Electronics > Repair

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

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--- Quote from: rh100605 on January 20, 2022, 12:04:00 pm ---Before replacing the triac, try shorting the input and output (A1/A2 )of the triac and see if the motor runs at high speed continuously
This is easy to do as the triac pin to the outer of the pcb should be shorted to the middle cut off pin (solder bridge?). This leaves the gate open...
Then reassemble the tool and switch on/off at the mains with the tool switch on. This experiment will eliminate the motor wiring + thermal fuse except for the control/feedback wire that connects to one of the brushes with a phosphor bronze tab.
--- End quote ---

Interesting! Did that, and as you guessed it ran at full speed continuously - at least that's some kind of result.

So you think that would mean the triac is fine?

If I understand you correctly, the phosphor bronze tab, which is connected to the board via a blue wire, supplies the feedback. It is soldered to the board under some glue, but i'll dig it out and see if there is continuity. I'd be surprised if there wasn't given the way it's glued in like that. The electrolytic is also glued down, but testing in circuit there is no short. Maybe I need to dig that out too and test it out of circuit.

@rh100605 Well, measured from the bronze tab to R5 and R2, as per your schematic, and got continuity so that seems to be connected ok. I'll try getting the electrolytic out...

Congratulations on proving, by shorting the triac, that the field winding, thermal fuse, armature, power switch and associated wiring is OK.

I do not think that the Triac is faulty. The cycling is due to the mains supply to the motor being cut off by the triac, the gate switching signal that occurs at mains frequency  is being stopped by the circuitry on the board.
Checking the continuity of the feedback wire (blue) is not enough to prove feedback is supplied to the board. Check the fully assembled motor from the thermal fuse or brass brush casing to the components on the board. This will confirm that the phosphor bronze tab is contacting the brush casing when the motor is assembled. Also try to check to see if the plastic case moves the brush casing when assembled.

You need specialist tools to find out if the Electrolytic capacitor on the board is faulty, just replace it ( noting the polarity before removal). with a 105 degC version from Panasonic.

I was late in replying as I was able to see your two posts on the 21st Jan only on my phone ( app)  not on my laptop (web) as I had not realised a second page had been started.......


--- Quote from: rh100605 on February 02, 2022, 03:06:36 pm ---I was late in replying as I was able to see your two posts on the 21st Jan only on my phone ( app)  not on my laptop (web) as I had not realised a second page had been started.......

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@rh100605 Ha! No problem - this has been rumbling on for a while now - I really appreciate your interest as I would like to get it working again, and more to the point, for me, to work it out (with a little help!). Should be simple, but obvs not simple enough for me  ;D

I will check continuity with it assembled, and get a new cap - I don't have any 105C caps, tho I though I might put in a standard 85C one just to see if it works.

Will have to wait till next week now tho.

Thanks again.


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