[ARC5] Lopsided modulation

[Fuqua, Bill L]

  There is nothing wrong with lopsided sidebands. The audio will sound just fine.

Look at CHU, they transmit a carrier with one sideband.

73

Bill wa4lav


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From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of AKLDGUY . <neilb0627 at gmail.com>
Sent: Sunday, February 25, 2018 2:46 PM
To: Brian Clarke; ARC-5 List
Subject: Re: [ARC5] Lopsided modulation

Modulator is homebrew:
* Dynamic mic salvaged from Kenwood TR-9130, with input circuitry closely adhering to the Kenwood schematic -->
* 3 stages of MPF-102 FET amplification to 50k pot for level setting -->
* LM-380 power amp driving the 8 ohm winding of a small audio line transformer connected backwards (Jaycar MM-1900). The 20k and common taps feed the modulator grids, with 5k tap used as center tap -->
* pair of 2E26s in AB1, screens regulated at 150V, bias via center tap from 3x9 volt batteries decoupled with 47 uF 35V electrolytic -->
* Thordarson 75 watt modulation transformer T-11M75 set for 8k:8k ratio, appears to be a NOS item.

I have had a report that my audio is good, but want to fix the unequal sideband level. Checking with CRO is not an option, a multimeter is all I have.

Neil ZL1ANM


On 25/02/2018 7:58 PM, "Brian Clarke" <brianclarke01 at optusnet.com.au<mailto:brianclarke01 at optusnet.com.au>> wrote:
Hello Neil,
With what are you modulating? Shrader says that voice modulation is always lopsided. If you're using a close-mic technique with the original carbon mic, that will increase the lopsidedness.
As others have commented, the stiffness of your power supply may be a cause. As you are both using plate+screen modulation. use a CRO to look at the DC rail after the modulation transformer where it feeds to transmitters. Also check the bias on the 1625 finals and compare with the PA output - like, is the carrier symmetrical?
73 de Brian, VK2GCE


On Sun, 25 Feb 2018 09:38:20 +1300, you asked:



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?


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