Are there any electronics experts in the house?

Norvin

Some years ago I bought a Miele CM5100 bean to cup coffee machine. After lending it out to various people it came back as not working. It has a switching power supply on the same PCB as the chips that control the various functions, unlike on later machines where both roles are separated. The issue is power related, there isn’t a dc output on some of the pins on the socket on the RH side of the board. I replaced the electrolytic capacitors and one diode but the problem remains. I have reached the limit of my abilities.
The whole lot was put in a box and stored in the garage a couple of years ago but it’s time to do something about it. Does anyone on the board have the ability to fix this? I would be happy to pay.

DavecUK

Norvin I have heard defective transformers are common on Miele Mainboards. Isa it worth checking and possibly replacing the transformer. If there is any details on it, you could most likely substitute something similar

I assume it’s that cream coloured thing in the lower centre of the board.

Norvin

Thanks Dave.
Looking at my two year old notes, it’s a Flyback transformer with two outputs, a 9v which serves a 5v circuit and a 15v which serves a 12v circuit. The 12v output rapidly cycles from about 11.5v to 14.5v. There is no output on the 9v side but I think I remember that it appears if you disconnect a diode. That’s as far as I got, thinking that I would look out for a spares or repairs machine to cannibalise but one has never appeared at a reasonable price.

DavecUK

Norvin You could try buying 2 transformers and offboarding one of them if there is room?

Norvin

I suspect that the fault lies elsewhere.I read that when the output is cycling it’s due to a faulty board component. The 5v output returns when the diode is detached, suggesting that something in the circuit is shorting. I could try removing the transformer from the board and then testing the output if you think it’s worthwhile.
I was hoping that as switching power supplies are common someone on the forum may be experienced in fixing them.

Gagaryn

I expect that you have probably tried this but a spritz of IPA over the board is a handy quick hack for detecting shorted components - the shorted component will likely be warm causing the IPA to evaporate quicker than elsewhere. If I was a betting man I’d be looking at the switching transistors.

Norvin

Gagaryn Thanks.
I’ve just tried that. Didn’t have any IPA but used contact cleaner. There are two items that I have identified as power MOSFETs on the board near the small capacitors on the DC side of the board, a 50v and a 30v (marked F7306). The 30v one dries much faster than the 50v. Would it be worth replacing it?

Gagaryn

Norvin Yup, I’d give it a try, they are cheap enough that there is little risk. Looks like these might be suitable - https://www.ebay.co.uk/itm/152309318173?hash=item237657c21d:g:CScAAOSwScBf-CLt

Norvin

Gagaryn
Just ordered, thanks.
I’ll have my old Lucas service part ready in case I let the smoke out.

Gagaryn

Norvin Ah, the good old magic smoke, and real Lucas stuff, not Britpart repro. Look after it - getting valuable now.