[ARC5] SCR-274N side tone level?

[Richard Knoppow]

    It appears that the side tone is generated by an audio 
oscillator which feeds the headphones through a winding on its 
transformer. Since it is designed to feed an 8000 ohm load such a 
load presented by a variable resistor should not affect it nor 
should it affect the waveform delivered to the modulator grid. It 
may be that you partially shorted the transformer.  A resistive 
attenuator that looks like about an 8K load on the transformer 
side can be made for whatever attenuation is desired for proper 
level into the phones. Probably the source impedance at the 
phones is of no consequence but it can be made 8K if desired 
(another resistor). A pot can be used but will have non-constant 
loading on the transformer although if the transformer works 
properly with an open circuit a pot of somewhat larger than 8K 
can be  used with the phones on the wiper, with perhaps, a series 
resistor between the phones and wiper.
    This should keep the audio oscillator happy and also present 
a reasonable load to the telephone side tone which comes from a 
different transformer with a similar circuit.

On 2/9/2019 9:05 PM, David Stinson wrote:
> Caution:
> If you try putting a pot across the sidetone winding, rather than 
> in series with it,
> 
> the impedance change this makes is reflected into the other 
> windings of the
> 
> modulation transformer and will change your modulation envelope.  
> It changed
> 
> mine drastically when I tried this.   In fact, any non-spec load 
> on the sidetone
> 
> winding causes tx modulation envelope effects. It’s been a lot of 
> years now
> 
> and I don’t remember the exact details, but ended-up abandoning 
> the sidetone
> 
> line altogether in my sets.

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