Apollo 50si shutting down intermittently - Ignition problems

[Bodiebod]

Hi all,

This is my first post so please give me the space to run off topic.....

We have a Myson Apollo 50Si which had started acting up...
It would ignite and run fine for a good while...
Then whilst running for no obvious reason would shut off the all the gas and after about 2 seconds relight the pilot and main burner. While this happened the fan kept running all the while.
This intermittent 'shut down' eventually became an event that would happen every 15 seconds!

Eventually it just plain refused to light up altogether and the symptoms became a 'problem'

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I searched high and low for a solution online, eventually solved it myself, but feel the need to publish what I learned.

Installation and fault finding manual was found:-

http://www.partsarena.co.uk/baxi/System/DATA/Dx/DS1/installation/3271/I51-3271/I51-3271.htm

pm me if link dead, I have my own copy now.

Running through the diagnostic flowchart indicated replace PCB - as pilot was not lighting now...

This I did with a 'remanufactured' one for £35 with £37.50 postage.... sending back your old board gave a refund of £30
Total cost just over £40 with 12 months warranty

http://www.allsparesdirect.co.uk/

I used their ebay shop...

http://stores.ebay.co.uk/All-Spares-Direct1?_trksid=p2047675.l2563

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This solved the ignition of the boiler and all was good

Until it started shutting down randomly again

Flowchart did not mention this kind of behaviour/fix

I took out the board and proceeded to draw out a circuit schematic as I was not going to be beaten by an engineer charging to turn up, drink tea and replace parts until everything works...to this point i would have been charged about £200 for a new PCB plus labour charges!

From the circuit diagram - Im not going to publish this as there will be to much effort to make a 'neat copy'....

I found that the PCB detects pilot flame being present by the fact that flame conducts a small current, then starts the main burner.

From the schematic, both pilot and main burner would be shut off if this small current was to disappear...clearly the PCB was thinking flame had gone out.

I checked the connections to the spark gap and everything was fine but still a problem....took out the electrode/ceramic/ignition wire and examined closely... all good. As it wasn't working (and I was really annoyed at this point ) I pulled on the wire and it just came apart!!

Problem solved! the wire had completely fractured an inch away from the ceramic where it had bent around a corner...high voltage would jump the break but the sensing current would not. Cyclic behaviour was from thermal expansion and contraction giving an intermittent connection.

I managed to fix this myself as I have high temp PTFE insulated wire and PTFE heatshrink sleeving - anything else WILL melt away so close to the burners.
A new one of these is actually about £40!

I can see why 'engineers' suggest new boilers as this episode would have been charged 3 visits plus £240... probably over £400

Sorry for the long post, but I hope this helps someone with the same problem