Refurbishment in Progress .. .. .

Hi Guys

I recently acquire a very small precision lathe to add to my workshop equipment, paying a very reasonable price because of a number of declared faults, most of which are mechanical and a matter of adjustments.
However, as is usually the case the Direction and Speed Control board is also faulty; It will drive the motor CCW, but neither CW or any speed control are functional.

Here's the board .. .. ..



As you can see, TDC amidst the big Transistors is a well barbecued resistor ? maybe ?
It still has a resistance @ 1K, as does it's near neighbour which is in perfect condition. The circuit is difficult to follow but each resistor appears to track one side to the two pale blue components adjacent to the potentiometer, and the other side to the pushbuttons; the top pushbutton is CCW, the bottom is CW. The middle is STOP.

I thought it might be worth replacing the barbecued item with an identical component, but I'm struggling to identify it .. .. .. Too many coloured rings for me .. .. .. .. .



Anybody decipher the markings for me please ??

Thanks

MM

The unusual color codings really piss me off; the learned from decades ago does not apply.
The 1K readings are for any of the resistors in circuit or with one pin lifted ? How difficult to chase a factory schematic or to redraw one, at least for the pertinent portions ?
Good educated guessing tells it is a resistor, 1070 Ohms / 1% tolerance / 50ppm degreesKelvin
They now need to add another bands for power dissipation, for material if carbon, metal film and whatever else to end as confetti.
I would pick select among many whatever gets closest, ordering qty one they will want to sell it for $9.99999 plus $19.99999 shipping + insurance + handling + tax plus whatever they want added

Hi XT

Yes, I tested with one end lifted @ 1K.
I've tried contact with the manufacturers for a schematic, but they've ignored me so far.
I can certainly trace and redraw, but it's quite a complex board, tracks disappear under components so it would be a DMM job which will take quite a while .. .. ..

Thanks

MM

Musicmanager said:

Hi XT

Yes, I tested with one end lifted @ 1K.
I've tried contact with the manufacturers for a schematic, but they've ignored me so far.
I can certainly trace and redraw, but it's quite a complex board, tracks disappear under components so it would be a DMM job which will take quite a while .. .. ..

Thanks

MM

Click to expand...

I would have said it's VERY, VERY unlikely to be available - the boards are usually bought-in, so the manufacturers probably don't even have a schematic for them. Servicing will be just replacing the board, as with washing machines etc. Even TV service no longer come with schematics, same thing now, board replacement only.

I have only found a couple of resistor manufacturers along decades revealing their body color meaning (light blue, beige, light grey) ; cannot tell what your light grey may be.

Replacing the board still poses its risks as external components (motors, actuators, wiring, switches...) can again burn/damage components.

Try confirming what peripherials are healthy and what can again destroy a replacement/repaired board.

What type of motor is it?.

My GUESS would be that one (Or more.) of the 4 mosfets have failed with a short between source and drain. (the 4 mosfets are all about 100 volts 4 amp rating.)
Is the motor direction controlled by the board or is there a DPDT reversing switch between the board and the motor ? Is there a transformer between the mains input and the board ? (I suspect this to be the case as the mosfets are not high enough voltage rating to work at mains voltages.)

Les.

If everything fails, another chance ? :
----> https://www.aliexpress.com/i/33046105815.html

Externet said:

If everything fails, another chance ? :
----> https://www.aliexpress.com/i/33046105815.html

Click to expand...

Nice find

Hi Guys

Thanks for the input .. .. .. ..

I also thought of the unlikelihood of a schematic available at the manufacturer, but, with my luck, if I didn't ask the question they would have been the only manufacturer in living history to offer free and uncumbered access to their files !

It is a very simple machine in many ways ... mains power into 115 - 115 / 28 - 0 -28 encased traffo, a bridge rectifier and a smoothing cap with a discharge resistor across the terminals, supplying approx. 40vdc to the circuit board.
The IC on the top rail of the board and left of the pushbutton is a 7812 V. Reg supplying the IC's on the LH & bottom edge of the board. Those IC's are a mixture of buffers & Op Amps.

The Motor in the user manual, is described as 42v, 60 Watt .. .. ..
I cannot see anything of the motor to check, it's completely encased in a tin shroud. It can be removed but progress on that is held up by a nylon spur gear that is stubbornly adhered to the shaft; I have the gear enclosed in a polythene bagful of WD40 & IPA in the hope of persuading it to fall off !

Les.. .. the suggestion that the 4 Mosfets have failed at some time is good, because the solder pads were coated with a lot of discoloured flux ( unlikely the rest of the board ) they clearly have been replaced recently.

The motor direction is controlled by the white pushbuttons, the middle button is STOP.

XT .. thank you for the find .. .. .. I did look at something similar as an easy alternative if I can't crack the problem.

I'm grateful for your help guys, TY

MM

This is what I'm working on .. .. .. . very small, 17 inches long, and quite precise .. .. .. .




MM

Morning

Bit of a progress report .. .. .

