It Sounds like your old Diplomat is fitted with a very old Honeywell Gas Control Valve, I assume he has repalced the Thermocouple?? if he has and the pilot goes out when turning the Control Valve Knob to Main Flame position it is likely that the Gas Control is U/S, in days gone by, an obsolete gas control could be repalced with an equivelent provided it provided the equivilent or better protection. HOWEVER nowadays this is frowned apon and would in fact be illegal, so i'm afraid if a simple Thermocouple dosen't do the job , you will probably be looking for a new boiler. sadly we are no longer allowed to be Engineers!
www.elanar.co.uk
May 2009
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