Proxxon MF70 went bang

[Bill PhinnParticipant @billphinn90025]

I've had this machine for about eight years, but it's not had a lot of use in that time (easily less than 100 hours). Today I was milling some brass with it, and it had been in use for about five minutes when a loud bang and some smoke emanated from the motor housing. The machine stopped working permanently.

Does anyone have any advice that might help me to diagnose the cause of this and get it working again? I am ill equipped, in more than one sense, to do anything but the most basic fault finding of electrical items.

Thanks for any help.

[Bill PhinnParticipant @billphinn90025]

[John RuddParticipant @johnrudd16576]

What is your location? Someone nearby who is savvy might be able to help…..

I'd offer to help….but I'm Newcastle…

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bill Phinn on 30/01/2019 20:18:57:

… Today I was milling some brass with it, and it had been in use for about five minutes when a loud bang and some smoke emanated from the motor housing. The machine stopped working permanently.

[ … ]

Does anyone have any advice that might help me to diagnose the cause of this and get it working again?

.

It might be worth having a look for brass chips in that motor housing.

MichaelG.

[Roger ClarkParticipant @rogerclark]

Where do you live Bill?

Roger

[John Paton 1Participant @johnpaton1]

usually there will be heat and smoke marks on the printed circuit board in the vicinity of the deceased item.

I would expect Loud bang, smoke and age of the machine would lead me to look first at any capacitors in the circuitry. Modern capacitors seem to fail after about 10 years and hate not being used. Look for largish cylindrical component with two terminals at one end or one at each end and annotated microfarads. If blown it will usually have discolouration and probably bulging at the ' plugged end'.

Alternatively it might be a rectangular capacitor with a wire at each end. These tend to rupture again with burn marks on one of the large faces – but these ones are generally used for motor interference suppression and often do not stop the motor from actually running unless they shorted out and blew a fuse or transistor/ thyristor when they failed.

Also possible transistor / thyristor has blown ( smaller rectangular black or dark grey component with three wires in Aline passing out of the underside and usually an aluminium heat sink on one side) Sometimes this will show as darkening or burn marks but does not usually cause smoke. It would probably show as having one side face ruptured if there was a bang.

Like the transsistor a diode could also have failed – typically a black cylindrical 'fat pellet' shaped component maybe 5mm long and 3mm diameter with a single fairly thick wire out of each end – often grouped as a bank of 4 components. Again expect signs of having ruptured if there was smoke and a bang.

If you think you have found the suspect post a photo for further advice!

[Martin WParticipant @martinw]

Hi

If you can get somebody to help or have at least have look at it then there are some circuit diagrams here which give the part numbers for the semi conductors and values for the passive components. It would appear that there are two variants one with a very simplistic speed controller and the other a much more sophisticated controller.

Looking at your picture I suspect that you have the former and the rectifier appears to be a bridge rectifier which is probably a fairly squat square unit with 4 wires, at minimum, attached.

Again if you can give us a general indication as to where you are then I suspect will be willing to help. If it's any use I live in Berkshire in the Reading area.

Good luck and hope you get it fixed soon.

Martin W

[Bill PhinnParticipant @billphinn90025]

 

Many thanks to everyone for your replies. It's very generous of people to be prepared to look at it in person. I live in Manchester. If it came down to it, I could put the (very small) machine in the back of my car and do the travelling myself. Incidentally I did try several times to add my location to my profile, but, in spite of pressing the save button each time, this change appears not to want to take place.

I've added some more photos here and in my album, which hopefully convey a tiny bit more information to the initiated.

I have tried to extract the circuit board with attached wiring from the housing so that we can see what we're dealing with, but the speed control shaft passing through the wall of the housing is preventing me from lifting things upwards and I can't see any easy way of removing the external knob from the shaft (I've tried gripping the shaft with needle nose pliers while pulling the knob away from it, but it is stuck fast). I can't fathom out another way of getting the circuit board out at present short of desoldering the connections.

I can't see any obvious blackening of internal components; only the copper windings glimpsed though the cut-aways in the motor body looks quite dirty, but this may just be accumulated dust.

Any ongoing advice is greatly appreciated.

Edited By Bill Phinn on 31/01/2019 01:36:22

[Michael GilliganParticipant @michaelgilligan61133]

Looking at your detail photo, Bill … I would say it is almost certain that the control knob just pulls straight off the splined shaft. [ Although the plastic surfaces may have stuck together over time ]

Given the easy access to the motor terminals, I would be tempted to just touch them with a suitable DC voltage and see if the motor turns. … Wise electronics experts may disagree of course !!

