[ARC5] A discussion on AGC.

[Kenneth G. Gordon]

I would like to begin a discussion on the details of the AGC system(s) implemented in the 
AN/ARC-5 receivers. I find it very interesting.

First, however, I want to make clear to all the "denizens" of this forum that when I put the 
term ARC-5 in quotes, like this "ARC-5", I intend to indicate the common generic term for 
these receivers which is used, incorrectly, to mean ALL of the series, despite their many 
differences.

When I wish to talk about a specific series, I will use terms like "SCR-274N" or "BC-454" or 
"R-23A/ARC-5" or "AN/ARC-5", or "ARA", etc.

So, now that that is out of the way (I hope) here goes.

In my past review of the various AN/ARC-5 receiver schematics, I see that the diode in the 
12SF7 is used in one type (NAV or COMM) while the diode in the 12SR7 is used in the other 
type.

Since in that type in which the 12SR7's diode is used, the BFO voltage would severely effect 
the AGC voltage, I would assume that that particular type of receiver was not used often for 
CW. 

Whereas since the diode in the 12SF7 is well ahead of the injection point for the BFO, that 
AGC would NOT be (as) effected by the BFO voltage, and thus the AGC could be left on 
constantly without the need to turn the BFO off when the AGC is in operation.

Furthermore, I think the AGC voltage being taken from the 12SF7's diode would not be as 
great as if that voltage were taken off the circuit further along in the chain, the AGC voltage 
would be greater and more effective from the diode in the 12SR7, which would be needed for 
the NAV system.

Also, as I understand it, there was, essentially, no AGC implemented at all in the SCR-274N  
series receivers, and although I know very little about the ARA series, I would assume the 
same thing is true with those. However, oddly, those receivers had all but the source for AGC 
voltage built into the receivers, but it was not used in those.

Mike Murphy WU2D in the most recent (April) issue of ER Magazine detailed a completely 
reversible way to add AGC to the SCR-274N receivers. However, in his mod, the AGC must 
be disabled when the BFO is turned on.

Anyway, there. I have thrown a bunch of info into the pot.

Let's see if anyone else here is interested enough to input some data or ideas.

Ken W7EKB

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus