Thanks to everyone on this thread, especially Agile, and the fantastic reverse engineered wiring schematic from underwurlde -
//www.diynot.com/forums/viewtopic.php?t=207176&start=0
You helped me to diagnose a faulty APS on my Baxi Solo 3 PFL 50. I was concerned that the replacement main control board that I had fitted 16 months ago was faulty and I would have to fork out another €200. I was able to confirm using the multimeter that the APS was not resetting when the fan stopped, until the boiler cooled down again.
Not being one to waste scarce cash, I wanted to see if I could repair the APS itself (€50 to €60). It consists of a differential pressure diaphragm actuator which operates a microswitch attached to the side of the APS.
Blowing gently into the positive pressure tube confirmed the diaphragm was working ok, but the switch contacts were not returning to their normal position when the pressure was removed. It looked as if the actuator wasn't pushing the small cam which operates the microswitch enough to toggle it. The movement of the cam is measured in millimetres and the movement of the actuator was just short of the tipping point of the switch by a fraction of a millimetre.
I carefully removed the small melted plastic domed heads on the retaining posts on the microswitch, allowing me to lift it off the body of the APS. Examining the small green plastic cam showed that it was quite loose, very worn and perished looking. I would guess that the high temperatures that it operates at would degrade the plastic.
I was lucky enough to have another microswitch that I had recovered from my old dishwasher before scrapping it (Hate throwing anything away
). It had the same electrical characteristics and rating as the faulty one (VERY IMPORTANT). I popped it on to the APS and tested its operation again with the multimeter - it worked as per spec. The plastic retaining posts were re-melted with a heated small tipped screwdriver to ensure the microswitch didn't fall off.
Apologies if the above is a bit long-winded, I just want others to be able to follow it blow-by-blow. Anyway, on re-assembling everything, the boiler is working fine again and it cost me nothing but time! As an DIY kind of person I can devote time to these things, but not cash. I know things are different for those who make their living at this.
Thanks again to the forum users and I hope this feedback may prove useful to others.
Best regards,
Mark
Pic 1: APS with faulty microswitch removed - note retaining posts
Pic 2: APS with old (black) and new (white) microswitches.
Pic 3: APS with new microswitch fitted and plastic retaining posts melted to ensure it doesn't fall off.