[ARC5] A discussion on AGC.

[K5MYJ]

When SSB first came into common use in the late 50's most receiver had the common diode detector. 

We were taught to turn off the AVC when we turned on the VFO. Then we manually controlled the signal level with the RF gain control.

For CW operation the signal level was usually not critical but for SSB it seemed very critical.

In 1960 I bought a NC-109. It has both a diode detector for CW and a product detector for SSB. It was the first easy tuning SSB receiver I owned.

Today I own a SX-111 Mk1. It has a product detector and is easy to tune. But the original SX-111 did not have the product detector.

I guess receivers that were not intended for SSB were cheaper if the only used the simple diode detector.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark" 
  ----- Original Message ----- 
  From: J Mcvey via ARC5 
  To: WA5CAB at cs.com ; Arc5 at mailman.qth.net 
  Sent: Wednesday, May 10, 2017 2:24 PM
  Subject: Re: [ARC5] A discussion on AGC.


  "The hardest to understand feature is ..., 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."


  Yeah, can someone explain how they are adjusting the coupling with the second diode? Does the diode cap  shunt some of the cathode signal to ground, thus attenuating it a bit?

  I'm used to seeing "gimmick" caps or very tiny ceramics that couple point A to point B in a direct way.





  On Wednesday, May 10, 2017 4:54 PM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net> wrote:




  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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