[R-390] The Penultimate R-390* Ballast Replacement

[ToddRoberts2001 at aol.com]

In a message dated 2/13/2005 1:42:21 PM Eastern Standard Time, 
tetrode at comcast.net writes:
Sounds good Todd.
The 12.6 VDC supply doesn't even need to be husky, for only a 300 ma load
any of the common 78xx style 3 or 4 terminal regulators in the TO-220
packages would suffice, and even the best regulator would only need a 723
and a pass transistor. I wouldn't even bother with connecting the ground
return to the ballast tube socket, just use any chassis ground connection on
the back panel.

I've read all kinds of over-worrying about electronic regulator "noise" or
oscillations from voltage regulator IC's in radios, but it's really a
non-issue. As long as you follow the bypassing suggestions in the app notes
for the part, use good parts, and check things with a scope it'll be fine.

Only *once* did I have a problem with a 3 term regulator inside an HF
receiver. My TMC GPR-90 has a whole bunch of solid state mods and has a +/-
12 VDC regulated supply under the chassis to run them. After I installed it
I heard some 300 kHz carriers that I didn't hear before. It turned out to be
one of the 3 terminal regulators oscillating (and I hadn't followed my own
advice about checking it with a scope!) which happened to be a TI part. Took
it out and installed a Motorola device in its place and it's been clean for
years.

John

Thanks for the comments!  I was thinking of something along the lines of a 
12.6 VAC  1 or 2 amp filament transformer with a full-wave bridge bypassed with 
caps on each leg and a decent-size filter cap running into a 723 regulator 
(well bypassed) and a single 2N3055 pass transistor. The power supply output may 
sag for a second or 2 with the turn-on inrush current but that is fine and 
self-limits the current at turn-on. Put the parts into a nice small aluminum 
mini-box or open chassis with perhaps a small heatsink and an umbilical cord and 
tuck away near the receiver. Would be a fun project to build! 73 Todd WD4NGG