Likely cause of boiler transformer failure?

Well, obviously I do not know the boiler specifically, but it sounds to me like a thermal link has blown due to overheating of the transformer. It is likely that something in the power supply circuit has put a short on the secondary, or the transformer merely has broken down with shorted turns. I'd have thought if there had been a fan or other bad component then you might actually see it or hear it, ie lack of sound if its failed, so short of trying to get a transformer and replacing the old board, I'd imagine just checking every so often for overheating might be all one can do. When this happened to a different appliance here, it turned out to be a capacitor and a bridge rectifier which had gone, and one could actually measure this on the old pcb. It was a shame nobody thought to fit a hi tech device like a fuse in the low voltage side of the ac from the transformer. grin. Brian

Hard to say from here... might be worth having a chat with the guys at CET and see what the likely causes are. (you can also send em your dead PCB and get a recon in its place for the next time!)

the only thing that kills transformers is too much heat due to too much current or voltage. So current overload on the 2ndary side is the answer. Post the schematic.

NT

Mathew Newton pretended :

It was probably inadequate design or inadequate rating it was such a common fault. You can actually buy, or could, identical replacement transformers to avoid the cost of a new PCB. I repaired one that way, but added extra ventilation around the transformer.

Another regular fault, is the fan and especially the wiring to to the fan, which runs up the rear of the boiler. The heat from the boiler melts the insulation.

Internal thermal fuse has given up.

I'd tend towards age and border line design. It takes quite a while for plastic to change colour under heat without getting smelly. Especially when there is an air gap like for the cover. The transformer case was blue but is now mostly and evenly dark.

As you have a working board how warm/hot does the transformer on that one run?

NT

Or hand warm for a long time, like 12 years...

As soon as it failed I thought of Geoff!

I've actually bought another transformer so will give that a go.

Unfortunately I don't have a schematic.

Yes, just bought one. One other company I contacted (YHDC in China) so they don't keep one as a stock item but would make me one in 3-4 weeks!

That's not good to hear, but good to know - thanks!

Good question; I'll do some testing. I did wonder about the location of the PCB as whilst it is underneath the combustion chamber it is never the less inside the casing and so it wouldn't surprise me if it got close to, or exceed, the 50C rating that I've seen printed on some replacement transformers.

I don't know any material that's charred or blackened by being hand warm for 12 years. You're outdoing yourself.

NT

Mathew Newton was thinking very hard :

It might be worth your while if you intend to keep the boiler a while, to take those cables out and replace the centre section with heat resistant wires. Its a short loom, of four or five wires, which unplugs at each end. Just snip close to the plugs, then solder in and sleeve heat resistant wires. I suspect that cable failing, might be the cause of the transformer burning out..

Mathew Newton explained :

I have a couple of spare fans for that boiler in my hut, if they might be of use to you. One is a used working one, the other brand new unused. I bought one when the boiler suggested a fan fault, before I found the over heated and shorted fan loom running behind the boiler.

They are free for collection.