[ARC5] SCR-274N side tone level?
Robert Downs
wa5cab at cs.com
Sun Feb 10 04:25:26 EST 2019
Richard,
You are talking about the CW sidetone and Dave was talking about the voice sidetone. Two different circuits in the BC-456.
Robert Downs
-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Richard Knoppow
Sent: Sunday, February 10, 2019 01:06
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] SCR-274N side tone level?
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
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