[ARC5] SCR-274N side tone level?

[Brian Clarke]

Hello Dave,

 

I can accept that the reflected impedance of a load can affect the impedances of other windings on the same core. Whether the potentiometer is in series or parallel is immaterial – the reflected impedance will change, unless you change the design by putting a low resistance damping load on the sidetone winding and then a high resistance pot across or in series with that.

 

However, the designers could never have guessed how many sets of headphones would be connected across the sidetone winding. Further, the pilot or co-pilot choosing TEL A, TEL B or none on the receiver remote controls or a radio operator choosing similarly on the receiver racks, would have quite  unpredictable effects.

 

So, the better design approach would be to feed the sidetone output from the modulator only to an intercom amplifier and then have a ground tech set up the output levels to be delivered from a low impedance transformer output winding in the intercom amplifier for each aircraft’s complement of crew and mission(s). In fact, I’m presently rebuilding an intercom amplifier that has four screwdriver-adjustable output level pots. The problem we Johnny-come-latelies have is that we are using these sets in armchair-comfortable conditions that don’t resemble the ambient noise of a normal mission. So, the sidetone we get is bound to be loud.

 

As a side effect, many WWII and subsequent radio users suffered nasty hearing damage, particularly on AM sets where there was no squelch; they lost significant upper audio frequency acuity. Squelch on FM only came into mil service much later. I may be on shaky ground here, but I vaguely recall seeing an RAAF modification for squelch using a 12H6 mounted under the chassis of an SCR-522 receiver (BC-624?). A Lamb design, perhaps?

 

73 de Brian, VK2GCE.

 

OnSunday, 10 February 2019 4:05 PM, you said:

 

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.

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