[ARC5] Lopsided modulation

Sun Feb 25 20:40:46 EST 2018

The sidebands are asymmetrical levelwise. I think the PA padding capacitor
being offtune wouldn't be the cause - there's no way the circuitry here
could be so sharp.

My friend about 5 miles from me and I are both running ARC-5s on the local
AM net on 75 meters. Some of us on the net don't use receivers. We access
the SDR receiver which is permanently set on the net's frequency and is
located at Musick Point Memorial Radio Station on the other side of the
harbour from me.

This Musick Point station has acres of ground and wonderful antennas. It
has a Wikipedia site. Check it out.

Its SDR receiver has an online waterfall display and S-meter. My friend has
long been puzzled why his SCR-274 (modified for plate & screen modulation)
shows the lower sideband much higher level than the upper. No other
stations show like that. Interest in solving the problem was renewed when
my ARC-5 transmissions showed the same effect when I began operating on the
net. I wondered why this is peculiar to the Command transmitters.

The most promising suggestion so far is that of Bill Fuqua - simultaneous
AM and FM results in the enhancement of one sideband and the partial
cancellation of the other. I'm unwilling to add a buffer stage between the
VFO and PA as the transmitter is absolutely stock unmodified. The best I
can hope to do is regulate the VFO B+ in case FM is being introduced via
unsuppressed audio on the main dynamotor (550V) line. As I mentioned
earlier, the MD7 modulator schematic shows only a small decoupling
capacitor (C55 = 1.2 uF) at the dynamotor output, and there is no further
decoupling. C57 (0.05 uF doesn't count, it's only a spike suppressor). I
followed this decoupling arrangement in my homebrew modulator, and more
microfarads may be necessary.

Neil ZL1ANM

On 26/02/2018 12:14 PM, "Robert Downs" <wa5cab at cs.com> wrote:

Neil,

When you say that the sidebands are assymetrical, do you mean that if you
had the rig on some F sub o and modulated it with a single e KC audio
signal, that the amplitude of the signal at F sub o plus 3 KC would be
lower than that at F sub o minus 3 KC?  If that’s the case, it could also
be that the PA tuning is not tracking the MO frequency.  However, I don’t
have a feel for how sharp the PA tank tuning is.  It could just be too
broad to be the culprit.  But it’s an easy check.  Just remove the bottom
cover and the padder lock screw.  Fire it up.  And rock the padder tuning
back and forth.

Robert Downs - Houston

WA5CAB

*From:* arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] *On
Behalf Of *AKLDGUY .
*Sent:* Saturday, February 24, 2018 16:08 PM
*To:* ARC-5 List

*Subject:* Re: [ARC5] Lopsided modulation

FM does not seem to be present in either his signal or mine. The carriers
are straight lines on the waterfall display. Actually, there is one
operator on the net whose carrier is a fuzzy line when he speaks, but his
sidebands are symmetrical. I'm stumped.

Neil ZL1ANM

On 25/02/2018 10:30 AM, "Richard Knoppow" <1oldlens1 at ix.netcom.com> wrote:

    That affects the upward vs downward modulation but not the strength of
the sidebands. A frequent cause of asymetrical sidebands is incidental FM.
If the oscillator is being affected by the modulation it can be the cause.
    The scope display will not show assymetry of the upper vs lower
sidebands although it will certainly show assymetry of upward vs downward
modulation.

On 2/24/2018 1:18 PM, Jim Wiley wrote:

One reason could be that your voice in itself is non-symmetrical.  If you
can do so easily, try reversing the "polarity" if the modulation
transformer, and see if that helps.  If it does, your can leave it set the
way that works best for you. Monitoring the modulation envelope with an
oscilloscope is always  the best way.

- Jim, KL7CC

On 2/24/2018 11:38 AM, AKLDGUY . wrote:

A friend and I are running Command transmitters on the local 75 meter AM
net, with full plate and screen modulation. We monitor our transmissions on
the local SDR receiver site which has a waterfall display. In both cases,
the lower sideband shows as considerably stronger than the upper, and this
has us perplexed. Our stations are the only ones affected.

I have never seen anything in the literature to explain this. Downward
modulation is a problem with AM, but articles do not indicate whether
lopsided modulation results.

My transmitter is the AN/ARC-5, absolutely unmodified and running 40W input
due to dynamotor constraint. His is the SCR-274 modified for plate & screen
modulation and running twice that.

Anyone suggest a reason for the unequal sidebands?

Neil ZL1ANM

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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