No heat in hot end

[tim meh]

Hi CTC community, I've had a CTC replicator for 5 years and it's been reasonably reliable with some upgrades until now.

The printer turns on fine and the thermistor still reads temperature, however after replacing the thermistor and hot end parts several times with different parts I can't get the hot end to heat up at all - it just sits at 21 degrees.

I've tried switching the active head to both left and right with a several spare parts (heater cartridge & thermistor), but no change. 

Some people seem to suggest changing the MAX6675 chip next, is this the best next step?

Additionally if people think that swapping out the motherboard is the better, has anyone got any links to a suitable replacement? I'm based in Melbourne, Australia. 

Thanks in advance, appreciate the help! I'm at a bit of a loss about how to approach it :)