Improving Yaesu AGC
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I've been impressed with the AGC performance in the Collins 51S-3 and also
the German KW.E.A. recievers. The TS-500 has no AGC amplifier at all, and the
performance is very good. The Drake 2-B has a 6AT6/6AV6(6BF6) type triode as
the AGC amplifier and this too is excellent. The Drake TR-4 uses half an
ECC83/12AX7 as a DC amplifier and in the 2-C and R-4 series a quasi-linear
transistor is used.

I believe that this indicates that the AGC amplifier should operate as a
linear DC amplifier in order to achieve good sound quality.

In order to improve the Yaesu series transceivers, replace the AGC transistor
with an operational amplifier such as the CA3140E. Set the circuit up with a
gain of 20 to 30 and some form of voltage delay such that it is 20dB from the
noise level before the AGC starts, and matters should be improved.

Note that the use of silicon diodes will add to the voltage delay.

73 Jan-Martin, LA8AK

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