Worcester Bosch 28cdi RSF PCB problem.

Hi all,

Just wondered if someone out there can help with a problem I'm working on relating to a WB PCB. One week ago boiler started to 'play up' . On DHW demand, boiler would fire up for a couple of minutes, make a slight rumbling noise then stop - exhibiting both red lights flashing slowly: Before taking boiler apart, my brother tried dumping some water from the boiler via the PRV. This worked for a couple of days. After which, same fault symptoms... + pressure dial loosing pressure. Loss of pressure due to a faulty PRV caused by some dislodged scale within the valve not allowing the the rubber washer in the valve to make a completely watertight seal. Also, I observed that the failure indicators on boiler were again both red lights flashing: 'Fail POINT G' Primary Overheat in Instruction & Service manual. I followed the faulting logic & arrived at last comment box stating: "Fault caused by PCB or primary heat exchanger." I inspected the PCB & spotted a burnt Power transistor. Therefore I assumed faulty PCB. I purchased a replacement. before fitting I checked all possible electrical connections on ST connector: pos 1&2: 1.7=CE=A9; 5&6: 0=CE=A9; 7&8: 78=CE= =A9; 9&10:

78=CE=A9; 12&13: 10.45K=CE=A9; 14&15: open; 17&18: 11.65K=CE=A9; 19&20: OK.= Impedance on transformer windings checked against readings from a new transformer. Same values...! Pump & Fan checked for short ckts. ALL OK. fitted new PCB & turned on. On DHW demand - boiler fired up with pump working but packed up after approx 15 seconds - I heard a "pop" coming from PCB location..... On inspection, yes I had blown my replacement PCB :-( Any ideas ??? its not obvious !!
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Gilly
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