[MrFixr55]

Hi RadioSvit,

Other than the tone control, your C8-20 and 9K have similar Audio, Detector and IF Stages. The big difference is in the separate Oscillator and Mixer instead of (what we call in the US) the pentagrid convertor. However, on the broadcast band (540 - 1700 KHZ), performance should be the same.

I am big on dividing the radio between the Audio Amplifier and the rest of the radio and verifying the performance of the audio circuit. If you don't object to modifying the circuit, put a Radio - Phone switch in the C8-20 in a manner similar to the 9K. Pipe the same audio program from a computer, CD, etc. into both radios. If the Audio Amplifiers both sound satisfactory, troubleshoot the rest of the radio. You may want to leave the switch in. RCA sold a "Radio - Phono" Switch accessory for any radio back in the 1940s. Although it is "cheating" I like to do this because the quality of the sound is better than going through the radio circuit.

Is sensitivity an issue? I didn't see any response from the tuning eye when you were tuning the radio. Measure AVC voltages in both radios and compare. (Junction of R31, R32 and the Tubing Eye input on the C8-20, similar location on the 9K but my schematic is not clear enough to read the numbers). I think the voltages should be similar. If AVC does not go very negative on strong stations, troubleshoot the Antenna, RF and Convertor coils. another trick is to connect the antenna directly to the 6A8 Grid cap. If the sound is much better, then investigate everything before that point back to the antenna connection.

Hope that helps!