[R-390] NEED HELP R-390

[David C. Hallam]

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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