Scout Model 555 RF Gain Control Mod

From the Ten-Tec Reflector 11/16/2007

I just completed development and test of a really simple mod. I had developed the mod that uses the mic. gain control but have since removed that mod. This new mod is really simple and I did no drilling.

  1. Remove the ground stud from the rear panel. In its place mount a sub-miniature toggle switch. (These are the really tiny switches, not to be confused with the merely "miniature" ones. The hole in the rear panel will have to be enlarged very slightly, but the two solder lugs on the grounding bus inside the radio will fit on the switch bushing just fine.
  2. Run a wire from the "R" (regulated receive voltage) pin of connector 9 on the IF/Audio board to one of the switch lugs.
  3. From the other switch lug, connect a wire with a resistor in series with it to the emitter of Q5 on the IF/AF board. Its a little tricky finding this point, but if you study the board and the schematic you will locate it. (Hint: Find U2-5 and trace the circuit run to R15 and then to Q5-E).
  4. Resistor value selection is critical: 3.3k-ohm gives lots of attenuation. 3.9k drops the IF gain only a little. I settled on 3.6k as the best compromise. Caution: Use a good quality resistor. I tried some 3.6k resistors that permanently shifted value up to 4k after soldering! I finally put in a 15k in parallel with a 4.7k resistor.

I put shrink sleeving over the resistor leads and just tied the wiring down to the exisitng harness with a couple of small cable ties.

Now you have a handy attenuator switch. The resistor forms a voltage divider with R80 to put about 4.65 to 4.75 volts of bias on the IF amps. The AGC will still work, though the mod seems to increase AGC time constant a bit.



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