[ARC5] AVC in SCR-274N receivers

[Jack Antonio]

I may have this wrong, or it's not the whole story, but
I think it is something like this.

The "AVC" in the SCR-274N receivers wasn't intended
to be an AVC, that is, it is not supposed to keep audio
output constant over a wide range of input signals.

The intent was to keep the receiver from blocking on
very strong signals.  Or at least to not have the
audio output decrease as the signal gets very strong.

The reasoning for this is that in flying the
old LF radio ranges, (and this is where I am
on shaky ground), noting
increasing and decreasing audio levels
was an important part of the procedure. IIRC
it had to do with not obscuring the cone of
silence when directly overhead the range station.

Jack Antonio WA7DIA/4




On 5/7/2014 12:58 PM, Ian Wilson wrote:
> Changed the Subject since I think this is an interesting topic
> in its own right.
>
> I would have thought that 100k is just too small to generate enough
> grid bias (from grid current flow in the last IF stage) to provide a
> noticeable amount of AVC.
>
> Also, AVC could be picked off the audio detector stage.
>
> I seem to recall reading somewhere that the receivers were designed
> with AVC, but that this was not connected up in their final form.
>
> This would be consistent with the original connection of the lower
> end of the last IF grid resistor to the AVC network. The 100k/0.05uF
> filter would remove most of the audio from the AVC voltage.
>
> The complete story would be interesting. Apologies if this is readily
> accessible elsewhere and I have overlooked it.
>
> 73, ian K3IMW
>
>