Archibald Tarquin Blenkinsopp Esq wrote:
From the rear.

1. Mains in 6. ALM
2.Mains In 7.ALM
3 Common 8. RT
4. SSR+ 9.TC-
5.SSR- 10. TC+


6 I strapped to 1, with 7 taken to a 240Vac relay

Hang on a mo. According to a different datasheet:
https://www.mpja.com/download/rex-c100.pdf

Power input is 6 and 7, alarm relay contacts are 1/2.
Thermocouple 8/9 or thermistor 8/9/10, output 4/5, 3 can part of alarm or triac output.

If you put the relay contacts 1/2 across the mains that might explain
things...

Of course, in the Cambrian explosion of clones who knows how yours has
evolved.

Much testing and head scratching later, I dropped in my last
controller which unfortunately has no alarm relay.

It works!

Did these all come from the same vendor?

Theo