I have now succeeded in modifying my QCX-30 with a switch to Duoband 30 / 40m. The occasion was the temporarily difficult conditions at 30m. The biggest problem was the small space in my home-made housing.
I acted like this:
The switch is an Augat ASE62R (DIP, 6 changer)
The built-in low-pass filter is connected in parallel with 4 capacitors. The blocking effect for the 2nd harmonic (14 MHz, approx. 44dB) differs only slightly from the 30m filter according to RFSIM.
In addition, the resonant circuit in the built-in antenna tuner (simplified Z-match) selects Rx and Tx
The RX circuit is connected in parallel with a capacitor + trimmer for exact adjustment to 40m.
The keying was not changed.
Result: good sensitivity on both bands, unchanged output power, now on both bands.
The VFO frequency is recalled from memory as the band is changed.
A first test brought a 40m-QSO.
vy 73 Reiner DL8LRZ