firstly, (if applicable) try moving or reinserting any ribbon cables. i
had an early sony which had erratic picture and this cured it.

The mode switch may need cleaning or replacing - I have not needed to
do this so cant be more specific.

And as Maarten says, do check the A/ C tape lever - very common problem
on sonys. make a note of how far down the retaining nut is screwed, as
if the lever is put back at the wrong height it will crinkle the tape
down the edge. It sounds like this may be a cause of your woes, since
if the counter wont move then the pulses aren't being picked up at the
AC head.

good luck
Ben