[ARC5] Lopsided modulation

[AKLDGUY .]

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> 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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