Tom contributed this solid state
replacement for the VFO ballast tube. It is great that knowledgeable
people share their ideas and hint for this hobby.
Thanks again Tom.
Sent: Thursday, December 13, 2012 10:22 PM
Subject: NC-303: Solid stating the 4H4-C ballast tube.
You've done a wonderful job on the NC-303! I
have done a total of five restorations on the NC-300/303's so far,
with two more left to do.
You can solid state the 4H4-C VFO filament ballast
tube. It's fairly easy, and reversible: I added a ground terminal to
one of the 4H4-C tube socket pins. Then I took the base off an old
Octal tube and built it up.
I took the AC input, and half wave rectified it,
feeding the pulsating DC to a filter cap. I then ran this to a 7805
voltage regulator. The trick is to take the ground pin of the 7805,
and add two diodes in series with it.
adds 1.4 volts to the 7805, for 6.4 volts DC. Close enough to 6.3
volts, and it won't drift at all!
It is necessary to heat sink the 7805, as you're
dissipating a fair amount of power.
I did this for a radio that had no 4H4-C, and it
Best of Luck to you on your radio projects.
Tom, PhD, CETma
I really appreciate the sharing of
ideas, hints and tricks. Everyone is free to send me their ideas for
posting on this web site.