[R-390] W0BT and N6PY mods in ER Magazine

[Masters Andy]

Good evening list.  Recently, I decided to modify my
R-390A based on the September 2006 issue of ER
magazine. 

I made the following mods:

1. N6PY's noise limiter circuit. Result-it does work
more effectively with less apparent distortion when
the limiter is turned on.  It also works nicely when
the BFO is turned on.

2. N6PY's detector mod.  This was a bit more
challenging but well worth the effort. SSB sounds
fabulous now on the receiver. I initially ended up
with increased distortion on AM but, based on the
advice of Ray, N0DMS, I changed out the disc .01 cap I
was using as C2A to a mylar and then changed its value
to .022 which worked better for me-passing more low
end audio.  The distortion went away.  

3. W0BT AGC mod. A big step but also well worth the
effort.  My R-390A came to me already with the Rick
Mish SSB AGC Mod on the back terminals and the
Langford AGC mod.  Removing the Mish mod was no big
deal but moving from an undocumented Lankford mod to
the new circuit was tedious.  There is a typo on page
32, item 6 (should always read .1uf) but the article
is very easy to follow and mine worked the first time.

I did have a few questions about the alignment
procedure but Clark and Ray wrote the procedure out
correctly and my issues were a short between the
headsets...

In aligning my unit, I used my TEK 2465A scope to
monitor the IF output waveform and the audio out
waveform at the speaker while using a RMS Voltmeter to
measure the line output on the top side of the line
out pot.  I looked for symmetry in the modulated
signal and tuned T503 and Z503 as suggested in the
article. It does make a difference. 

I made a serious mistake of trying to tweak T501 and
T502 on my IF board.  My slugs on these two
transformers are tight and when I applied lug nut
torque to T501, I promptly twisted the coil form right
around ripping the transformer wires right off the
vertical wires that hold the caps and resistors in
place on the coil forms.  If you have never resoldered
those thin wires-it is no fun.

Over all result is a much better AGC system, a noise
limiter that functions better, and a nice SSB product
detector that sounds really good.

I have also discovered some issues in my audio amp (a
Kleronomos mod audio amp).  I can see nice flat
waveform on the IF from about 40hz through 6+Khz on
the IF with my signal generator and using the 8 or 16
khz IF filters, but the audio is only flat from about
80hz through basically 3Khz and then it rolls off
through 6+khz passing audio out to about 10 Khz.  The
issue seems to be in the audio amp but I don't know
where yet.

All of this to say if your thinking about doing these
mods, they do work.

Dutch Masters NU5O

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