Electronics > Repair

Dremel 400 failing with constant start-stop-start-stop

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I have a Dremel 4000 which I use when working on jewellery with a flex shaft.

Everything was fine until Monday, when after a couple of hours work it failed. I turned it on after changing the mandrel I was using and it proceeded to start-stop-start-stop as if one of the brushes in the motor was not getting any power.

I had changed the speed/motor control board last summer and everything has been fine up until now.

Taking it apart, I cannot see anything obviously wrong.

I've made short video showing the problem, and a quick look inside:

There are 3 chips on the board
The markings on the 3 pin chip are:
GF135 824
(a T4 3560 TRIAC?)

One of the other two has the following:
830166 (could be 830156 but I think 830166, hard to read)
I have found a U2008B 8 pin chip which is a phase control circuit, applications including motor control (datasheet: https://www.mouser.com/datasheet/2/36/doc4712-29429.pdf)

The last chip has the markings:
I can't find anything on this - no idea.

Has anyone any idea what is going wrong here? Is it the TRIAC or is there something else happening?

All the diodes check out fine, but that is limit of what I know how to test...

Any help gratefully received.


What about the brushes themselves, are they in good shape?  The commutator too? 

It could be a bad connection e.g. a broken wire.  If so, its normally making contact, but the reaction torque from the motor starting up causes it to loose contact, stopping the motor, repeat ad nauseam.

beside broken wire/bad potentiometer, check all smd's for correct pin soldering, you may have a cracked resistor or capacitor
take them out one by one (start with R and C) or just resolder all, takes 15 mins
in vibrating machines, sometimes you can't see cracked devices, it happened to me once with a resistor, with multimeter it was ok, when i put the soldering iron, he split literally in 2 pieces
but as momeone said already, start with wires/ potentiometer

The brushes are fine, relatively new and the commutator seems ok, tho i would have thought that if there was a problem with it it would effect both brushes/sides of the circuit and not just one side. I have also checked the wire connections and having disassembled and reassembled several times in the last few days with the problem remaining exactly the same I am confident that those connections are also good.

I will check the other components, resoldering where i can - it's all quite fiddly tho.


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