Miele intermittent failure

[aardvaarkk]

Good call! The coil is at least 1 megohm… And maybe intermittent (hard to tell) that seems far too high?
(Funnily enough I fixed a coil on an electric piano last month. I spent hours unwinding and rewinding it.). Is this the kind of component one can buy?
thanks

[electrofix]

you should find plenty of them on ebay, you will have to get the whole brush holder assy. If you get stuck am sure i have a few scrap motors i can get one off

Dave

[aardvaarkk]

Hi Dave — bad news.I got a replacement board from Ebay but it still doesn’t work.

The tacho coil on that board is about 500 ohms, great, however the board is a slightly different design to mine, and the coil is a different shape too (I don’t think it would fit in the old board, but I didn’t try.) The new board fits properly around the motor, but: there’s a 10 way connector. The 8 way which supplies juice fits nicely, but the 2 way next to it which connects to the tacho is not quite the same (the registration notch is displaced.) I jury rigged it by making up some crimp spade connectors to bridge to the connector. So it should work perfectly, even if I need to make it more robust later. But the machine does exactly the same as before, as if the tacho were open circuit. Motor turns for a few seconds and then the warning lights flash, just as before.

Not sure what to do next, but we’re running out of clean laundry!
Thanks again for all your assistance.

[electrofix]

whats the number on the side of the motor Mrt37 or the like ?

Dave

[aardvaarkk]

It’ll be quite awkward to get to there (so far I’ve done everything from the front) – will take some time. What did you have in mind when I’ve found it?

[electrofix]

to see if i have one

got a few scrap motors around. got one here MRT37 606/2 tacho measures 500 ohms approx
just found 2 more same motor

Dave

[aardvaarkk]

I’ve done some more investigation.
The only number I can find on the motor is 6201Z, here on the end of the shaft:
https://www.dropbox.com/s/nkzqot3gdj2flo8/IMG_20210305_161130685.jpg?dl=0
I managed to get the side off (and broke a few plastic clips in the process 🙁 ) but didn’t really find much else.
Here’s all I can see of the motor from the panel side:
https://www.dropbox.com/s/okzf6hh0982aaaz/IMG_20210305_154631241.jpg?dl=0
https://www.dropbox.com/s/us2upiz42xuz6sl/IMG_20210305_154640743.jpg?dl=0
Here’s the broken but correct tacho:
https://www.dropbox.com/s/qu4c6lnnbzo5mb0/IMG_20210305_155244704.jpg?dl=0
https://www.dropbox.com/s/2m8f7w9yp2b8p5x/IMG_20210305_155403328.jpg?dl=0
Here’s the wrong controller (left) and the correct but broken controller (right):
https://www.dropbox.com/s/il9gy281ydt8l5r/IMG_20210305_155503042.jpg?dl=0
The “wrong” tacho looks quite different, the coil apparently isn’t intended to go around the shaft, just sits at the end of it.
The “wrong” tacho is 500 ohms (seems correct), but is generating less than a millivolt when I spin the drum by hand — that’s way too low, I imagine. I don’t know what it ought to be, though.
Any thoughts? Thanks.

I did find a manufacturer’s circuit diagram tucked away under the lid — is this standard practice? It shows the connections and component names, but almost no part numbers or component values. If it’s useful to anyone I could scan and post it?

[electrofix]

the motor number will probably be underneath it facing the floor

the tacho i have is not like yours but the magnet seem different . its a lot thicker

will try to think on to check next week but changing the magnet may cure this

Dave

[aardvaarkk]

Ok, the plate is underneath. I did the mirror, torch and camera trick:
https://www.dropbox.com/s/ene8it9i1iknhg4/IMG_20210306_093240283.jpg?dl=0
Type Mrt 37 – 606/2 exactly as you said.
Should I return the alternative tacho, or am I going to need it?

[electrofix]

return it and pm me your address

will take the back end off one of my scrap motors and will send you the magnet as well

my guess is the unit you have may have worked if you had the correct magnet

its only a theory but your old tacho is completly different so is your magnet

Dave

[aardvaarkk]

Doh, I’m an idiot! I said:
> but is generating less than a millivolt when I spin the drum by hand
I’ve just realised that I measured it DC, but it must be AC… Too late now, it’s gone back.

[electrofix]

we have all done things like that lol

Dave

[aardvaarkk]

Thanks for sending the parts Dave (not arrived yet.)
To replace the magnet is it as simple as undoing this screw, or will that release the motor too?
https://www.dropbox.com/s/nkzqot3gdj…30685.jpg?dl=0
Cheers

[electrofix]

yes undo screw. its tight and has a compound to stop it coming loose. i had to wrap cardboard round the motor pulley and clamp in the vice to get it off. make sure you use the correct size driver as if it ruins the screw head you could have problems

you might need to remove the motor to do this, its not s bad job as both sides are removable to get at the bolts

Dave

[aardvaarkk]

Success! We now have a working machine. Thank you so much for your help, Dave.

Summary: the tacho coil had gone open circuit. Thus the machine didn’t know that motor was turning or not, so refused to operate (presumably in case it was jammed.) Also, not knowing if the motor was turning it also refused to open the door for safety. I replaced the motor control unit, and it just works.

[Author]

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