>> I haven't seen any reply to my take on it - I would still like to
>> see a resistor between gate and cathode to lower the sensitivity
>> to stray hum and EM stuff triggering the SCR.
>
>that will prevent false triggering.... it will not fix triggering
>not happening.

Well, I get the impression that something is permanently triggering the SCR, 
like a nearby cellphone tower, or mains noise induced into the circuit, or 
...

The gate will be a very high impedance point in that circuit, so anything 
could trigger the SCR, and there will be only small voltages around it if 
the noise is continuous. 

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