[TenTec] Delta II AGC Improvements

Wayne Montague montaw@inforamp.net
Sat, 12 Apr 1997 13:25:08 -0400

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 -

- 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
- the AGC never, ever 'sounded' right. Lots of popcorn noise.

To fix the 'popcorn' noise, make the following changes to the IF
Board 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 too
low 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 M
ohm 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

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.


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