Mouser Electronics has created a mobile web edition for its online store. This is a handy tool for searching, cross-referencing, and ordering parts. Mouser has a large stock and they ship quickly. Time once was that you could run down to the local electronics store and get just about anything you needed. Even Radio Shack carried a fair amount of small parts, tools, connectors, and so on. Since then, the local electronics shop has closed and Radio Shack’s inventory gets smaller every year. Using a large parts supply company like Mouser or Allied is necessary if any type of troubleshooting and repair is undertaken.
I like the Mouser Mobile site because there is no app to download and install. One simply points the web browser on any mobile device to mouser.com and it will automatically redirect to the mobile site. If that does not work, then m.mouser.com will. The mobile website is easy to browse around, and if needed, a quick call can be made by hitting the little phone icon. Here is more in a video:
If one considers paradise an FM35A. Going through another iteration of blown transmitter fuses for WEBE, Bridgeport, CT. Yesterday, I spent the afternoon examining the transmitter and found several interesting things:
Fresh arc tracks on the PA cavity and PA loading capacitor
The shoes and bars in the high-voltage contactor were severely pitted
One of the mains phases (middle) in the high voltage supply appears to be heating up, likely due to a loose connection.
Discolored wire on buss bar
I checked and re-tightened all of the mains connections. Apparently, this is an old problem, as the Allen screw was tight. Interestingly, the fuse that was blown was on the red phase, which is different from what it was last time.
I spent the afternoon filing and sanding off the arc track marks in the PA cavity. It is very important to file flat all sharp points that were the result of arcing. Any sharp points will induce corona. I also filed down all of the contacts in a high voltage contactor, which took a fair amount of time. These are soft copper shoes and bars that had so much pitting and carbon I wonder how they didn’t catch on fire. I filed them flat. We were back on the 35A transmitter at full power by 4:30 pm.
If this happens again, I will bring my megger out and check the insulation on the wire between the disconnect switch and the HV power supply.
When I left the site at 5:30, I felt like we did some good work.
The rumble of thunder this morning let me know that lightning season is upon us here in the Northeast and likely the rest of the country as well. I used to enjoy the odd summer thunderstorm, especially the late afternoon pop-ups that cool off a hot summer’s day. Now whenever I see lightning or hear thunder I wonder if the phone is going to ring. Chances are good that it will not, as I invested many hours of my time and my previous employer’s money into lightning protection at the transmitter sites.
upper atmosphere lightning depiction
I go on the assumption that all tall steel towers will get struck, oftentimes repeatedly, during any particular electrical storm. Back in the day, I took a course by Polyphaser called “The Grounds for Lightning and EMP Protection.” It was a great primer on how to ground and bond equipment at a transmitter site to eliminate current flow, which is the cause of all EMP and lightning-induced failures. When I was in military communications, no expense was spared as they took uptime very seriously. Any downtime was a personal affront to the commanding officer of the unit in question.
Lightning is DC however, it behaves more like 10 KHz – 2 MHz AC. Therefore, lightning and EMP grounding systems need to be designed and installed to accommodate DC through 10 KHz AC voltages. This is easily done by choosing the correct conductors, ground bus bars, and bonding systems.
The other path lighting takes into a transmitter is through the AC mains. Utility company high voltage primary feeds act like large antennas for lightning-induced EMP. Fortunately, much of that is filtered out by the step-down transformers just before the building service entrance. It is still possible, however, for some impulse voltage to make it through the transformers and into the service entrance panel. On older tube type transmitters, this could often damage the plate voltage power supply because of the voltage multiplication factor of the plate transformer. Oftentimes, the transformer secondary would have “holes” punched through the insulation to ground. This is an expensive and time-consuming repair.
I would conservatively estimate that for every $10.00 spent on lightning protection, $1,000.00 dollars worth of damage and downtime is saved. Overall, a pretty good return on investment.
The basics for lightning ground bonding are thus:
Use the lowest inductance wire possible. The industry standard is #2 solid copper, however, if bonded properly, there will be very little current flow inside the transmitter building, so if #2 is not available, then any solid wire up to #8 will work. Tower ground bonding should be as heavy as possible.
Ground all guy anchor points, and bond all guy wires together and to the same ground rod or ground rod system.
Keep the bonding conductors as straight as possible, bends should be long sweeping turns to minimize series inductance.
All metal equipment should be bonded, no rack, telco demark, electrical panel, dummy load, bulkhead entrance grounding bus, combiner, door frame, etc should be left unbonded.
All coax outer shields should be grounded where it comes into the building.
All coax cables should go through a toroid before being connected to the transmitter.
All outdoor bonding connections should be exothermically (CAD welded) bonded to ground rods.
All grounding must go to a common ground point, AKA star grounding point. No individual ground points should be allowed in the building.
Multiple ground rods were installed around the outside perimeter of the building as deeply as possible. Some mountaintop transmitter sites may require special grounding material (Bentonite) and or to have a ground well drilled. Ground conductors should have as much surface area contact with Earth as possible.
The whole idea is to present a low-resistance ground path and keep all of the equipment at the same potential to minimize the current flow between equipment.
For the electrical building service entrance, a series surge protector installed before the service panel is the best method. Several are made and they need to be sized for the building service. A fallback is a parallel surge protector that will provide some protection. On the AC mains connections, any series inductance that can be added to increase resistance to the lightning pulse is good. All AC mains connections to the transmitter should go through a toroid before they are connected to the transmitter. This is a good idea for remote control and mod monitor wiring as well.
Or Star Quad Microphone Cable, depending on who is making it.
Star Quad Microphone Cable Diagram
This has been around for quite a while, but many studio/broadcast engineers don’t understand it or don’t use it for some reason. Microphones and mic pickups produce relatively low signals when compared to line-level audio. Most microphone preamps have a gain of +50 dB, which means any noise gets amplified and even small things can become major problems quickly.
Gepco MP1201 Quadstar Microphone Cable
Under general conditions, most balanced shield twisted pair (STP) audio cable such as the standard Belden 8450 is adequate for stationary microphone cable for short runs. When the cable is not permanently fixed in place, as in hand-held microphones, microphones mounted on booms, or other nonfixed microphone applications, then a flexible cable must be used. Star Quad cable has better noise specifications than standard flexible microphone cable.
The advantage of Star Quad cable for low-impedance microphones (150 ohms) is that the parallel twisted pairs significantly reduce inductive reactance. In AC circuits, inductive reactance acts as a low pass filter, gradually rolling off as the frequency is increased. This effect is cumulative, the longer the cable run, the more inductive reactance is added to the circuit. The result is microphone audio can have smeared or ill defined high frequency audio.
parallel inductance formula
Using two parallel twisted pairs is similar to parallel resistors when it dealing with inductive reactance, it halves the value.
In addition to reducing inductive reactance, the tighter twist found in Star Quad cables reduces the CMRR by about 20 dB. The Star-Quad configuration keeps the conductors in the same relative position to each other as the cable is flexed and moved around. All of this makes it superior to standard STP microphone cable.
Several companies manufacture Quad Star cables:
Belden: 1192A
Canare: L-4E6S
Gepco: MP1201
Mogami W2534
Cardas 4X24
The price of Star Quad cables runs about 40-60 cents per foot (more for the Belden, much more for Cardas) if purchased in bulk. That is about the same range for two conductor mic cables.
As good as this cable is, I don’t think they had this in mind when they made it:
I wonder what the centripetal force on that cable is when the microphone is in full motion. Also, I’d bet that SM58 was none the worse for were after it’s crowd surfing moment.