I replaced the toasted resistor with a 1k - 2 watt version, to give me a bit of warning if it was heating up; I put a pair of LED's, red and green, and a 1k CL resistor with opposite polarity across the output terminals and applied 15vdc to the input pins.

The pushbuttons for CW & CCW worked OK but, I then adjusted the speed control pot resulting in fishy smell, no smoke, but then only one direction button worked.

I switched off and left it for a while, tried again, same result .. worked for a while then back to one side only. I can't find any significant heat, but something is warming up I suspect.

The chinese replacement board is looking more attractive by the minute ( Yes AG, I know it'll be full of rice ! )

MM

Hi MM, I don't think the speed controller board suggested by Externet is suitable. It is designed motors that require about 180 volts DC as used an larger lathes. I think it would be worth checking the 4 mosfets fo source to drain shorts without removing them from the board.

Les.

Musicmanager said:

I have the gear enclosed in a polythene bagful of WD40 & IPA in the hope of persuading it to fall off !

Click to expand...

I'll keep my eye on eye on the news, see if there's any reports of a big fireball

Be aware that upsizing R wattage sometimes not prudent. Some designs of
equipment used them as "fuses" so if they overheated they typically
failed open. Note carbon comp have a negative tempco which aggravates the
increase in current thru them. In this practice - tempco a good thing. Not an
encouraged practice on the part of designers to not use real fuses, just a fact
of life.

The blue components, w/o seeing closeup I have some that are caps. Maybe part
of a snubber the 1K and the caps, just a guess. If circuit is a snubber upsizing the R
wattage fine.


The NMOS and PMOS MOSFETS are 100V, I would have thought thats a little marginal in a
motor running at 40V. Maybe an experienced power designer on forum can comment on this.
And if snubber failed, the caps in particular, or the snubber R shorted, that would have
aggravated this problem. Cap fails, Pdiss in R rises, V transient on MOSFETS rises, blow
MOSFET gates, a cascading effect.





Regards, Dana.

There are some dipped tanatalums on the board, known for failures in
aged equipment.



The smallest one looks distressed in pic....?

They are polarized so be careful when replacing one if you do.


Regards, Dana.

Hi Guys

Les Jones said:

I don't think the speed controller board suggested by Externet is suitable. It is designed motors that require about 180 volts DC as used an larger lathes. I think it would be worth checking the 4 mosfets fo source to drain shorts without removing them from the board.

Click to expand...

Yes, I think I agree with that .. If I go for a replacement I will make sure of suitability.
I'll test the Mosfets in due course.

Nigel Goodwin said:

I'll keep my eye on eye on the news, see if there's any reports of a big fireball

Click to expand...

Oh Ye of little faith !
I was able to gently ease the spur gear from the motor shaft around 6 am this morning, after an overnight soak; it was only a small bag with a limited quantity of liquids, so only a small fireball Keeps the neighbours on their toes !!


Dana - thank you for that. I wouldn't leave the oversized resistor in place for long; It was just to give me a margin to detect the heat before another barbecue occurred.
The pale blue components are MKT capacitors; only set of markings are visible and the value is unreadable, but they could be replaced; I probably have some in stock.
Whilst typing this your message about Tantalum tantrums has arrived .. I'll check .. TY

Lots of help again, appreciated TY

MM

One thought, as parts are replaced the PCB traces get weakened somewhat in the area
of soldering, pads you are working on.

I often clip leads off old component leads and solder in new one to the old ones cutoff lead
stubs, to avoid PCB trace lifting.

Keep leads short as before to minimize lead L which loves creating transients.

Just a thought.


Regards, Dana.

Afternoon

I've done some Mosfet testing and there does indeed seem to be some leakage .. .. .. .Dana made a valid point about Tantulum Tantrums which is leading me to consider that's a considerable de-solder / re-solder operation, if indeed I can find the right bits.

I'm getting closer to a replacement board I think .. .. .. . any help with identifying a good choice would be appreciated.

Thanks

MM

Musicmanager said:

Hi Guys

I recently acquire a very small precision lathe to add to my workshop equipment, paying a very reasonable price because of a number of declared faults, most of which are mechanical and a matter of adjustments.
However, as is usually the case the Direction and Speed Control board is also faulty; It will drive the motor CCW, but neither CW or any speed control are functional.

Here's the board .. .. ..

View attachment 136951

As you can see, TDC amidst the big Transistors is a well barbecued resistor ? maybe ?
It still has a resistance @ 1K, as does it's near neighbour which is in perfect condition. The circuit is difficult to follow but each resistor appears to track one side to the two pale blue components adjacent to the potentiometer, and the other side to the pushbuttons; the top pushbutton is CCW, the bottom is CW. The middle is STOP.

I thought it might be worth replacing the barbecued item with an identical component, but I'm struggling to identify it .. .. .. Too many coloured rings for me .. .. .. .. .

View attachment 136952

Anybody decipher the markings for me please ??

Thanks

MM

Click to expand...

If you can measure the good resistor to determine the value, we might be able to decipher the color code.

Can you take a good snapshot of the back of board?

A snapshot of the top with better lighting might help also.

Is there a PCB part number somewhere on the board?