I wouldn't know that today is National Radio Day if it weren't for the explosion.
The view outside the double windows of the Upper Management Lounge lit up with a large red/orange/yellow flash accompanied instantly by a loud BANG which I took to be a lightning strike.
Quickly shutting down we went into offline safety mode with local KTRS 550 kHz 5 kW on the TECSON PL-310 where a conversation about adult pimples gave way to mention of National Radio Day.
Maybe the explosion was daylight fireworks.
Casualty
Being in a prolonged thunderstorm watch since the lightning strike, we decided to return to the air during the present lull, and find that our EDM FM Transmitter is broken, likely having inducted a static discharge
The AMT5000 survived, even though one of its ground radials runs right up toward ground zero where the strike was seen.
In addition to repairing the EDM we'll order another one since these have proven to be excellent transmitters for part 15 work.
Temporarily maybe the Wholehouse 2.0 can be used to stay afloat.
Follow Up
The Wholehouse 2.0 is a "junk box" item for KDX, but got put into use after searching for the appropriate audio cables.
The RF output is clipped to 1/2 of the horizontal dipole since the Wholehouse 2.0, like the C.Crane FMs, provide no RF circuit ground point.
At our main frequency down in the educational band the Wholehouse 2.0 puts out plenty of hash with distorted audio on the spurs, yet it is certified and the junk measures exactly 20 dB below the main carrier, in compliance with 15.209.
This did not pass our own standard, wanting to prevent nearby neighbors from receiving interference to their favorite low-band stations, so we moved to the high-band and there are no spurious transmissions observed.
Having a backup transmitter matters.
Protecting your transmitters from static on the antenna matters too.
A 2.2uH coil across the antenna to ground will do this.
Also a surge protector that SHUTS DOWN if the surge protection is inoperative or overwhelmed by even a lightning strike to the house that will protect your equipment.
The ones that just have a light that tells you if the protection is working isn't good enough.
Carl, when you get your new EDM, if you can, obtain a 2.2uH inductor and connect it on the circuit board at the antenna to ground.
Good Advice from Mark
Later today I will open the EDM to see what the output section looks like, but I am aware that the Ramsey FM kits also have a sensitive and tiny output transistor.
The FM25b was famous for blowing finals even by simply touching the antenna.
The FM35b uses the same output transistor but added an inductor across the RF output EXACTLY LIKE MARK SUGGESTS, so I guess they learned their lesson about making the transmitter less prone to blow-out.
Meanwhile, the Wholehouse 2.0 is keeping KDX on the air for now.
Forgot to Mention
The Wholehouse 2.0 at its high-band FM channel has been dubbed translator station K293FM.
Here's a Question
My guess is that the static impulse from lightning detected by an antenna and discharged into a transmitter's RF output can blow the final transistor EVEN IF THE TRANSMITTER IS TURNED OFF.
What is your opinion about this?
Yes Carl. It is pointless to shut down power.
Points and Pointlessness
OK Radio Legend, based on your statement the policy is slightly shifted for KDX... there is no point turning transmitters off due to lightning, since they remain vulnerable either way.
I do think that disconnecting our ground radial from the AMT5000 is still worth doing, since it runs 100' beneath the surface of the ground on the outdoor side (the transmitter is indoors), and might absorb lightning striking the ground. It might even bring fire into the house (?)
Now let's talk about the computers. Radio people in California have advised totally closing and disconnecting computer equipment from all outdoor cables and wiring.
Going on, the antenna for the temporary Wholehouse 2.0 transmitter has been changed from horizontal to vertical, and I plan to post a thread about our maze of FM sending and receiving antennas and the engineering challenges.
This morning again we were subjected to hours of storm, residue of which lingers even though we are risking operation.
Oh, and I opened the EDM FM Transmitter to see that the output transister is indeed a very tiny device with legs on 4-sides, followed by a filter circuit of several caps and 2-coils.
We should also check the Ramsey FM35b, which is located 35-feet farther away from the recent lightning zone.
To answer Carl's question.....YES!! doesn't matter if the transmitter is on or off.
Even a shock from your finger if you touch the antenna without first discharging your static electricity sends 6000 volts into the final transistor and it won't like that too much.
A 2.2 uH coil (easy to do) at the antenna to ground will shunt the charge on the antenna to ground, saving the output. The coil will be a short with DC but with A/C at FM frequencies it has a very high impedance and doesn't affect your output.
If you notice the SStran's you have there's what is a gas filled component across the antenna/ground, explained in the manual and that works like an MOV but has a finite life. The inductor is the best way. That's how the Decade is protected.
Those Ramsey kits with the GAL-5 were notorious for this but can be protected the same way.
Mark That As Useful
Mark says a lot of useful things. Take for example:
"If you notice the SStran’s you have there’s what is a gas filled component across the antenna/ground, explained in the manual and that works like an MOV but has a finite life."
I knew about the gas filled part, but was not aware of its finite life expectancy.
By the way, is there a test of the gas-filled part to determine if it's still effective?
That output on the EDM is a surface mount transistor and replaceable but hard to do so when you get the new one do the coil yourself.
The gas filled thing is like an MOV and like power bar surge protectors have a limited capacity, each spike absorbed leaves less capacity and a big one can push it to the max fast. The best surge protectors are the ones that shut of power when the surge protection is gone. The one like what the SStran has, you don't know if it's working or not.
The coil is best....works indefinitly.
As for a test, not sure, maybe you can check with a multi-tester, but don't know what reading you are supposed to get.
Too bad SStran is not there anymore to ask.
Safety During Electrical Storm Practices and Procedures
AMradioLegend teaches that: "It is pointless to shut down power."
In the case of our recently lightning damaged EDM FM Transmitter, we have come to realize that because of the burst of voltage discharging between sky and ground, our nearby horizontal dipole antenna picked-up a large voltage which destroyed the output transistor.
Although simply shutting off power to the transmitter wouldn't have prevented the damage, we strongly suspect that had we ALSO disconnected the antenna during the storm we would have probably saved the transmitter.
The comment period is now open and operators are standing by.