I am working with a L293D Dual Motor Drive Shield from DK Electronics. It has been working fine for over a year. Today, I discovered a problem. Power is not getting to the DC motor connected to M1 if the motor is to turn forward. It works fine in reverse. If pressure is applied to the screw holding the first wire in place, the motor turns forward. Releasing the pressure causes the wheel to stop turning.
Tightening the screw does not change anything.
A volt meter shows no voltage until pressure is applied to the screw.
In the image below, M1 is controlled with the top two connectors in the blue block on the left, and the screw in question is at the top of the block.