Micromotor (used for jewellery) failed - can't work it out - Page 1
Forum section: electronics / repair

author: Cykar
author_rank: Contributor
timestamp: July 21, 2022, 05:22:00 pm
content: Hi all,I have STRONG (that's the brand name lol) 207A micromotor. It's used in jewellery production, but can also be found for sale as dental equipment.It no longer works. The circuit didn't look too complex so thought I'd try to fix it, but have completely failed.Any help greatly appreciated.What is it?It consists of a handpiece in which you fit various tools and within which there is a motor. The speed of the motor is controlled by the main unit, a small box which sits on the workbench. On the front there is a speed control which works only when in "hand" mode, an on/off switch, a switch to change the direction, an "over-voltage" led, and the socket to connect the handpiece. On the back there is the cable to plug it into the mains, a switch to choose between "hand" or "foot" control, and a socket to connect the foot pedal. I do not use it in "foot" mode, the pedal is disconnected as the slide pot in the unit has worn out.Symptoms:Initially the hand piece would start when the unit is turned on at MAX speed or possibly higher since a simple sanding roll would bend out with the forces involved. The speed control on the unit would make no difference to the speed, and after a few seconds the over-voltage led would come on and the whole thing stop.After testing it a few times, it now doesn't work at all and the over-voltage led has blown, but the led in the on/off switch still lights.I have previously changed the speed control pot on the front since when we were given it it was stuck at a single speed (although not over fast as was the more recent symptom) and it worked well for a few months until these new problems occurred.What I've tried:I have tried changing the voltage regulator (LM317T), but this made no difference. I have made an attempt at reverse engineering the circuit which I've attached - please forgive the hand drawing, haven't had time to put it in a program, this was just quicker and easier. It's of course possible i've made some mistakes despite my checking and re-checking, so if there seems anything obviously wrong or that you need clarifying, please let me know.The ac mains voltage comes through a fuse into a transformer which outputs about 34V into the circuit, going into a bridge rectifier outputting about 44V.Measuring the voltage output from the LM317 I am constantly getting around 44.5V regardless of what speed is set. The speed pot itself measures fine.I have measured and tested all the passive components, the diodes, caps and resistors - all fine.Other components are a BDW93C darlington transistor, the LM317T variable voltage regulator, a CRO8M SCR, and a transistor (i presume) marked ALY, which with a bit of googling appears to be a KTC3875 NPN transistor, and there is the bridge rectifier which measures good.The motor itself measures about 11ohms between the two pins that are connected to the control unit, so the windings are not open, or shorted. I'm assuming they are ok.I am really at a loss as to what could be wrong.I could just blindly change all the active components, and hope for the best, but would rather understand what is going wrong. All the resistors are SMD, as is the transistor and the SCR. Thinking about it now, measuring the resistors in circuit probably isn't going to work, so maybe one o them has shorted?I hope someone can point me in the right direction as to what to do. The unit costs about £400 so really can't afford to replace it.Thanks for your help.Intitially the motor in the handpiece

author: Andy Watson
author_rank: Super Contributor
timestamp: July 22, 2022, 12:14:12 am
content: It appears from your circuit diagram that you have a variable-voltage power supply with a little added fault-protection. The voltage reference - i.e. speed control - is derived from the LM317. You should be able to measure the voltage on its pins and confirm that it is regulating. In particular, the output should follow the "adj" terminal plus about 1.23VWith regard to your tracing of the circuit: D3 is definitely the wrong-way around. Something odd is happening around the fault-detection circuit: The bottom end of VR1 should not be planted in the ground. I'm guesing that R9 should connect with the base of KTC3875 rather than its collector?

author: james_s
author_rank: Super Contributor
timestamp: July 22, 2022, 12:35:24 am
content: Is this just a brushed DC motor? If so then a motor fault is the first thing I would suspect. I have had to fix a few cordless tools where carbon dust from the brushes fouled the commutator causing the motor to draw excessive current. Try powering the motor from a bench supply or try using a similar motor or incandescent lamp load connected to the controller to isolate the fault.

author: Cykar
author_rank: Contributor
timestamp: July 22, 2022, 11:23:04 am
content: @Andy Watson, You are right about D3, I have got it the wrong way round on the schematic. VR1 though, which is a trimmer, is definitely connected to ground as far as i can tell. I have attached photos of the board itself after removing the heatsink for the BDW93C, hoping that might help.As to the KTC3875 transistor, there seem to be different numbers on the pins in different datasheets, tho what those pins are remains the same if you see what i mean. But given what you have said I looked again and have gone through, buzzing things out, and have redrawn those parts of the schematic. There is a difference, but R9 does seem to be connected to the collector, and only to the base via a 10k resistor (R4).Testing the voltages on the LM317 with the motor/handpiece connected, (and the foot pedal disconnected), initially i get about 17V from the output pin 2. After a second or so it leaps up to 43, and the over-volatage LED comes on.Wondering if @james_s is right and there's a problem with the motor itself. Seems to make no difference as to what the initial speed is set at. How can I test the motor?

