference of between being back on the air or driving down the mountain to look for an in between series adaptor. A couple of recommended additions include a 7/8″ and 1 5/8″ EIA flange to type N male.
TPI-3000A inside
They can be a little pricey, however, I have seen several for sale on eBay for less than $100.00. The key to not loosing the various little parts to this kit is to write a little note detailing the date and location where the adaptor was used, then stuff it in the empty hole. Hopefully, when permanent repairs are made, the adaptor will be retrieved.
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.
Last year, the main antenna and transmission line for WSPK was replaced. I was, therefore, somewhat surprised to hear that there was an issue with the new transmission line. And yet, problems there are. Most likely, some ne’er do well has shot the transmission line with a bullet making a hole, which, when it rains, allows in water. Said water then accumulates in the bend at the bottom of the tower. When enough water is present to fill the gap between the center conductor and the outer conductor, this happens:
Foward and Reflected power meters, WSPK, Mount Beacon, NY
For those of you keeping score at home, that’s 980 watts forward, 375 watts reflected or about 4:1 VSWR. Obviously not a good load, in fact the transmitter shut down. Fortunately, the backup transmitter and antenna system worked flawlessly.
This began happening last month, usually after a heavy rain storm. Thus, I went out to the base of the tower and shook the transmission line and sure enough, water was sloshing around in there. Last time time it happened, a tower crew was summoned to inspect the line. Inspect it they did, but did they find any holes? No, they did not. Perhaps the issue is with the antenna itself, in which case the entire thing will have to be removed from the tower and lowered to the ground. In the mean time, my boss drilled a small “weep hole” at the bottom of the bend where the line comes off the tower.
I uncovered this weep hole and pressurized the line and viola, lots of water came out:
WSPK 1 5/8 inch air dielectric transmission line, Mount Beacon, NY
A bit unconventional, but effective nonetheless. The first video is of the water dripping out:
The second video is of me walking back into the transmitter building to pressurize the line:
Everything is very noisy because it is Monday, when all the generators on site exercise. There are five diesel generators running while I was videoing recording this.
I would estimate about 6 ounces of water came out through the weep hole, most of it landing on the ladder underneath. After the water was drained out, the transmitter came back on at full power and normal VSWR.
A temporary fix to get the station back on the air. The real repair work will begin when the antenna comes down to be inspected.
Update: The tower climbers did find a hole in the transmission line, just below the flange that connects to the antenna. It looks like a pencil sized gash just before the line bends back to the tower. Lightning? Rifle bullet? Damage while installing? We can’t really tell. They installed a patch over the hole which holds about 3 psi line pressure. We then used a vacuum pump to evacuate the line, then recharged with dry nitrogen.
Regarding Pedro’s question below in the comments: Since we found this problem quickly and were able to evacuate the line, there should not be any corrosion, that is our hope. Time will tell
A piece of vintage gear from the late 1970’s, the Optimod 8000 was and still is a good sounding box. I have often thought that these processors would make an excellent internet audio processor using the test jacks on the back of the unit. The audio on these jacks is unbalanced and has 75 µS pre emphasis. It would be easy enough to make a de-emphasis network and create balanced audio with a 10K:600 ohm transformer. Some experimentation may be required with the transformer primary impedance value. Orban notes that not less then 1 MΩ impedance should be connected to the test jacks. For the internet station looking to copy the “FM radio” sound, this unit would do the job nicely.
The 75 µS de-emphasis network would look something like this:
75 microsecond de-emphasis network, unbalanced to balanced audio conversion
In this case, the values for the de-emphasis network are fairly critical, therefore 1% or better tolerances for the resistors and capacitors is required.
Even better, an LPFM or some other radio station on a budget could acquire one of these for relatively little on eBay or somewhere else. With a little TLC, most of these units can be rebuilt and put back into service. I would recommend that some type of limiter be used in front of it, such as a Texar Audio Prism or CRL SEP-800.
Some classical music stations prefer these units. I have noticed that they have a nice, mellow, open sound. Not at all fatiguing and yet still offer a nice easy 10 dB gain reduction. There is also a modification that can slow down the release time on the gain reduction. More gain reduction, AKA compression, can be had with something else in front of the unit.
The best part about these units, there is no rebooting, no processor lock ups, software glitches or any of that non-sense. Additionally, a quick look at the front of the unit shows very few user controls, making it almost impossible to screw up and sound bad. They are well built and so long as the electrolytic capacitors are changed out, fairly bullet proof. Other processors, not so much.
Optimod 8000A under test
This is an Optimod 8000A that I decided to put through its paces.
Really, how much more do you need? I recorded this on the camera microphone using a replica table radio, seen near the end of the video on the right hand side of the frame.
I used the Technics SL-1200 MKII turntable through an ATI P100 turntable preamp into the Optimod. The Optimod is feeding a BE FX-30 exciter running 15 watts into a dummy load. The Optimod is running about 5-7 dB gain reduction, which is enough in my mind. The BE FX-30 is still just about the best sounding analog exciter every made.
Rechipped Optimod 8000A, TL071 opamp
This unit has been re-capped and re-chipped at one point. The re-chipping follows the Orban recommendation; the 4558 and 1556 opamps are replaced by TL071CP and TL072CP respectively, and the uA 709 and 301A opamps are left in the unit. A good thing to remember, the uA709 and 301A opamps can be replaced by TLO71cP opamps in the event of failure. The Texas Instruments TL0 series opamps are very good and readily available.
Optmod 8000A input and limiter board
Overall, this unit is in good condition, however, like many such units, it is missing its brown “Optimod” cover, which goes over the input/output controls.
Manual is available at the Orban ftp site: ftp.orban.com.
