Chinese Minilathe, she die , muchous sorrow :(

[DeadLatheOwner]

7x12 Chinese minilathe.

Managed to strip a tooth or six off the hi/low gearbox. Got replacement gears, stripped it all down, started putting it back together. Fitted the motor, connected it back up to control box, switched on.

Slight "PHHHUUTT" from somewhere inside the motor or control box. Motor does nothing now, does not respond to control box.

Inside is a controller board and a daughter board that has the display plugged in. Ive been round with a multimeter, both boards have power, at various points, except the motor connector, fuse ok, mains light is lit, power on switches and speed pot. The display boards works, display lights and responds to chuck being turned by hand. Motor has continuity by an ohmmeter across brushes and across the connector plug when shaft is rotated. The two SCR's and three power rectifiers have been replaced by S8010 and D8010 devices (800v 10A isolated tag TO220 devices). You can here a relay clicking when you flick the reverse switch.


This is the board:

SO, im pretty much stuck now, and tending to the idea the board microelectronics, ie one of the tiny chips on board, is kaput.

Anyone got any ideas, or a plan B, apart from £85 for a new board?

 

[Ken I]

Normally if you hear a PFUTT - you will normally find some evidence of the blow up - black mark on the board or pinhole in the device that blew - go over every part with a magnifying glass.

Sorry I don't recognise that board and can't be specific.

Ken

 

[DeadLatheOwner]

Yes, cant see anything tho, no smoke, no scorching, no pinholes, nothing.

this is what the end says:

ive done some googling, seems to be used in a lot of small workshop machines as well as lathes, eg millers.

Im baffled, theres nothing visible and the only likely devices (ie the power switching seminconductors ive replaced.

 

[John Rudd]

I have the circuit diagram for the KBIC controller that looks similar to yours....

I can mail you a copy if you want it, if you think it will help you...

Check the power resistor to make sure it isnt open circuit...That is used to sense the load current through the motor and it is o/c the no current to the motor...

 

[Don1966]

The only thing I can suggest is to check the voltage reference across your speed pot to see if it is there. If it is not there one of your trimmers could be burnt. It would have to be the maximum speed trimmer or it would go wide open if mininum speed pot was open. Or the reference Zener diode that feed the pot, and you should be able to track it if you get a diagram. Other then that is your circuit it self, see if the chips have voltage on them to make sure the power supply is functioning.
I want to correct about the power supply, I should of said the power conditioning circuit that gives you the 12v,15v or 5v to operate the circuit.

Regards don

 

[fcheslop]

The only things i can suggest are why did it go phut in the first place is the wiring correct ? is there any voltage going to the motor? with and without the motor connected ? are the output devices being switched on.Some times its better to start from the begging again
best wishes

 

[John Rudd]

I've emailed the schematics of the KBIC controller to the op...

The motor can be substituted by using a 100 watt 240v lamp...which is better for testing the speed controller rather than using the motor ( which would initially appear as a dead short on the output, whereas the lamp offers more cold resistance)

 

[DeadLatheOwner]

The only things i can suggest are why did it go phut in the first place is the wiring correct ? is there any voltage going to the motor? with and without the motor connected ? are the output devices being switched on.Some times its better to start from the begging again
best wishes

Ive no idea what went phut, it was very quiet, and there no trace of carboning anywhere.

Theres no voltage to the motor off the board connectors, ive checked continuity thru the motor and lead, its ok.

I think the power bridge cant be receiving analogue control signals, but ill check with a voltmeter tomorrow. Having seen a circuit diagram now, my money is on the quad op-amp being u/s.

 

[kf2qd]

Check the brushes. If the brushes aren't making good contact the motor won't run, or run erratically. And it is a whole lot cheaper that replacing a board that might not be bad.

 

[n4zou]

Not long ago the starter on my motorcycle started acting erratic. At first it acted like the battery had gone bad. After replacing the battery I found the problem was still there. I pulled the starter motor apart and found a small burr was preventing the brush from pressing down on the commuter.

 

[steamer]

Hey Deadlatheowner,

Could you just repair the board?....assuming it's the board...It appears you know your way around electronics is all.

Following along

Dave

 

[DeadLatheOwner]

Hey Deadlatheowner,

Could you just repair the board?....assuming it's the board...It appears you know your way around electronics is all.

Following along

Dave

Well i used to to TV and Radio repairs in the 50's/60's/70's, when it was all valves, and few transistors, and i also built a few bits of disco equipment, light displays using SCR's/Triacs and 74 series logic, and i was a radio ham, but my involvement in electronics finished about 1975, havent picked up a soldering iron since, i went off into mainframe computing, COBOL programming and the early days of the internet for the next 25 years.

