Quote Originally Posted by scopit View Post
The power supply is a regulated DC 6V 300mA plug type. On the board have a 7805 regulator with 10uf + 100nF decoupling capacitors. I have 100nF at the opto encoder supply.
That 6v adaptor won't give a lot of headroom to the 5v reg if it is a standard 7805 style one - this might cause inconsistencies if it is borderline on the regulation and cannot supply the current for the opto LED. I always design at least 2 and ideally over 3v more input than the output of the reg.
Do you have a 9v DC plug pack you could try ?

Bill.