Hi. Just assembled mk138 and the 470uf capacitor has blown. Any ideas what I have done wrong? I felt I had installed everything correctly double checking the polarity etc. Any advice welcome. Ander34 
You either inverted the polarity of the cap (very likely) or you applied a too high voltage (less likely).
Thank you for your reply : Have replaced 470uf capacitor but Iam still having trouble. I have 12vdc between pins 4 & 11 and the relay operates when T1 outer pins are shorted. However that is about it - I cannot get it to operate from the thermistor.Your help would be greatly appreciated.
ps. I believe the origional problem was caused by my first transformer - IT WAS 12v AC ! Whoops!
Make sure IC has not been inverted.
Replace IC if necessary
Should get 5V at pin 8 (if not, check ZD1/R1)
Check if you get adjustable level at pin 12.
Check level at pin 13 when sensor is connected.