Apologies - I started to reply to this earlier before work but somehow lost my post. This is an abridged version:


I'd be concentrating in the first instance on establishing the logic supply (the +5V) to the L293 - without this you're going no-where. Add decoupling caps if you've not done so already (ref. the LM7805 datasheet for detail). If you can, measure the current drawn by the L293 - the standard 7805 can source 1A, and will current limit somewhere above that (by dropping the V-out). The L293 datasheet suggests the logic supply should be 60mA, max - above this and you have a problem - either wiring, or a knackered L293. Any metal tab on the 7805 is internally connected to common (ground/0V) - so take care that this doesn't short to something that it shouldn't.


I wouldn't even worry about the motor wiring etc until you stabilise the 5V logic supply.


If you find you keep blowing the L293s, I would consider removing the motor and channel-by-channel testing the circuit/switches with a DMM to ground.


The circuit is trivial (I mean that in the sense that there's little to go wrong) - it should be straightforward to diagnose with the above. If you report back we can take it further.