[Central-Electronics] Notes on the 10A and Phasing Rig alignment

[Robert Nickels]

I decided to spend as much time as it took to really get to know the 10A 
while I had it on the bench, and thought I'd document a few findings and 
suggestions for the benefit of others since the same basic circuitry is 
used on many phasing type SSB rigs.

Repair/restoration:
The filter cap on the output of the HV filter was open, which created a 
problem I hadn't seen before -  the B+ line would oscillate at about 55 
khz rate!    We all know about replacing old electrolytics, I just got 
complacent because the HV was clean under light loading.   Just a 
reminder to do the basic restoration things before tackling alignment - 
   distortion in the audio section will made proper alignment impossible.

With the transmitter and 458 VFO running I had low (1-2 watts) power 
output but the signal didn't sound clean and there wasn't much sideband 
suppression.

Alignment:
If you don't have a real solid understanding of phasing rig alignment, 
let me suggest the excellent article "How to Adjust Phasing-Type S.S.B. 
Exciters" by Robert Ehrlich, WØJSM, which appears in the Nov. 1956 QST 
and in the 3rd edition of "Single Sideband for the Radio Amateur", 
published by ARRL.  I seem to learn something new every time I re-read 
it (which is often!)

I wanted to make sure the RF phase shifter was set as close to 90 
degrees as I could, so I first used my dual-trace scope in X-Y mode to 
generale a Lissajous pattern (attach scope probes to the wiper of each 
of the carrier balance pots).   When two sine waves are 90 degrees out 
of phase, the display will be a circle.   Adjusting the red core 
(primary) made the circle larger (and disappear, when the oscillator 
drops out); adjusting the green coil makes the circle round.   It's a 
quick way to make sure you've got the coils in the ballpark.

The amplitude of the drive signals to the audio phase shift module must 
be in a 2:7 voltage ratio.  My 10A did this with 1400 and 400 ohm 
precision resistors in a voltage divider,  but I added a 500 ohm pot 
(this AF BAL pot was added in later CE models) to make it possible to 
tweak the exact ratio.   I found that when you're trying to eke the last 
bit of sideband suppression out, this adjustment is critical.

The Ehrlich article describes three methods for making the opposite 
sideband suppression adjustments:
1.  Observing the "ripple pattern" on a scope connected to the 
transmitter output
2.  Using a receiver tuned to the appropriate signal
3.  Observing the RF output in PM mode to fine-tune the RF phase shifter.

I experimented using all three, but I really recommend a 4th technique 
that I discovered in an old RSGB publication.  It is attributed to 
ZL2AMJ who is the inventor of the "Tuna Tin Two", a brilliantly simple 
phasing rig, so all credit goes to him -  he describes it better than I 
could.   I highly recommend building this little circuit before you 
start alignment.  Instead of headphones,  connecting the gizmo to an 
amplified computer speaker will make it a lot easier to hear the audio 
tones.

Download:  Aligning Phasing-type SSB Exciters:  
http://smg.photobucket.com/albums/v652/ranickel/Vintage%20SSB/?action=view&current=PHASINGRIGALIGNMENT.jpg    
or:  http://tinyurl.com/449l92u**

With audio sine wave input at the correct frequency and level,  AF BAL 
(2:7 ratio) pot mid-range, and carrier nulled, go back and forth between 
the green RF coil and the AF BAL (in the cathode of the AF Modulator) to 
null out the opposite sideband.  This is indicated by minimum ripple on 
the scope and/or a null in the high pitched tone on the audio gizmo; I 
found the secret is to make very small coil adjustments to these two 
controls only,  and flip back and forth between sidebands to optimize 
the settings.   Once you start to see a fairly ripple-free display,  I 
suggest doing the test described in the last part of the Ehrlich 
article:  Set up the transmitter for PM mode operation and inject tone 
for 20-30% modulation.   If the "humps"  on the scope display are 
exactly the same height, that's confirmation that you've got the RF 
phaser set for exactly 90 degrees.   Lock it down, then tweak the two AF 
BAL pots for deepest null on either sideband.  When the high-pitched 
tone virtually disappears on the audio gizmo, you know you're done.

Having fiddled with this process for hours, I believe that a tech who is 
skilled with use of the audio gizmo could align a phasing exciter to 
sideband suppression of 30 dB or better in a minute or two, without even 
looking at a scope.  I found that I could make much finer null 
adjustments based on the audio tone than by looking at the scope pattern.

A couple of RF signal references:  I found that with proper alignment, 
full output (5-6 watts) was obtained with 1-2 volts P-P RF on both pins 
2 (VFO) and 7 (9 mhz excitation) of the 6BA7 mixer tube.    I moved the 
parasitic suppressor from the grid to the plate of the 6AG7 final as per 
a 10B change notice but have not done any of the mixer changes yet.

While the current crop of SDRs offer impressive capabilities, they need 
a PC that typically contains over 10 million transistors.  The 10A is a 
great reminder of what can be done with a handful of cheap tubes and 
clever yet simple circuits that can still be maintained over 50 years 
after it was designed and built.

73, Bob W9RAN