[R-390] NEED HELP R-390

[Barry]

That's what I was referring to as a "reasonable replacement."  I'm thinking
of modifying this one for 12BA6 operation.  The only thing I don't like
about this is it means there is a dependency between the IF and PTO modules.
Not too bad and if the jumper is replaced with a 3TF7, it won't burn the
filaments out so it's relatively "safe".  I may try the 12BH7/12BY7 route
too.

I'm sort of planning on modifying the power supply for 12BW4s so this might
be my "modded" radio.

Thanks,

Barry - N4BUQ

----- Original Message ----- 
From: "David C. Hallam" <dhallam at rapidsys.com>
To: <R-390 at mailman.qth.net>
Sent: Wednesday, June 15, 2005 12:42 PM
Subject: RE: [R-390] NEED HELP R-390


> Why replace the ballast tube at all?  Dallas Lankford's work showed that
the
> ballast tube didn't do a very good job of stabilizing the PTO filament
> voltage anyway.  The resistor or 12BA6 substitutions require no
> modifications and do just as good or better job.  Besides they cost less
> than the $20-$30 you have to pay for a new ballast tube.  Unless a person
is
> a fanatic about original condition, I don't see any reason to keep the
> ballast tube.
>
> David C. Hallam
> KC2JD
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Barry
> Sent: Wednesday, June 15, 2005 12:07 PM
> To: R-390 at mailman.qth.net
> Subject: Re: [R-390] NEED HELP R-390
>
>
> I wonder what the output voltage on your PTO runs with the 3TF7 in place?
> Both of mine only put out a couple of volts, but I think they're supposed
to
> generate about 7 volts.  Apparently it takes very little injection signal
to
> keep things going and even my 2 or 3 volt units provide plenty of signal.
> Not sure, though, how long the loop will continue after mine loses
filament
> power.  Perhaps I'll try that.
>
> Sadly, one of my 3TF7s died recently.  I was aligning the IF and in the
> process of pulling the xfmr shields at the time and I think something
> shorted to the shield in the process.  Yeah, I should've turned off the
> power when doing this, but I didn't think there would be a point that
would
> touch anything.  At any rate, the 3TF7 died when I heard the "buzz" while
> pulling the shield off of one of the transformers.  Since the 3TF7 only
> regulates the two filaments, I'm not sure how this would have caused the
> 3TF7 to blow.  Perhaps it was just its time to go?  At any rate, it is now
> replaced with my trusty 47-ohm resistor bundle until I find another
> reasonable replacement.  :(
>
> Barry - N4BUQ
>
> ----- Original Message -----
> From: "Tim Shoppa" <tshoppa at wmata.com>
> To: <R-390 at mailman.qth.net>
> Sent: Wednesday, June 15, 2005 10:46 AM
> Subject: Re: [R-390] NEED HELP R-390
>
>
> >> HOW DOES A R-390 RECEIVE WHEN BALLEST TUBE [TUR]3TF7 IS WEAK OR BAD?
>
> > The PTO will not work.
>
> I've pulled the (good) ballast tube from a working R-390A and I was
> surprised how
> long the PTO continued to "work":
>
> A few seconds after pulling the tube, the PTO begins to drift enough to be
> noticeable.
> For the first coupld of seconds, I don't hear the PTO drift even when
> listening to CW.
>
> For ten or so seconds after that, the PTO drifts maybe a few hundred Hz
but
> keeps on working.
>
> There's another few seconds where the PTO is drifting way way out from
where
> it originally was oscillating.
>
> After about 15 seconds, finally the PTO stops oscillating and the radio
goes
> quiet.
>
> My guess is that there is enough "gain margin" in the PTO design that even
> though loop
> gain drops dramatically as filament emission drops, still the oscillator
> keeps on running.
>
> Tim.
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