[G6ONEDave]

Re the SCR test reading. If this is a thyristor then you should not get any resistance reading between the anode and cathode when out of circuit, you will however get a low reading between the gate and anode, which may be the same both ways round. If the thyristor is showing a dc resistance between anode and cathode when in an off state (such as out of circuit) it is faulty. The same is generally true for a triac. These devices conduct when a drive signal is presented to the gate.. Check the volts on the cathode when powered to see if you have anything. If not check input volts on the anode and if there is a correct voltage there either you don't have any gate drive signal or if unlucky the thyristor has gone o/c so won't conduct. The anode input may be an output if the supply is a negative one. Sorry don't have a circuit to hand, so don't know which way round your device is connected. If the gate drive is missing, have look for a diac in the circuitry (BR100), it looks like a signal diode with a black band round it's centre, unfortunately the diac won't give a resistance reading either way round unless it is faulty, so you will have to substitute it for a powered test. Hope this helps out, otherwise a circuit would be useful to help diagnose the problem.