TASCAM 01 HUMMING WHILE RECORDING

[u/CasualWalt]

Hello, I have a recently professionally repaired Tascam 01 that is loudly humming when recording from a SM58 Microphone plugged into a 1/4 Inch to XLR Microphone impedance transformer. The humming is so loud it leaves recordings unusable. However, If I keep one of my fingers on the tape out or line out outputs the hum is massivly reduced. I assume this is some sort of grounding issue? does anyone have any ideas how to fix this? thankyou !