author: james_s
author_rank: Super Contributor
timestamp: July 22, 2022, 05:07:17 pm
content: Testing the motor is easy, just supply it with a DC voltage and measure the current it draws, or do like I suggested and use a lamp or other load to test the controller.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 24, 2022, 03:41:19 pm
content: If the lower end of VR1 is planted in the ground - and the photos appear to confirm that it is - then there is no "over voltage" protection. The unit trips on over-current. This is important because it suggests that the power-supply is still attempting to work and is being tripped-out because the motor won't turn.Does the motor spin freely? If it does, then do as James suggests and try putting voltage across it - not too much - check that it spins and doesn't overheat.There's still somethnig odd about Q2. I think it might not be a transistor - perhaps something more complicated, like the AME8500. The 2.93V version is marked ALY, but the pin-out doesn't make sense.

author: Cykar
author_rank: Contributor
timestamp: July 27, 2022, 01:04:13 pm
content: Tested the motor with a poor mans power supply built from an old computer PSU. The motor worked absolutely fine at 3.3, 5, and 12V. Used roughly 10-30mA.So the motor is fine. Didn't get hot, though I didn't do an exhaustive test over time, but just turning the motor by hand, and having disassembled it to check bearings and connections, it all moves freely.I see what you mean by it not being "over voltage" as over current would definitely make more sense, although it is marked as "OV-LED" on the circuit board, but is marked "0.V" (not sure if these are zero or letter "O") on the front of the box next to the LED. The only reference I've found to anything about it just states the unit has an "Automatic overload protection system". Not very usefully any user manual I have found does not mark what the LED is, although it appears on the diagrams.Interesting to note that when turned on the motor does not attempt to turn before the LED comes on.Not sure what to do now. Have tried replacing the LM317, and the SCR. Could try changing the BDW93C, or replacing the "transistor" with a KTC3875?Any thoughts?

author: james_s
author_rank: Super Contributor
timestamp: July 27, 2022, 05:43:54 pm
content: Have you checked the current sense resistor? I suppose it's possible it has gone up in value causing the current limit to be much lower than it should be. I agree that it sounds like the motor is ok.

author: Cykar
author_rank: Contributor
timestamp: July 28, 2022, 03:57:46 pm
content: Going away tomorrow, so i'll get back to it in a week or so. Really bugging me as the circuit seems so simple, but the device is so expensive! Really need to get it fixed.I'll post if I work out any more and will see if anyone else has any idea.

author: james_s
author_rank: Super Contributor
timestamp: July 28, 2022, 05:14:45 pm
content: I've dealt with situations like this a few times and it can be frustrating. Stepping away for a few days may help you reach a solution. I usually facepalm when I do finally figure it out because it ends up being something stupid simple that was staring me in the face the whole time.

author: jpenasar
author_rank: Newbie
timestamp: February 24, 2024, 10:33:23 pm
content: Hi, I know it's been long since the last post, but now I'm facing the exact same problem, the output voltaje is always 42 V and i can't find the problem in the circuit. Did you find out the solution? Thanks in advance.

author: djboby
author_rank: Newbie
timestamp: April 17, 2025, 12:14:49 am
content: In hand mode, if the potentiometer is faulty, the motor receives the 42v directly. In fact, it receives a voltage that can be adjusted from VR1. Also in foot pedal mode, if the potentiometer inside the pedal is faulty, the voltage received by the motor is maximum or more precisely the one adjusted from the potentiometer on the back of the pedal.I have repaired two devices that had the same problem with the potentiometer that varies the speed.The schematic was corrected.

author: djboby
author_rank: Newbie
timestamp: April 17, 2025, 06:15:41 am
content: I forgot to mention that in the diagram presented in the attachment, the connectors are seen from the back, the opposite side to the one with the components.

author: picburner
author_rank: Frequent Contributor
timestamp: April 18, 2025, 04:30:06 am
content: Finally an understandable electric schematic.The part you forgot is the connection between the Vin of Q3 and the collector of Q4, otherwise the LM317 is not powered.

author: djboby
author_rank: Newbie
timestamp: April 21, 2025, 09:38:03 pm
content: You are right.Thanks.The schematic was corrected.