Several days ago I assembled (tr)uSDX bought from Up Tech Store on aliexpress. Very nice piece of kit, but unfortunately it had all sorts of ringing and oscillating issues in receive mode, either SSB or CW. In fact, on CW mode the issue was even more pronounced.
Initial observation: at volume +11 very noticeable ringing with intermittent audio oscillation while on the speaker, but less so when using headphones. With the volume set at +12 it would simply oscillate and receiver performance is ruined, even when using headphones. Triggering any of the switches on vol +11 would cause short damped oscillation in the audio range. My first thought was that there was Class II MLCC in the audio path both creating and picking up vibrations from the PCB. Btw. MLCCs that are not Class I (NP0, C0G) are known also as "singing capacitors".
Cause of the issue: Investigation of the schematic diagram led me to the QSD filtering capacitors C25-C28. Here is a link to the video where I tried to record the piezoelectric effect on those capacitors https://www.youtube.com/watch?v=jImurYa7SOg .
Fixing the issue: Well, I didn't have a small enough 470nF at hand, only some 100nF 63V polyester capacitors in 5mm pitch (7.2x2.5x6.5 mm size), bundled them together in a 2x2 pack with some painter tape, used a 5 short pieces of some insulated thin wire and placed them on the unpopulated part of the Main PCB left of the opamp (if J1 is left/up).
Results: Clean audio on both speaker and headphones, even heard VU2HW on 7120k with my 1m dia magloop prototype with all sorts of European strong stations creating pile-up. AGC attack time is a bit too slow for my taste, but that's just me
. Also, it seems like some aliasing is introduced, probably 100nF is not enough. The receiver is usable up to volume +13 with att2 0, too much THD with volume 14-16, I'm not even sure if it oscillates anymore. But now it's a joy to listen to SSB stations with a filter at 4k, more than listening on my ft-450 
Things left to inspect: When in CW mode, there is some ringing noticeable, especially when a filter is set to 200Hz or lower. Slight tapping on C22 and C29 exhibits the same piezo effect as was with QSD capacitors, but is less pronounced. Before changing QSD capacitors I couldn't discover that. Unfortunately, checking that will probably wait for next weekend since I'm away from my shack these days.
Sidenote: Not all MLCCs have that piezo effect in the audio range. NP0 or C0G capacitors are from CLASS I and are allowed as such to be used in the audio path. There are numerous papers and application notes on that. But I don't think there are any over some 1nF even in 0805 sizes, let alone 0603 used in this transceiver. But Class II is totaly a different beast under some 30kHz, especially when in SMD format mounted on FR-4. I used Superlux HD_681EVO headphones and Yaesu FT-450 during fixing this issue.
