Arcs and Sparks in the Viking 500 - RT's Radio Home Page
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Arcs and Sparks in the Viking 500
Amateur Radio > E.F. Johnson Restoration > Viking 500 Specifics
Years ago, owners of aging Viking Five Hundred transmitters bitterly complained of arcing and blown fuses.
1. EF Johnson engineers knew arcing would be a problem when un-keying the Five Hundred. To counteract this, they incorporated a "slow-release" relay into the design which allowed residual energy in the tank circuit to bleed off prior to disengagement of the relay. Over the decades, this relay eventually lost this magnetism resulting in this residual energy arcing to whatever point of lowest resistance it found.
Rodger, WQ9E came up with a simple but ingenius circuit to restore this delay action to the relay. It uses a single diode to isolate relay RY-302 from the DC relay circuit, and a 10 uF electrolytic capacitor to hold RY-302 the relay in the energized/closed position for a fraction of a second. This is a necessary circuit to add. This should not be seen as a "modification" since it actually restores original function to the transmitter.
2. When first inspecting 500 #1, the modulated high voltage plug/jack located on the power supply chassis had partially melted due to arcing. The area surrounding the jack was blackened, so the carbon track certainly made an easy path for further arcing. All the HV pin plugs and jacks were replaced.
My copy of the Viking Five Hundred manual calls for .025" spark gap for protection of the modulation transformer. Thankfully, Mike Harrison, KE0ZU sent a revised manual page that called for .018"! (May have saved a modulation transformer). About a year into operation of this transmoitter, this gap began occasional arcing on modulation peaks. These grew more frequent accompanied by blowing fuses. Kevin, WA6JKN, suggested the increased arcing was due to the gap surfaces becoming roughened by the arcing making them more susceptible to further arcing--he was right. The gap itselt is made using domed cap nuts; after polishing these cap nuts to a mirror finish on a buffing wheel, the arcing ceased.
The plan for the new power supply for 500#2 includes using highly polished stainless steel cap nuts for the spark gap, these spaced at .018".