MichaelG.

[Bill PhinnParticipant @billphinn90025]

Inspired brutality, Michael. Thank you!

[John RuddParticipant @johnrudd16576]

Bill, if you can disconnect the wires from the motor and connect them to a domestic light bulb ( it must be an incandescent type, not led or cfl) you can test the functionality of the speed control board…the bulb's brightness should vary with turning the pot….

With a 12volt supply ( battery, bench power supply etc..) connected to the motor, the motor should spin freely….

Edited By John Rudd on 31/01/2019 08:43:17

[Andy CarruthersParticipant @andycarruthers33275]

Stating the obvious here…

Have you checked the fuse?

I can help diagnose, am near Scunthorpe…

[John HaineParticipant @johnhaine32865]

There is nothing obviously burned out on the PCB. Michael's suggestions are sensible. But I'd start with testing the motor, if that turns (only slowly on 12V) then maybe resign yourself to a new controller. There are lots of controllers for these high voltage DC motors on eBay, such as this one:

f:1&frcectupt=true”>https://www.ebay.co.uk/itm/NEW-AC-220V-500W-Knob-Motor-Speed-Controller-DC-0-220V-Adjustable-Lathe-Control/292937976391?hash=item443476ca47:g:c2gAAOSwhzhcT7b5:rk:5f:1&frcectupt=true

[SillyOldDufferModerator @sillyoldduffer]

Here's the circuit diagram, which I found here:

A bang with smoke often makes diagnosis easy because it leaves obvious damage, so it's odd I can't see any. Possibly because it's one of the components with a cover – like the chokes L1, L2 and L3.

My guess would be L1 because it's completely hidden inside the octagonal black case marked CMC/14881. But L2 and L3 are also candidates, not least because they might be designed dual-purpose as fuses as well as EMC filters. Easy enough to check continuity of these with a multimeter if you have one, or a kind person steps in.

Looking at the spec of the Proxxon, it's clearly not meant for heavy work!!! Looks as if it's for modellers specialising in plastic, wood or chaps working on PCBs.  I think it would be quite easy for a metalworker to push it too hard, even with brass.

Dave

 

 

Edited By SillyOldDuffer on 31/01/2019 10:49:35

[Stuart BridgerParticipant @stuartbridger82290]

In my experience, a choke is the least likely component to fail. Its a passive component, basically a coil of copper wire. Not saying it can't fail, but I don't think I ever replaced one during my time in electronic repair. Much more likley to be the the Triac or the bridge rectifier.

[Martin WParticipant @martinw]

Again here is another site with the circuit diagram plus a picture of the pcb identifying the components. Dave beat me to posting as I was rummaging through other pages on the net looking for any other data/information, at least it is not the same site as he has identified.

Martin W

[SillyOldDufferModerator @sillyoldduffer]

Posted by Stuart Bridger on 31/01/2019 10:40:01:

In my experience, a choke is the least likely component to fail. Its a passive component, basically a coil of copper wire. Not saying it can't fail, but I don't think I ever replaced one during my time in electronic repair. Much more likley to be the the Triac or the bridge rectifier.

Yes that's generally true, but I was focussing on what might explain a bang and smoke without visible damage. I think it likely that L1 is wound on a toroid (ie making live and neutral windings more stressed because they aren't on separate bobbins.) Also, note my suggestion that L2 & L3 might have been dual purposed as fuses.

Main reason for mentioning the possibility is that coils are easy to check with a multimeter.

I wouldn't bet the farm on my suggestion though – the rectifier and triac have got to be high on the list, as is the motor itself.

Dave

[John HaineParticipant @johnhaine32865]

The circuit diagram confirms the motor is a 220V model. Simple speed controller for these are widely available on eBay, as in the link I gave. If the motor is OK surely better to just junk the present controller, bring out a cable from the motor and use a new controller? If the motor isn't OK then bets are off!

[Mike PooleParticipant @mikepoole82104]

A very close inspection can be required to find the bang and smoke source, my NAD amplifier went bang and poured out smoke. After taking it outside to clear the smoke I opened it up and could not see any signs of damage. Eventually I found the capacitor across the mains on the power switch had all but disappeared. The amp powered up and is still working fine, I suppose I should replace the cap but I rarely use the amp these days. I have also known semiconductors split and let the smoke out but the only evidence is a crack in the plastic or ceramic package. I was once inundated with weld timers that all had extensive component and track damage, it turned out that the temperature sensor for the weld transformer had gone down to a phase on the weld transformer and they tried 7 timers before tracking the fault down, all the timers lived again but 12v cmos does not like 240V.

