Potterton Profile 50 control board fault?

This was quite an unusually one for me as my normal work is designing in car multimedia systems!

A work friends boiler decided to stop working last week! After finding out he couldn’t get a plumber for some weeks he asked me to take a look at it!

The symptoms were: boiler would kick in fan would run pilot would ignite then that was it! No main burner supply! The first steps were to check all the sensors – this was quite unnecessary I later learned, as if any were at fault not even the pilot ignites!. The PCB was removed and with lack of a diagram I basically worked out how it works, Firstly when applying mains voltage to the control PCB it generates a spark and also enables two relays to switch over, one is normally closed and one normally open, the latter feeds the pilot valve that allow gas to the pilot lamp thus the pilot ignites, at this point all is well! The negative end of the High voltage transformer (Spark generator) feeds a signal to a transistor circuit that should switch off the first relay this then feeds the main burner valve and cause the boiler to run normally. Unfortunately this was not doing this in this instance. Changing the first transistor on the PCB cured the fault. The transistors are BC307B’s just a cheep general-purpose PNP transistor! Probably available from Maplin electronics for about 50 pence or so! It is worth changing all three of them just in case. We did get a quote for the PCB, its about £100! Note there should be a delay between the first relay clicking and the second of a few seconds, The spark generator is running permanently in normal operation. I would also guess that if the spark gap is to wide this may upset the PCB’s operation. Hope this helps.