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

[John KA1XC]

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




----- Original Message ----- 
From: <ToddRoberts2001 at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, February 13, 2005 12:54 PM
Subject: Re: [R-390] The Penultimate R-390* Ballast Replacement


> Most of the 3TF7 substitutes I have read about involve buildng something
> small enough to plug directly into the 3TF7 socket or on a nearby bracket.
This
> comes with problems of heat dissipation or else installing unsightly
brackets
> near the I.F. subchassis. Some of the recent ideas got me thinking - Why
not
> build a small separate power supply module with a
> well-regulated/filtered/bypassed 12.6 VDC output that could be placed next
to the receiver and the only
> connection would be a small umbilical cable with a 9-pin plug that plugs
directly
> into the 3TF7 socket? You could run the umbilical through the side of the
R390A
> chassis thru one of the large holes and tuck the power supply and cord out
of
> the way next to the receiver. This layout is similar to the way some
> audiophile preamps use a separate power-supply module with an umbilical.
You could
> modify one or two pins of the 3TF7 socket with a jumper to ground to
provide a
> ground return for the 12.6VDC so you wouldn't have to tie down a separate
ground
> lead with a terminal and screw somewhere else on the chassis. When you
plug
> in the umbilical it breaks the 25.2 VAC circuit and connects the 12.6VDC
> circuit and ground. The ground pins would have no effect on the original
3TF7 if you
> wanted to plug one back in.  This way you could build a nice little husky
> separate  regulated/filtered/bypassed 12.6 VDC power supply and not have
to
> miniaturize it or compromize the performance. If you want to go back to a
3TF7 just
> unplug the DC supply 9 pin plug and put the 3TF7 back in. No unsightly
> permanent wires or brackets hanging off the I.F. subchassis.  The
regulated DC
> supply should give the ultimate in stability and pure DC on the filaments
of the
> BFO/PTO tubes. 73 Todd WD4NGG
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