i have a sony walkman f10 i replaced the belt and put my walkman back together and the fm isnt working anymore. the red light still lights up when its on . 

Normally selecting between tape and radio functions should work fine...unless you haven't aligned the slider properly while fixing things back together....so it slides without actually contacting the switch

In a more bothersome scenario the flexible pcb connectors from the radio PCB in the lid to the main board which cross the hinge portion might have gotten damaged...

hope this helps

Yes very likely you have a tear in the flex cable as mentioned. There's a tutorial on this forum that shows you a method of repair.

Where?

stickied at the top:

wm & wm F 10/10II/20/30 w800 troubleshooting

this is the most helpfull bit... ( the wm10/10II/20/30, their radio variations F10/F10II/F20/F30 and the back to back wm w800 (double deck wm10) all use the same belt (3-315-405-00) approx 49 mm dia, 0.8/0.9mm square cut. )

Not so certain about repairing the membrane flat wire... that's really hard or delicate.