Radio Shack TRS-80 PC-3 Owner's Manual page 20

If you get no response from any key, even when the above operation is performed, push the RESET button without any key.
With this operation, the program, data and all reserved contents are cleared, so do not press the RESET button without any key
unless the above trouble occurs.
RESET button
Figure 7.
Contrast Contro I
Figure 8.
18
Turn the control in the direction of the arrow for a brighter
display, and turn it in the opposite direction for a dimmer
display.
Adjust it so that the display is easy to see.