An AGC Mod for the Delta II and Argonaut II

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

very long AGC decay times. I ended up using 'fast' on SSB and 'slow' was useless

the bar graph would drop to about 3 blocks and then take 10 seconds to drop the last 3 on an S9+ signal

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

Change R17 from 1.5K to 3.3K

Change R16 from 10K to 33K

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,

connect a 1 to 1.5 Mohm resistor from the emitter of Q4 to ground. This provides a DC return for Q4 and turns the Eb junction 'on' a bit.

All resistors are 1/8 watt. The above values bring the Delta II AGC values more in line with those used in the Scout and Paragon 585.

Re adjustment of R15 and R93 are required after this change.

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

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