Hi @michaeljin5600
power hum in the audio is usually caused by leaky electrolytic capacitors on the power rails.
Theses are used to isolate the power from the audio.
Check if the electrolytic capacitors in the recorder are OK i.e. not domed, bulging or leaking.
Here's the [document|41538|service manual] that will help.
It has the schematics and also more importantly the parts list for al the parts, which is handy if a component is damaged and you don't know whhat it's type or value is.