[ARC5] A discussion on AGC.

[WA5CAB at cs.com]

The hardest to understand feature is that 1st, the BFO injection is via 
stray capacitance shown as always being less than 2 mmf..  But the two 
schematics show the capacitance C33 as coupling the BFO plate to the 12SR7 2nd diode 
plate in the COMM receivers and the BFO plate to the 2nd IF grid in the NAV 
receivers.  In the COMM receivers, Note 5 says that C45 from the 2nd diode 
plate Pin 5 is not present in R-25 and the 2nd diode pin is left floating.  
It is present to ground in the R-26.  It is not present in the R-27 and Pin 
5 is left floating.

In a message dated 05/10/2017 10:58:44 AM Central Daylight Time, 
kgordon2006 at frontier.com writes: 
> On 10 May 2017 at 2:39, WA5CAB at cs.com wrote:
> 
> >Both variants of the AN/ARC-5's take the AGC voltage off of the plate of 
> the 12SF7,
> 
> I am having a bit of difficulty with this, which is why I have not replied 
> before now. The AGC 
> diode receives its signal from the last IF in both cases, this is true, 
> but the BFO is injected in 
> the same tube in which the AGC diode resides when the 12SR7 is used for 
> AGC, but there 
> are several tuned circuits between the BFO and the AGC when the 12SF7's 
> diode is used for 
> AGC.
> 
> I still maintain that the reason this was done is that the COMM receivers 
> were more likely to 
> be used for CW (as seldom as that was used) than the NAV receivers.
> 
> Perhaps we need to take a working example of each receiver and measure the 
> effect of the 
> BFO on the AGC. Does the AGC voltage rise more in the one case than in the 
> other when 
> the BFO is turned on?
> 
> In both cases, the AGC is never turned OFF.
> 
> > The difference is that the NAV receivers use the rectifier diode in the 
> 12SR7 and the 
> >COMM receivers use the one in the 12SF7.  Why the difference, no one 
> alive today 
> >knows.    
> 
> See above. ;-)
> 
> Ken W7EKB
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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