[ARC5] Lopsided modulation

Mon Feb 26 00:48:37 EST 2018

Is this problem only happening with ARC-5 type transmitters? How do they sound on ARC-5 type receivers?

The ARC-5 type transmitters are not like most other HF tube transmitters. The oscillator and finals are gang tuned. There is no dipping the plate to resonance like most other transmitters.

The SDR may be seeing something not seen by normal AM receivers. 

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
  ----- Original Message ----- 
  From: AKLDGUY . 
  To: WA5CAB at cs.com ; ARC-5 List 
  Sent: Sunday, February 25, 2018 5:40 PM
  Subject: Re: [ARC5] Lopsided modulation

  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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