omnipure 12 high temp alarm wont reset?

we have a high temp alarm on the control panel, it will not reset. Relay cr37 opens and closes very quickly on depressing reset. The cells look ok, clean and in good condition, ive shorted out the temp s/w but it still comes on when 3 pos s/w is turned either way to auto or hand??

Just after this happened i isolated the supply, checked the relays are all ok, 4.5kohm approx on each, checked connections etc. On restoring power and turning to hand the F5 fuse blows?? different symptom now.

Any ideas appreciated.