[R-390] R-390 Langford SSB Mod

[Bill Feldmann]

Hi All,

Some of you maybe considering or have the Langford SSB modification in your
R-390's and be interested in my experience installing it in my non-A R-390.

I'm very impressed with this simple and easy to do modification which is
easily reversible if you are not happy with its performance.  On very strong
SSB signals it does have a very slight tendency for overload but is a great
modification if you plan to mostly use your R-390 for AM reception but
occasionally to monitor SSB.  If you experience any overload just back off
the RF gain a bit.

If you are planning to mostly work SSB than a full blown product detector
maybe a little better, but not much.  Also if you are using a non-A R-390
like mine and the SSB station your receiving has poor opposite side band or
carrier suppression there will be a slight distortion due to the beating
effect due to the less steep shape factor of the LC filters compared to the
mechanical filters of the A model.

I did run into one little bug that degraded the audio of AM signals but only
with fast attack AGC.  This was caused by some audio getting onto the AGC
line due to the decreased impedance between the AVC rectifier tube and the
AGC circuit by adding the diodes across the 180K and 220K resistors.  It was
most noticeable when listening to music by a reverb type sound on low
pitched instruments such as drums.

I easily solved by adding a .5uf cap of at least 25 volt between pins 1 and
2 of the AVC switch, S104 on a non-A or pins 9 and 10 of S107 on a A model.
This completely by passes audio off the AVC line and only slightly slows
down the fast AVC, which I think is too fast to begin with.  I put the cap
on the AVC switch to not affect the time constants of the medium or slow AVC
which I didn't want any slower.  After adding the cap the fidelity of AM is
excellent in all AVC modes.  This was the only chance to the original
Langford modification I found necessary.

For those of you not familiar with this simple and effective modification
let me summarize it here.  Carefully remove the IF module after unplugging
the power and BNC signal connectors, loosening the two clamps and uncoupling
the band width and BFO shafts, then loosening the three green screws
securing the module to the chassis.

Add a 1N4148 diode across each of the 180K and 220K resistors between the
AGC rectifier tubes anode and the AGC circuit, R556 and R557 on a non-A or
R546 and 547 on a A model.  Install the diodes so their cathodes are toward
the plate of the AGC rectifier tube.  This will convert the AGC circuit to
fast attack slow decay.

Add a 47pf, at least 300 volt, dipped mica cap across the 10pf output cap
for the BFO that is connected to the BFO tubes plate, C536 on a non-A or
C535 on a A model.  This increased value of 57pf for the coupling cap will
increase the BFO injection to the audio detector.

After checking your work reinstall the IF module and test the radio on AM
and SSB stations.  Offset the BFO +1 to 2kc for LSB in the 2 or 4kc
selectivity positions.  For USB offset the BFO -1 or 2kc.   Adjust the BFO
and main tuning dial for best audio sound.  Then if you also notice the
slight distortion of AM signals in the fast AGC mode, add a cap to the AGC
switch pins as described above.

Physically installing the cap was the hardest part of the modification on my
R-390 because of the BFO bellows covering the area of the cap but using a
small soldering tool hemostats and some patience was possible.   Installing
the diodes was easy on my R-390 because the two resistor are easy to get to
on TB502 located on the back wall of the IF module.

Hope my experience is helpful,

Bill N6PY