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R4 SERIES AGC MODIFICATION

(R4, R4A, R4B, R4C)

By: Mark Gilger, WB0IQK

The stock R4 series receivers have excellent AGC characteristics. Even though the stock AGC performs very well, I thought there was room for improvement. This modification will enable the user to slow the AGC response, when the AGC switch is in the “Slow” position.

The following modification puts the new capacitor in parallel with the original. This modification will need for the user to experiment with several value capacitors to meet their listing needs.  The values to try would be starting at .22 uf, .33 uf, or .47 uf and any combination in between. The higher the value, the slower the AGC time response. In my case, I prefer to use two .22uf caps in parallel (.44 uf ). This makes for very pleasing audio characteristics.

 

R4 & R4A Modifications:

  • Locate the AGC function switch, located on the bottom of the cabinet.

  • Locate and isolate Capacitor C109, which is a .47uf.

  • Cut the leads of the new capacitor, either a .22, .33 or .47 uf to length.

  • Carefully solder each end of the new capacitor to each end of C109.

R4B Modifications:

  • Locate the AGC function switch, located on the bottom of the cabinet.

  • Locate the Receiver Sensitivity pot, located on a small circuit board just to the right of the AGC switch looking from the front.

  • Locate and isolate Capacitor C109, which is a .22uf mounted on the front of the board.

  • Cut the leads of the new capacitor, either a .22, .33 or .47 uf to length.

  • Carefully solder each end of the new capacitor to each end of C109 on the back side of the circuit board.

 

                        

R4C Modifications:

  • Locate and isolate Capacitor C74.

  • Cut the leads of the new capacitor, either a .22, .33 or .47 uf to length.

  • Carefully solder each end of the new capacitor to each end of C74.

 

Advantage & Disadvantages:

Advantage: A side benfit to slowing down the AGC response has been that the receivers background noice has been reduced. I suspect this is because the receiver cannot respond fast enough to the quick noise pulses, so it blocks them. It’s like having a built in noise blanker.

Disadvantage: When switching from transmit to receive, there is a short lag period before you can hear a weak received signal. The slower the AGC response, the longer this lag. In my case, on my R4B it’s about 1 – 2 seconds.