[R-390] Another (perhaps) silly ballast question

[Barry]

I started this mod last night, but was too lazy to do it all the way.
Instead, I soldered a jumper wire from the 12.6V center tap and plugged it
into pin 7 of the ballast tube socket.  Naturally it worked fine.  I plan to
run it neatly into the plugs and sockets, but was just anxious to try this.

One thing I found very interesting was what happens when disconnecting the
jumper while the radio is running.  The BFO fades rather rapidly (about 5
seconds or so), but the PTO continues to run for about 15 seconds.
Naturally, when the BFO is warming up or dying out, the change in output
signal is easily recognized, but after the BFO dies, it is difficult to tell
if the PTO is drifting or not without a counter connected somewhere.  I'm
curious as to whether the stability issues reported by those who have
performed tests with regulated filament lines vs. not regulated lines are
more due to the PTO or the BFO?

Anyway, it looks like my search for a ballast tube now is moot.  Using the
12.6V line just makes sense -- at least for me.  In a way, I a bit surprised
the engineers way-back-when didn't run this as an option.  In the field, if
the ballast were to go out, there wasn't anything to do except either find a
resistor or wait for a replacement ballast.  If they had run the 12V line to
blank position on the ballast socket (unused pins are available on both
plugs), if the ballast fails, the radio could have been temporarily restored
to operational status with a paper clip.  I realize each inch of wire
amounted to extra cost and they were trying to save on costs where possible
and maybe it just wasn't a critical enough of an issue at the time given the
fact that ballast tubes were in plentiful supply.  Also, ballast tubes seem
to last indefinitely as long as they're not abused, so maybe they figured it
wasn't an issue.  Dunno.  I'm just happy the transformer has a 12.6V tap!

Oh, by the way, there was some discussion as to a suitable resistor to use
for the ballast (in case that's the route you go).  I looked at my setup
last night and I have four 180-ohm resistors in parallel yielding 45 ohms.
Just a tad closer to the actual value needed than the standard 47 ohm
resistor and it allows using smaller wattage resistors to equally dissipate
the heat.  Mine happen to be higher wattage resistors (3-watts each, I
think), but a bundle of four 2-watt resistors would be well over the
required rating.

Happy Ballasting,

Barry - N4BUQ (hoping nobody goes "ballast"ic on me for this post...)

----- Original Message ----- 
From: "Barry" <n4buq at aol.com>
To: "R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Wednesday, January 04, 2006 3:13 PM
Subject: Re: [R-390] Another (perhaps) silly ballast question


> Well, I was going to wait until I tried it to make sure there weren't any
> "gotchas", but it appears someone else (Dan?) has mentioned this recently.
>
> I kept trying to think of a clever way to get 12.6VAC to the oscillator
> tubes -- even to the point of adding a filament transformer somewhere (it
> would be cheaper than a ballast tube costs these days and last longer) -- 
> when it occurred to me that if the 26.2VAC winding were center-tapped,
that
> voltage could be used.  That's when I looked at the schematic (and
confirmed
> on the unit) that the 26.2VAC winding is indeed center-tapped.  This tap
can
> be run through unused pins from the PS deck to the IF deck.  From the IF
> deck plug, I plan to run the wire to an unused pin on the ballast tube
> socket and plug in a jumper from that pin to pin 7.  This allows the
> original oscillator tubes to be used and if someone decides they want to
use
> a ballast tube, all that will be necessary is to unplug the jumper and
plug
> in the ballast tube.
>
> Barry - N4BUQ
>
>
> > Oh?  Well TELL us what the plan is!
> >
> > Roy
> >
> >
> >
> > - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> > 7130 Panorama Drive, Derwood MD 20855
> > Home: 301-330-8828 Cell 301-928-7794
> > Work: Voice: 301-975-3254,  Fax: 301-948-6213
> > roy.morgan at nist.gov --
>
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