Problem:
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.
Solution:
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.
Implementation:
- 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.
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