Thank you for your modification of C24. It helped a lot.
Nevertheless I still had the same ringing Kuba documented in his post #44926. It turns out that T/R switching transients generated in the QSD mixer enter the audio chain. These fast spikes cause ringing of the CW filter.
A simple RC low pass filter, fitted after R27 reduces these transients. Ringing is reduced close to the RX noise level. Even with max. audio gain clicks or thumps are no longer audible anymore. The attached oscillograms show the results of tests on a dummy load.
This is a summary of all my modifications for click/thump reduction:
• C21, C22 changed to 0,47uF – 1uF; standard on PCB Rev 4, QCX assembly Rev. 1.11 and beyond
• C24 added 100uF parallel to existing 10uF capacitor; will be standard standard in QCX+
• R59 changed to 33k; reduces CW filter overload at high sidetone settings
• added C54 10nF; eliminates small spike
• added RC low pass (3.3k + 47nF) between wiper of R27 and input of CW filter
One additional modification, which may or may not apply to a standard QCX:
In my QCX I replaced T1 by a broadband transformer (5 turns trifilar on FT37-61) for multi-band operation. With this non-selective input the sidetone became distorted with RF output of more than 3.5 W. Residual RF at the input of the QSD caused overload in the mixer and/or in the pre-amp stages. Obviously the isolation of the T/R switch Q5 was insufficient. A 47 Ohm resistor parallel to the primary side of T1 solved the problem, without significantly reducing the RX sensitivity.
73 Wolfgang DK4RW