Soft 404
I reviewed those types of adapters back in 2019 - they do work, although some of my readers do note some strange behaviour on Linux.
https://goughlui.com/2019/02/03/tested-generic-sintechi-fc1307a-based-sd-to-ide-adapter-sd35vc0/
- Gough
Since, in your words, it's in need of some TLC I recommend you start with this...
https://en.wikipedia.org/wiki/CrazySexyCool
Playing a wav file with a 100 Hz tone :
Looks reasonable enough.
Great update project. How big of an SD card did you use? I noticed the clip lead power connection, is this because of the battery's health or the lack of a proper charging supply? You might be able to do a quick and dirty USB charging, by stealing power from the USB connector and crossing over to barrel jack. Good luck with your future upgrades.
That's great to see the 3rd harmonic way own into the noise floor!