baxi solo 3 circuit diagram

[underwurlde]

You are an absolute star, sir!

Thank you!

Although I haven't fixed it this time, it has confirmed that R123 is 1K (the coloured bands are so cooked they have almost lost all their colour)

Does it measure out as still being 1K. If it is truely cooked, can you not replace it? Maplin might be able to supply such a resistor and you could replace it with a higher wattage version?

I was interested to see that your board is also "toasted" around R123 (near the fuse) - obviously this one runs hot.

Indeed. To be honest, I think the design of these boards is **** poor. As you can see they are basically analogue computers, a thing of the past really.

I was so tempted to design my own control board and use the obvious solution of using a processor, but I think my wife would've chucked me out even earlier if I'd have started 'mucking about' with our boiler! I had visions of providing a link to a laptop that would've been able to run diagnostics on the system to pin-point any problems - this would've been quite simple to do! (Quite a bit of effort, but none the less not difficult to achieve).

I really hesitate to say this after all the time you've obviously spent on them, but if you wanted to make them even more useful, adding the component identifiers to the "Mk II" and "Mk III" diagrams (like you have in the Mk I) would make them truly complete.

I would've done that, but all the component idents on the later PCBs are printed under the components and without removing them all (which I guess I could've done) I could not see what the idents were. To do that now will not be possible becasue I sold my MKII PCB on eBay and the MKIII is currently in use in my boiler. Apologies for that then!

Thanks again,

Andy

PS it does make me laugh that these PCBs cost £150 to replace, for a control board that must cost less than a tenner to make!

 

[chuzzlewit]

Does it measure out as still being 1K.

Yeah, popped one end out and it reads 999, so unfortunately not a simple fix. The "Boiler on" light comes on when heat is called for, but that is it. No other lights, no fan, no ignition spark, nada.

The current board was a recon. one off Ebay for £40 (sounds dodgy but they seem pretty pukka, 1 yr guarantee and they take your old board back to recondition). Just checked the receipt and it lasted 16 months so am debating whether it was false economy, since I can get a new one for around double that.

I'd debug it if I had the time, but the wife and kids are starting to bleat about living back in the 50s, so no other option than to replace the board in quick time. I will do a couple of other checks first but 99% sure it's the board (fuse is OK).

all the component idents on the later PCBs are printed under the components and without removing them all .. I could not see what the idents were

Fair point!

 

[DP]

wow. Supeb. This is what I call talented.

 

[underwurlde]

Yeah, popped one end out and it reads 999

Then it's fine!

Are you getting the basic on-board supply voltages (see circuit diagram).

Question: Does the pilot light come on? There is a wierd little part of the circuit that detects ions coming off the flame of the pilot light. If the circuit does not detect this, then nothing else happens (this is because the main gas supply, should it get switched on would not then ignite if there is no pilot light - this would not be good for obvious reasons).

If you are not getting a pilot light, i'm GUESSING that the spark electrode may be out of position or the HT supply is dodgy. What I think happens is that when first connected, the system open the pilot light gas supply valve and then generates some sparks to ignite it. If after a few seconds the ions from the pilot light are not detected, the pilot light gas supply is switched off and the system then does nothing.

Remember, as what happened to me, it may not be the control board that is at fault.

Andy

PS. No, I am not a gas engineer. I am not Corgi registered and no I don't know what I am talking about and yes, I am making 'educated' guesses!

 

[chuzzlewit]

Does the pilot light come on?

No there is absolutely nothing happening at all, no fan, no ignition attempts (clicking). I think the ignition sequence from the PCB perspective is:

Call for heat received. Provide power to circulator pump. Provide power to fan. Check pressure switch input (verifies fan is on), activate pilot gas and ignition, check for pilot flame, if pilot present, activate main burner gas (& stop ignition spark). Obviously most/all this overridden if the Overheat input is active.

This is in line with the following I nicked from another forum:

1) 'Call for heat' signal arrives at boiler
2) Boiler thermostat checked - must be calling for heat
3) APS checked - must be in 'relaxed' condition (proves APS is not jammed ON)
4) Fan starts
5) APS checked - must be in 'exercised' condition (proves fan is running)
6) Ignition spark begins
7) Pilot solenoid energised
8) Gas arrives at pilot light
9) Pilot flame ignites
10) Electronic flame detection device detects pilot flame
11) Ignition spark ceases
12) Main burner solenoid energised
13) Gas arrives at main burners
14) Pilot flame ignites main burners

 

[underwurlde]

Have you recently had your boiler serviced? One other thing that was wrong on my boiler was the venting fan had siezed. Also the APS sensor can get clogged up with dust & crap. Again, I got a repacement reconditioned fan from eBay for about £40 (if memory serves) instead of the £168 asking price for a new one. It's still OK after a year....

I just get the feeling (dunno why) that the problem may not the control board. I see no reason not to trust a reconditioned one - I do think they check them before sending them out.

Andy

 

[chuzzlewit]

I'll keep this brief as the thread has wandered somewhat...
I've verified that there is power to the circulating pump but no power to the fan. This isolates it to the bottom left are of the Mk II circuit diagram, which I think is the closest to my board. Next suspect would be the fan relay, but I really don't have the time to remove, test, locate a pin for pin replacement, order and fit before the familiy mutinies!

So have confirmed it's the PCB at fault, but many thanks for your suggestions, I'll bear then in mind for next time!

Martin

 

[chuzzlewit]

Managed to get a new PCB off ebay for £70 and it now works a treat - toasty again!

 

[tfboy]

Bit of a ressurection from the dead, but a big thank you to underwurlde for doing this great work.

My "Mk2" version has an issue with the boiler overrun - it never stops the pump. All components on the PCB look OK physically. Issue is that I never get more than about 9V on the relay coil so it never kills the live feed to the pump.

BTW, the relays on my Mk2 are 24V, not 12V. You might want to ammend the comment in the schematic

 

[slarty]

I know this is an old thread but my old boiler (circa 1999) is in need of some TLC. is there a higher resolution schematic for the MkII the .jpg 800 x 506 pixels is not easy to read the legends. BTW fantastic job with the reverse engineering. Fan light on fan spinning (after replacing the burn out one at great expense), no ignition.