An AGC Mod for the Delta II and Argonaut II


As much I really like my Delta II, I never, ever liked the AGC in the radio or the action of the bar graph S Meter. I don't like bar graph S meters in radios, period, but sometimes compromises must be made.

A few things about the AGC really bothered me - the AGC never, ever 'sounded' right. Lots of popcorn noise

To fix the 'popcorn' noise, make the following changes to the IFBoard that contains the 2 MC1350's -

The above original values are those used in the schematic. The AGC on the MC1350 is controlled by current and R17 is simply toolow in value. When R17 is changed, R16 must be increased in value so that the voltage divider does not prevent the MC1350 from receiving AGC with the change in R17.

To fix the time constant AGC decay problem,

DO NOT trust the schematic diagram. Confirm values by visual inspection. My Delta II schematics have a number of mistakes and ommissions and if typical, can be trusted as a guide only.

The improvement after this change is remarkable and changes the receiver 'personality' to a real pleasure to listen to. The S meter bar graph action is much much smoother. Both fast and slow AGC settings become useable on CW and SSB respectively. This change for R17 and R16 has also been done by an Argonaut II owner and he reported similar improvements.

 

Wayne

VE3EFJ

montaw@inforamp.net