For most properly shielded equipment, the danger to equipment will not be directly. It will be the power conducted to the equipment from long wires
connected to that piece of equipment.
This is my experience in electronics, and dealing with surge and lighting induced damage on electronic equipment..
Radio receivers. If the receiver has a metal chassis, then the threat vectors are the connections to the outside world. The antenna. The power cord.
The speaker wire.
The first stage the antenna signal hits is the RF preamp stage. Older/cheaper units use a BJT. Newer/more expensive units use a MOSFET stage. That is
where the brunt of the EMP will be felt. A BJT will fail from the base voltage going past it’s max rated peak reverse voltage. or exceeding it’s
max instantaneous rated base current. One or the other depending on the pulse polarity. It will either fail in an open, or shorted state. MOSFETS will
fail from excess gate voltage causing a breakdown in the gate isolating layer. It will fail in a gate shorted condition. Repair can be affected by
replacement of the failed transistor. In an emergency situation, you can jumper the antenna lead to the output transformer of the first stage to
bypass it if no replacement parts are on hand. sensitivity will go down, but the unit will remain mostly functional.
Mitigation can be obtained by using a set of diodes or zener diodes back to back on the input. They should be considered sacrificial because they will
short when they take a solid hit. That will protect the input stage from the initial strike, and all subsequent strikes because the input will be
directly shorted. When the threat passes, then you can replace the diodes and resume normal operation.
Power cord. Will either enter a transformer, or rectifier bridge for a switching supply. Common mode surges will cause arcing in the transformer, or
arcing in the feedback circuits for a switching supply. Differential mode surges will cause excessive voltage across the rectifiers, excessive
voltages across the filter capacitors, and excessive voltages across the regulating components.
Transformers supplies. For common mode voltages, the primary threat is winding arc over. If the voltages from primary to secondary, or transformer
case becomes too great, then it will arc over, possibly damaging the transformer. For differential voltages. The transformer acts as a band pass
filter. The high frequency leading edge of the EMP will be muted. The low frequency component will cause excessive core magnetization and it will
cause the supply fuse to blow with no other damage to the supply. The part of the wave that falls between the two extremes will damage components
after the transformer. If the voltage goes past the maximum PIV of the filter diodes, then they will short. If there is enough energy, then the filter
caps may be stressed and fail. Or the regulators could be damaged from the spike in voltage.
Repair should be conducted by checking to see if the primary fuse is blown. If it is, put an incandescent light as a current limiter in line and see
if the power supply powers up as normal. Replace any blown components as needed.
Switching supplies. Common mode voltages may cause arc over in the feedback components. But normally it will cause arc over in the spark gap that most
switching supplies have in them for lighting protection. Differential mode voltage spikes will cause damage to the rectifier diode on the input, and
damage to the primary filter capacitor from excess voltage. Or damage to the switching transistor. Those failures will cause the input fuse to blow.
If the supply has a MOV, then it will most likely blow the input fuse with no other damage.
Use a light as a current limiter in place of the fuse and power up the circuit so you can isolate and repair the damage if possible.
For emergency use, you can isolate the internal supply and power the radio with an outside supply of suitable voltage and current capability. If the
radio is designed to operate on AC and DC, you may be able to just hook it to a 12V dc supply and operate it with no repair needed for the time
Mitigation can be done by using MOVs across input lines, and a MOV from each input to ground. Fuses on each line before the MOVs. The surge will be
caught by the mov’s and blow the fuses. Replace fuses and return to service.
Speaker wire. If you have an external speaker wire connected. The voltage surge will hit the final audio amp. Depending on the design of the audio
amp, it will either suffer internal damage(integrated amp), shorted transistors (component amp), or no damage at all if it uses catch diodes that will
sink the current to the supply rails.
If there is damage, then you can repair or replace the damaged components as needed.
For emergency service, you can isolate the AF power amplifier from the DC supply rail and use the headphone jack on the front of the radio with a set
of headphones. In a lot of radios the headphone jack is before the AF power amp that drives the speakers. So they will still work even with the power
Mitigation for radios with relatively isolated and short speaker wires can be done by hooking catch diodes between the output and the two power supply
rails. That will stop the output from swinging beyond power supply voltages and causing voltage reversing of components and resulting damage. If it is
a capacitor isolated output, and if the length of the speaker wire is longer, or hooked to other equipment, then you can use back to back zener diodes
to catch any input surge.
On radios with a plastic case, there could be damage with internal circuits that are not directly connected to external components. Damage and
repairs will depend on the type of radio, and it too vast to detail here.
Transmitters. Beyond the stated conditions listed for receivers. The only thing additional that can be damaged is the RF power amplifier stage that is
hooked to the antenna. The final power amplifier transistor may me shorted. That will cause the DC supply rail to be shorted, and the supply fuse to
blow. Test and repair as needed.
For emergency service you can bypass the final output transistor by hooking the driver stage transistor to the final output tank. Output power will be
lower, but the radio will still be functional for the most part.
Mitigation can be had by putting back to back zeners on the output. Again, the zeners will probably short in a strike, but they can easily be
edit on 7-4-2013 by Mr Tranny because: (no reason given)