Meters move but it doesn't record anything.
I assume you tried a different memory card as well as a re-format of the internal memory.
Here's a reset procedure for the D50 I once found online (but I've never used it):
You can reset the device by entering TEST/SERVICE mode. With the device off, turn the LIMITER switch to the off position. Turn on the device, hold A-B and DIVIDE while turning the LIMITER switch to the on position then release. Use FF or FR to scroll through the menu to Key Chk + Format. It'll ask for you to enter 18 key presses. Press FF,FR,MENU,LIGHT,STOP,PLAY,DIVIDE,REC,PAUSE,DISPLAY,A-B,DPC,HOLD,LCF,LIMITER,INPUT,MIC ATT, MIC L+R POSITION, and lastly turn the power off and on.