Mike

Edited By Mike Poole on 31/01/2019 11:41:54

[DalboyParticipant @dalboy]

Posted by SillyOldDuffer on 31/01/2019 10:25:34:

With the bang and small amount of smoke normally it is a sign of a capacitor gone. The normal one to go first is the C1 in the diagram above so check that first. Another point that fails is where the mains wire enters the machine if the wire is moved about on a regular basis the inner wire can break inside and have been known to cause problems to a test on the wire while moving it near the housing will give an indication of a failure. And yes it will go with a bang if it shorts there.

Always start from the plug and work through the circuit from there that way nothing is missed

Just as a background I use to repair electric tools for a living

[John Paton 1Participant @johnpaton1]

Good photos Bill but no obvious signs of smoke in there, nor of brass swarf having got into the motor /control housing.

I would agree with Mike and Derek – look at the capacitors first – starting with the big 'yellow brick' (C1 I believe) but I suspect if this has died it might not be easy to test as it will probably have blown 'open circuit' and capacitors are not easy to test without the right meter even when removed from the circuit. If you can unsolder them and have a look at their undersides there might be evidence of damaged casing or the smell of smoke.

The next items I would look at would be the bridge rectifier D3 – the black brick mainly because it is easy to test while in circuit – between each pair of pins there should be a resistance one way and short circuit / 0 ohms with the test meter probes connected the other way round (opposite polarity) . No harm checking the inductors L2 and L3 which should read as, effectively, short circuit (0 ohms) on a multimeter continuity test.

At first glance the rating of D3 at 1 amp looks a bit light for a lathe so that raises the thought that this might b a vulnerable component. That said the motor should only be drawing 0.5 Amp so strictly speaking the rectifier is not overloaded.

Triac also needs taking out of circuit and a slightly more specialist test instrument to check it.

Having got the board out of the housing can you smell smoke on it anywhere?

This is a very simple circuit and if you can find someone with even fairly basic electronics knowledge and a decent test meter they should be able to locate the fault and source replacement components.

[John Paton 1Participant @johnpaton1]

Oh, I should add that I do not suspect motor failure as that would normally die relatively slowly (lose power over a few seconds) as opposed to going with a bang which sounds like instantaneous failure.

[Bill PhinnParticipant @billphinn90025]

Many thanks to everyone for your helpful replies.

I replaced the fuse at the outset with a new 3A and then a second new 3A just in case. No joy.

The socket outlet the machine was plugged into is working normally, at least as evidenced by its ability to power other devices.

I connected a 100W incandescent bulb to the motor terminals, connecting the negative terminal to the screw fitting on the bulb and the positive terminal to the foot contact. No joy.

I don't have a 12v battery to hand; my drill batteries are all 18v. I've tried using a 1.5 volt AA battery in the hope that this might produce a flicker of movement in the motor but there was no sign of any movement.

I'm afraid I don't have a multimeter and would be pretty clueless anyway about how to test individual components on a circuit board.

If there is anything obvious I might be missing and that my almost non-existent electrical knowhow might allow me to explore, I would be grateful for the suggestion.

[Bill PhinnParticipant @billphinn90025]

 

Posted by John Paton 1 on 31/01/2019 14:16:58:

Oh, I should add that I do not suspect motor failure as that would normally die relatively slowly (lose power over a few seconds) as opposed to going with a bang which sounds like instantaneous failure.

John, I don't know whether it's relevant but over the course of about one minute before the machine actually terminated, the motor stuttered a little and an intermittent gritty sound, lasting only one or two seconds each time, came out of the housing before the motor picked up speed again as normal.

Eta: Derek, there is no apparent sign of anything defective with the power cord where it enters the machine.

Eta no.2: John Paton, I can't smell smoke on the circuit board now but there was certainly a strong smell of burned something or other when the failure occurred.

Edited By Bill Phinn on 31/01/2019 14:59:34

[John HaineParticipant @johnhaine32865]

Usually when something on a circuit board fails like that there is a strong acrid smell that doesn't dissipate, so I fear that the motor may be shot. Can you smell something from the motor itself?

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