[R-390] the saga cont. pt 10

[Shoppa, Tim]

Using slug tuning, with DC voltages for bias, AGC, plate, etc., on the coil is a genius move.

Trimmers and other variable capacitors are most advantageously mounted if one side is grounded. Otherwise you have to stand them off ground - it's doable (and in fact done often) but if you're clever you avoid it by grounding one side.

The balanced input in the 390A is not "just to tune out common-mode noise". Balanced antennas and feedline systems are a joy to use, you just have to build your whole station around them :-). The whole move to coax for antennas, post-WWII, was in my opinion misguided. (Yeah I have coax in the shack... but the antenna feed line is ladder line, all the way from the link-coupled balanced tuner to the sky!)

These are common themes in radios other than 390A's, too.

Tim N3QE

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of ka9egw at britewerkz.com
Sent: Sunday, September 26, 2010 6:49 PM
To: Flowertime01 at wmconnect.com; r-390 at mailman.qth.net
Subject: Re: [R-390] the saga cont. pt 10

tm 11-856A, figure 106, page 187, upper left corner....er...[scraping egg
off face]...dang, I must have misread the schematic.  Time to swipe the
FXYL's lighted magnifier...

Looks like the cap and trimmer form a capacitive divider across the primary
to make a "wannabe grounded center tap".  Yer right, there's no dc ground.

Hmm.  Virtual-grounded for what? Just to tune out common-mode noise?

Or is there a greater and more nefarious purpose I'm not catching?

73, Brian KA9EGW


-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of
Flowertime01 at wmconnect.com
Sent: Sunday, September 26, 2010 5:16 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] the saga cont. pt 10


Brian,

I am looking at the schematic in the TM. I do not see any ground points on
the coils in the RF deck cans.

The effects of the antenna trim will vary from octave to octave. No logic
to operation is known YMWV.

Just one pass through the receiver after pulling it apart and changing
tubes will not get you in good alignment. Make at least three full passes
before
you think you are getting close. Burn the tubes in for a month and then do
the alignment again. You can expect things to be way off after changing
tubes. A bath and or mechanical alignment will also leave thins way off.
Sound
all normal to me.

Roger.

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