[GreenKeys] Terminal unit (HAL ST-5000) adjustment

[Ethan Blanton]

Hi all,

I had wanted to participate in the RTTY Roundup as a heavy metal
station this year, but didn't get my ducks all in a row in time.
However, I did tune around the bands a bit while it was going on, and
I found that in a contest environment my station was not performing
very well.  I hooked a 'scope up to my TU and decided that maybe it
was the culprit; I'm not sure.

This note is a plea for the voice of experience.  Hopefully someone
can tell me what I'm doing right and wrong!

The equipment:

Drake 2-B
HAL ST-5000
Model 28 KSR

The scenario:

I have no trouble copying RTTY signals with reasonably high SNR on a
quiet band.  (See, e.g., https://www.youtube.com/watch?v=g3VbeuTIgRk
and pleas forgive my fumbling tuning in front of the camera!)  I can
finagle in lower SNR signals on a noisier band, but it gets difficult.
I recognize that my receiver selectivity (and AGC action) will limit
reception performance, but on the bands today I felt like I should
have been able to copy some signals that I could not.

I normally use the 2-B 500 Hz pass band filter centered around
something near 2260 (I use low tones) on a busy band.  Receive signals
are passed to the TU in parallel with the 2-BQ speaker through a
filament transformer (I have an audio transformer for the job, but
it's on my TODO).  This is the setup I was using during the RTTY
Roundup.

Furthermore, a 'scope on the ST-5000 showed greatly unbalanced mark and
space amplitudes, rather wide ellipses on the cross, and an angle
between the ellipse long axes of probably 15 degrees off
perpendicular.  The needle bounced significantly on a known good
(i.e., generated locally on a computer with a high quality sound card)
signal.

What I've done:

>From the above, I decided the ST-5000 had probably departed from "in
tune" some time in the past forty years, and that I would go ahead and
void its warranty.  I started by putting the 'scope on its mark and
space terminals and sweeping audio tones through its input.  I found
that the mark tone amplitude peak was within about 10 Hz of nominally
correct, and the space tone amplitude peak was between 10 and 20 Hz of
nominally correct.  The difference between these peaks, however, was
about a factor of two.

On opening the cover of the ST-5000, I found that there was a dab of
(still soft!) caulk on all of the relevant trimmers (which the manual
suggests not touching).  This leads me to believe that they've never
been adjusted.  My general experience is that things that have never
been adjusted are much less likely to be out of alignment than things
that have, but figuring I could aways put them back how I found them,
I peeled off the caulk and started twiddling.

Long story short, I adjusted the needle for "maximum deflection,
minimum bounce" and the oscilloscope pattern for "closest to
perpendicular, narrowest ellipses, closest to equal amplitude".  The
resulting pattern (and needle deflection) is viewable here:

    https://youtu.be/ZLtL_761vFU

The needle rests right about 0.6 with the provided input signal, and
the mark/space ellipses for narrow offset are roughly 6 V on the long
axis and 2 V on the short axis, nearly perpendicular and nearly
plumb/level.  The controls adjusted were MR (mark receive), WR (space
receive 850 Hz offset), NR (space receive 170 Hz offset), and SB
(space balance).  I have not yet addressed transmit alignment.

The questions:

The ST-5000 manual doesn't give any advice at all for adjusting these
controls, except to say that they interact.  I did experience
interactions, that's true.  :-)  However, I don't know if my alignment
targets are appropriate or not.  In particular, this alignment puts
the mark receive and space balance controls much closer to their
limits of travel than the factory alignment did, which concerns me.
Neither is *to* the limit, but the mark receive control is within 10%.
I did not check the values of either the trimmers or the fixed
resistors to if they are out of tolerance, but this is not a high
current circuit if I understand the schematic, so I cannot imagine
they're substiantially out.

Is this reasonable?  Is there a better method?  Does anyone have an
alignment manual for the ST-5000?

I also have an ST-6 I intend to revive, but its manual has alignment
information.  :-)

I'll be putting this on the air and trying it out to see how I did
*empirically* speaking, but as I don't anticipate bands as crowded
with RTTY signals as the RU for some time, it may take a while to
learn how effective my "gut feeling" alignment was.

Thanks and 73,
Ethan