Partial quote from my Collins manual
"When listening to strong signals, a reduction in background noise under
no-modulation conditions may be obtained by rotating the RF GAIN control
away from the maximum position. "
In the 75S-3B, reducing the RF gain increased the bias on the AGC line.
This had the effect of over riding the AGC voltage fluctuations. The S
meter reflected this action.
The side tone is obtained from the 32S transmitter and is adjustable
internally in the transmitter or by using the receiver volume control.
Jim
>Most interesting input, for me, was a quote from a Collins
>Radio S-line manual, not sure if each of you saw it, so here
>is the quote:
>
>"To copy CW with AGC, set R.F.GAIN to maximum and
>adjust A.F.GAIN for desired level. If AGC action is not
>desired, reduce R.F.GAIN setting until S-meter does not
>kick with signals, and leave A.F.GAIN set as above. This
>method of operating results in optimum signal- to- noise
>ratio as well as more pleasing sidetone level. The practice
>of operating with A.F.GAIN at maximum and adjusting
>R.F.GAIN will degrade signal- to- noise performance of the
>receiver."
>
>Note: I do not know if the above is the exact text from the
>Collins manual; and, I changed "avc" which was in the text
>I rcv'd to AGC, just for consistency in our discussion.
>
>Now, I find the above Collins bit to be unclear!! In the
>second sentence, they do not say to turn OFF the AGC,
>just to turn down the RF gain until the S-meter does not
>"kick". Does that "defeat" the Collins AGC action? And,
>"leave AF Gain set as above", does that mean set to
>comfotable listening level, or set someplace in particular,
>such as exactly where it was when the AGC was on, and
>the RF GAIN was at full on?
>
>Whatever is meant, Collins engineers seem to say this
>arrainges things for "optimum" signal to noise, and
>most pleasing sidetone level (sound volume, I guess).
>Really does seem to be saying to leave the audio gain
>right where it was set when the AGC was on, and the
>RF gain was full up.
>
>Anybody able to offer clarification on how the S-line
>actually was used by those in the know back then??
>
>Thanks again, and 73, Jim, KH7M
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