[ARC5] ARA heater configuration

[AKLDGUY .]

Ken, thanks for making that clearer.

73 de Neil ZL1ANM


On 7/5/16, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
> On 4 Jul 2016 at 11:09, Jack Antonio wrote:
>
>> There is a "gotcha" here.
>>
>> ...the AN/ARC-5 receivers use a 12SF7 as the second IF amp, which
>> has a different basing diagram from the 12SK7 used in the ARA and
>> SCR-274N.
>
> The 12SF7 has an additional diode internally to provide AGC voltage for the
>
> up-dated AGC circuit in the AN/ARC-5 receivers. Furthermore, the base
> diagram of the 12SF7 is enough different from the 12SK7 that they cannot be
>
> interchanged by simply plugging one into the socket wired for the other.
>
> The reason the AGC voltage is taken from the 2nd IF tube rather than from
> the detector which was most commonly done at that time-period is so that the
>
> AGC can be on continuously and it will not be effected by the BFO.
>
> Therefore, the AGC works whether or not the BFO is on. In fact, the AGC
> cannot be turned off: it is on continuously whenever the receiver is powered
>
> up.
>
> In almost all other receivers of the period and for some time afterwards,
> the
> AGC had to be turned off in order to use the BFO, and vice versa. Otherwise,
>
> the BFO signal-voltage would raise the AGC voltage so high that it would
> reduce the gain of the receiver.
>
> This is yet another example of the genius of the designers of these
> receivers.
>
> Ken W7EKB