Avoid the cutting and splicing and get a polarity reverser:

https://www.amazon.com/gp/product/B07PMHMPMP/ref=ppx\_yo\_dt\_b\_asin\_title\_o00\_s00?ie=UTF8&psc=1

A different universal AC adapter with a polarity reversing adapter. A while back I tried using AA batteries with AA to C adapters and I was getting the same results.

I would try something like this: https://www.amazon.com/Belker-Voltage-Universal-Household-Electronics/dp/B00ZWU5L0C

You might also need this cable. https://www.amazon.com/Belker-Reverse-Polarity-Converter-Keyboard/dp/B07PMHMPMP