[ARC5] Lopsided modulation

[Richard Knoppow]

     Is this a plate modulated rig, transformer makes it sound 
like one. I still think you could hear it if the oscillator is 
FMing. I think  you will be on the wrong track going after the 
audio. You might also see if the B+ for the oscillator is varying 
with the modulation. A scope is the test tool for this but a VOM 
will show variation on a steady tone to the microphone. The 
amount of FM will depend on the particular oscillator circuit. 
Even though the final amp is running AB1 the plate current is 
still varying with the modulation so it could affect the plate 
supply. Another experiment would be to run the oscillator from an 
independent supply.
     Also, you will not see the relative strength of the 
sidebands on a scope anyway. A spectrum analyzer is best but you 
can use a selective receiver, say one with a crystal filter in 
it, by modulating with a steady tone. You can separate the 
carrier and the two sidebands with that setup.
     BTW, someone posted that the carrier never went away in 
either AM or FM, this is true of AM but not FM where the carrier 
definitely nulls at certain combinations of modulating 
frequencies and modulation indices. In fact, carrier nulling is a 
method of calibrating the modulation index.

On 2/25/2018 11:46 AM, AKLDGUY . wrote:
> 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

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