Dremel multi - fault diagnosis & repair

[jkn]

I have a Dremel Multi (model 395) which until now has had only very light use. I recently had a 'proper' use for it - cutting a square hole in a steel plate (1mm think or so). I bought some diamond cutting wheels for the dremel off eBay and set to work. I wasn't going 'hell for leather' and was paying some attention to allowing the thing to stay cool-ish.

Don'tcha know it - in the middle of this job the Dremel has packed up. Or partly ... it stops after a few seconds, seemingly losing power. It might start up again after a few seconds, or if I wiggle the cable as it goes into the drill, or if I use the built in switch/speed control to cut the power, then try powering it up again. But then it stops again after a few seconds.

I've opened the thing up and all seems well - the brushes seem OK, the cable connections are tight, the commutator seems OK, etc.. I've seen that Dremel do a load of spares and I wonder if anyone has experience of what might have gone wrong. My current thoughts are maybe the speed control unit, but given my recent poor experience of Bench Grinder fault diagnosis (and that I guess this is the most expensive part to replace) I'm happy to learn of common problems with these units.

Thanks Jon N

[meow2222]

you need to use a multimeter and find out. What v is on the low v power line when its running and when it cuts out? etc. We cant tell you which bit's bad. I could gues but theres not much point.

NT

[Clive Mitchell]

In message , jkn writes

Have a good look at the switch or variable speed circuitry. They are annoyingly prone to causing problems when dust gets into the contacts. Not so great for a tool that makes dust as part of it's operation.

[jkn]

update - I did a bit of reading and investigating the dremel innards. The speed controller board is based on a surface mount BT134W Thyristor which not uncommonly blows. I'll add one to my next FEC order. In the meantime I have jury-rigged around the speed controller, and I'm running the dremel off a Variac instead. This works at least as well as the original internal one.

Cheers Jon N

[jkn]

Further update, which might be useful to others.

Even running with a variac the drill would cut out after some minutes work. This was fairly consistent and originally I attributed it to the drill getting warm, guessed that there was a thermal cutout operating somewhere. I'm cutting a 1mm or so piece of steel so I did think I might be overloading the motor. After half an hour's rest or so the motor would start up OK again.

Anyway, after a few of these cycles it seemed to pack up completely ;- (. I buzzed through the motor windings and one of the stators measured as open circuit. I was just about to throw it in the bin when I thought to check the connection between the stator wire and the pin into which the volts go. This is not soldered as I expected, but is a sort of cheap IDC knife-edge connection, for the enamelled wire. The connector fell off the wire as soon as I looked at it.

So I've remade this connection and things seem fine again. It looks like when manufactured the connector is forced down into its housing where it supposed to engage with the wire and 'cut' a connection. I did this a bit more thoroughly than they seemed to have done at the factory. If it fails again I'll probably solder it up.

I'm guessing that originally the drill warming up was causing this connection to fail after a while, and be re-made when the drill cooled down. Not very impressed with Dremel quality at the moment....

jon N