Hi Jbest, sorry I don't have regular web access & some time can be days.
The output stage of the MOC302x optocouplers work differentl to a transistor output opto. The output is a low power triac & once turned on will stay on untill its is current reduced below a threshold value (same as the SCR you are using).
The MOC3020 can be activated with no LED input current under some conditions like high rise time voltage (eg: switch bounce noise) This is false triggering but usually its spurious operation.
You probably need to check that the opto input is actually turning off properly or it could be the way the opto output is conneted to your SCR.
Try disconnecting the input to the opto,
If the SCR is still on then the scr to opto circuit is the problem.
If the SCR now stays off then it would be the LED in the opto is still being supplied current ie: not turning fully off.
The datasheet for the MOC302x/ NTE3048 opto will be a good resource, read it thru even if you dont understand parts of it at first, look at the application diagram.
Best thing you can do is sketch out a circuit scan/photo & attach it but it must be clear.
The photo is not clear enough to be any help.
12volt sunk thru one gate of the 7404 thru led to ground and used the other gate to trigger the 4 gates for the optos
The 7404 IC is a 5V (4.75 to 5.25 supply) device & has a maximum of 5.5V input.
Are you switching mains full wave, maybe a triac (like dual SCR) is more appropriate.
MOCxxx App note attached for info.
An example of DC applictaion of scr attached, C10/R12 is for false trigger protection from high dV/dT transients (eg: switch on) This circuit is a Crowbar OVP for a 50V power supply.
Tonig au