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The Chernobyl Resistor
Amateur Radio > Vintage Transmitters
If you are a E.F. Johnson Viking series transmitter fan, you have likely heard of the "Chernobyl Resistor". It is an 18,000 ohm, 2 Watt carbon composition resistor located inside the small VFO cabinet. The problem is the resistor is undersized for the load it carries, and it has been known to fail causing a small fire in the VFO. It seems another failure mode is where the resistor loses value which overheats the 0A2 regulator. Either of these failure modes will damage to the VFO.
Here is the aftermath of the failure of the 18K resistor in a Johnson Viking 500. The area burned in this photo surrounds the 0A2 regulator tube. It appears this was due to overheating of the 0A2 as a result of the failed 18K resistor. It is a well known problem among today's E.F. Johnson Viking community of enthuiasts.
It is advisable to any new owner of a Viking Ranger, Valiant or 500 to verify this resistor has been changed to at least a 5 watt resistor. Many users have used a 20K 5 watt resistor with good results. The voltage at the 0A2 should be 150 volts, or very close to that.