The RF GAIN function is different in FM and in SSB/CW modes. In FM mode the RF GAIN controls a PIN-diode at the input of the preamp. In SSB/CW the RF GAIN simply activates the IF AGC.
The use of the preamp input attenuator (the PIN-diode) can be useful also for SSB/CW e.g. if you have connected a mast head preamp with excessive gain. In this case the attenuator will help you to adjust the signal levels at the mixer, and thus reduce intermodulation and blocking.
The solution is to remove the SSB/CW RF GAIN for SSB/CW (this function is of little use anyway), and enable the FM RF GAIN even in SSB/CW.
- Cut the lead (FM8V) conneted to (pin5, J6, potmeter PCB, green) about 4 cm from the plug
- Connect the short plug end with R14 (on the potmeter PCB) by soldering it to the end that is connected to the plug (+8V)
This will enable the FM RF GAIN in SSB/CW.
- cut R30 open
This disables the SSB/CW RF GAIN, and you now have a variable input attenuation with a dynamic range of 10dB (IC275 with MI308/1nF) to 16 dB (IC475).
Note: The IC-275 might have a shared PIN diode for FM RF GAIN and TX/RX switching. If you choose to split up the RX/TX paths, you must install a new PIN diode (MI301 or equivalent) on the RF YGR PCB to implement this modification. See the note under the section dealing with spliting the RX/TX paths.