That R97 (3.3meg) is removed and S8, the AGC/Manual switch, has been rewired. It will fall toward zero as the output of the AGC detector drops. Reduced, thereby pulling the AGC line strongly negative, the new S-meter will The 1nxx diode on the left ensures that when the RF gain control is Which will be more effective on SSB reception.
If both are given, it means an existing part has been changed in value.ġ0 megohm resistors were added to provide for a slow discharge time constant Wherever part values are given, it is for a newly added part. Some further reconfiguration of the AGC circuit is required to smooth a fewĬomponent parts list numbers are used, the original part is unchanged. 003, I have left the original in place to make Remove some of the variations due to audio modulation on the incoming carrier. Second problem with the AGC is that the existing filtering is insufficient to So I settled on changing R66 to 4.7k, which reduces the amount of Of 4.7k caused some distortion to AM broadcast stations. My receiver, dropping the value of R66 in the AGC delay divider below a value Receiver to remain at maximum sensitivity until quite strong signals areĭeveloped AGC voltage finally overcomes the bias and is fed back to reduce the First, the bias applied to pin 5 of V14, the AGC detector, causes the There are three problem areas I wanted to address with the AGC changes. Is best to start with the AGC modifications, because both the S-meter and
Have to know enough about your particular model to make your own decisionsĪbout the best way to incorporate each particular change. Since this will not be a “cookbook” set of instructions, you will If you are working with a later model receiver. Modifications are for an SP-600 JX1, so there may be minor circuit differences Wire for ease of identification by any future restorer. New wiring was done with a single color of easily recognizable modern SP-600 and left all original wiring in place. Making the following modifications, I removed as little as possible from the
RF signal level meter is very non-linear and is not calibrated withĪGC employs a large amount of delay, which is partially responsible forĭetection with the existing circuitry is very poor, and the existing AGC Point where they could be easily reversible if some future custodian of theĪreas where I feel the SP-600 needs help are the following: My goal was to minimize the impact of the changes to the SP-600 to the Following modifications were produced in order to improve what the authorįeels are several shortcomings in the SP-600 receiver.
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