I dont see how you fix this stuff, apart from changing the power devices, everything else is too small to register on my vision these days!!

 

[Don1966]

You have had experience with circuits in the pass don't let these circuits intimidate you. The bridge control circuit uses a reference voltage to control it. Make sure it is there and your power conditioning circuits make sure the voltages are on there and getting to the chips (Reread my first post), you know your motor is good , you have no voltage going to it, you here a click when you turn it on. It like riding a bic you don't forget.

Regards Don

 

[John Rudd]

If you have access to a variable dc power supply, apply 22v to the board at a suitable point. Looking at the KBIC diagram I sent, this would be easiest at C19. Z2 is a 22v zener,this clamps the main rail.If this voltage is incorrect check the zener, if the voltage is low or missing then suspect the capacitor C19 or R17a/b or the two diodes d11/D12. Having an oscilloscope would help too..Check for pulses at the collector of Q2..This is the SCR driver...If you see pulses there, they should vary in frequency when the speed pot is altered.

You can check the operation of the board by loading with a 100watt light bulb..similar to testing the power supply in a tv ( I used to repair tv's and vcrs many years ago ) if all of this appears ok then the fault must be with either the external wiring or the motor

 

[lordedmond]

before you go any further please check that you have not pinched /nuded a wire somewhere when you put it back together from my past experience with these new fangled do dah's that the slightest fault to earth spells disaster,
it seems to me to be a coincidence that the tiny amount of magic smoke was released at the same time

please do not take this the wrong way

the wiring needs be proved before any replacement work is done else you will be bad to square one


get a 10 amp 400 v bridge rectifier wire it up with a 100 watt incandescent lamp in series with the motor to prove that

Stuart

 

[DeadLatheOwner]

Yes, i checked the mains cabling, there IS a pinch which has split the outer sheath but theres no copper exposed anywhere, so it wasnt that.

I did think of putting a 100w bulb in as a dummy load, ill do that next and go round with a volt meter. There isnt a lot or room to work in the control box, might have to pull it all out the box so i can see whats going on.

Ah an oscilloscope, i havent seen anyone use one of those for years. I bought one of the Heathkit kits (who remembers them?) when i was about 14, it cost me £75 which was a colossal amount in late Middle Ages when i was a lad!! It worked second time on assembly, first time was a dry soldered joint.

*edit*

That set me off on ebay, just to see what 'scopes fetch these days, and OMG theres an actual Heathkit OS-2 on sale, this is EXACTLY the one i bought and built at 14!!!!

http://www.ebay.co.uk/itm/Faulty-Vi...t_Measurement_Equipment_ET&hash=auc35bc16e850

i was impressed by THIS:

pocket scope, 3 volt, 3 inch LCD display, £45, holy moly, one of those back in 1965 would have looked like Star Trek come alive

Sorry, this is turning into a nostalgia thread [/oldfartmode =OFF]

 

[DeadLatheOwner]

ok, had a poke round with a multimeter.

Daughter board seems to be ok, the 240v into the transformer, 12v and 5v as per circuit diagram.

On the power bridge. theres 93v acrosss A-K on the SCR's, indicating they are not triggered. Theres 238v A-K across the other two power rectifiers, also indicating they are not conducting.

There thus appears to be no signal to the thyristor gates to make them trigger. There is no inhibit voltage on I2, so the SCR's should tigger in the presence of a trigger pulse.I reckon the quad op amp that drives the trigger transformer is as the technical term goes, shagged. That or Q3, the driver transistor.

 

[krv3000]

well its all a black art to me i will just stick with me drive belts

 

[DeadLatheOwner]

Im pretty convinced now that the op amp or Q3 has died, so i think thats about as far as it goes, replacing them is a non starter, looks like £85 for a new board

the lathe has been used for a couple of years to make brass reproduction meccano parts, and the swarf off the brass comes off in tiny chips less than 1mm long, almost like coarse dust, I reckon at some point in maneuvering the lathe about rebuilding it, some brass swarf has fell onto the board, causing a low voltage short thats killed the chip or the transistor.

 

[lordedmond]

now that does sound a plausible cause , hard on the wallet though


a good clean out and some thoughts on sealing the enclosure from that nasty brass swarf


its bad enough when its in your fingers but deadly on a pcb


if brass is your normal material I would consider remotely mounting the new controller board , if you screen the control wire it should be ok

Stuart