To null out a 301 lower the read head (no film) and apply pressure to the read switch.
While holding the read switch toggle the null switch. The Display should go "888" and then to "0.00" (+/-0.01)
The most common reason a 301 won't zero is a burnt out Read Lamp (301-21). To check cover the aperture
with film or paper, you should see a circle of Light when the unit is on.
If the display didn't give you the "888" when you hit the null switch it has failed or is disconnected.
If the display appears to be Locked up the light column plug may have a bad connection.
Service is recommended if these suggestions don't solve the problem.