How do digital sewing machines fail? Is there often a visuall clue on the circuit board?

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Could be anything from a broken wire to a so-called repairer induced problem and anywhere in between.
Basically much like how long is a piece of string.

 

Appliance circuit boards is a category too broad to generalize about. Some appliances may stay always powered and be more subject to AC power line surges. Some handle water or cooling/condensation and could have eventual water damage. Some heat up so eventual heat damage. Some are just made to the lowest price point possible and could have the basic old capacitor failure issues that any SMPS powered device has.

Then you have each design in a class of appliances, having it's own weaknesses which means unless you are going to research failure modes most common to the particular make and model (or closely related models) of appliance, it is more of a take it on a case by case basis by observing symptoms.

Before buying it I would at least check to see if any parts are available. What is the evidence it has a "main logic board failure" opposed to a switch, sensor, power supply to the board, etc?

No it is not likely to just be a burnt out resistor or diode on the logic board. Possible, sure, but I doubt much in the way of power resistors under thermal stress and a diode failure would be more likely in the power supply to the board rather than the board itself which might be more subject to a transistor failure but again it would be an investigation followed by the evidence of missing functionality and measurements with a multimeter.

It seems a bit like jumping off a cliff to me to buy this widget without already having a good idea what is wrong then hoping we can remotely assist in repair, is a bit different when someone already owns a widget and they're already invested vs throwing it away.

 

Well, you supplied next to zero info so you get what you came for, an answer as broad as your query.

 

You could at least give the brand and model number of that unit .

 

Sir MidAtlantian . . . . .

Any chance that this is being the PCB that you are finding residing within that unit of yours . . . .if so the problem is probbaly the TO-220 cased power semiconductor device located at the far left center . . . in its being shorted.
Pass on to us, all of the numbering / I.D. being atop it .
I think that I am reading the dual comparator IC as being of an LM 193/293/393 series
Along with one or both of the discrete xstrs being BC547's ?

The units motors go fast - go slow circuitry . . . .




73's de Edd . . . . .

If we actually are what we eat . . . . then, I'm definitely . . . . FAST, CHEAP and EASY.

 

Sir MidAtlantian . . . . .


The most likely failure would be the failure of that units IRF620 Power FET, with a shorting of its drain to source . of .pin 2 to 3 as per




Or possibly the glass bead diode just above it, or the blue paint tipped disc ceramic capacitor / or /MOV, just to its right side . . . . . or lastly, the conventional black cased power diode down just below them . . . . with any one of them being shorted.
I would need a good photo of all of the foil side of the board to confirm any circuitry interconnects.

I suspicion that the LM393 would be the very last suspect.

The 8805 symbolization on the IRF unit is telling us that the part is being of a manufacturing / marking date of the 5th week of year 1988.
Thus, dating your machine as being of no earlier than that date . . .~30 years.

ALSO they seem to be design deficient /close cutting in their choice of that IRF620 unit with it only being a 200VDC spec unit, with upwards of 170VDC coming fron the units FWB power supply from the 4 diodes nearby clustered around the central filter cap .
I would go for its big brother . . .an IRF820. with it being a 500 VDC rated unit.

2SC547 and 2SC557 numberings. . . . go on the 99 and 86/100 percent probability of being good . . . small sig transistors.


https://alltransistors.com/pdfview.php?doc=irf820pbf.pdf&dire=_upd-mosfet


Thaaaaaaaaaaaasit . . . . .


73's de Edd . . .


A man should be greater than some of his parts.

 

Sir MidAtlantian . . . . .

Now . . . . in my making a full implementation of your photos . . . GOOD PHOTOS ! . . . .
The black paired AC input inductors serve bidirectionally . . . both for filtering any possible sporaidic fast rising incoming AC high voltage line spike(s) and also back filtering the AC line feed from RFI-EMI-CRAP generated by this unit.

You unplugged the foot / speed control, so that sort of . . but not totally . . . eliminated the spring loaded foot control from having a mechanical breakage and its resultant swinging into full max . . . or minimum. . . . speed.

Your main point of interest would now be in referring to the mark ups now made and noting that the fused AC power in, feeds into the four 1N5400 series rectifier diodes which are configured as a FWB unit , which store up their produced DC into the BLACK central filter capacitor.
Looking at the top photo . . . the created RED ARROW DC flows down thru the machines motor and thru it and up to the Drain of the power FET and thru it, and out its Source for a ground return path to the power supply.
That is being . . . . . IF . . . . . there is being a variable width pulse train at its Gate, which is being provided by the two discrete driver transistors and the frontal variable pulse generator as being configured within the '393, in conjunction with feed back from the variable resistance of the foot / speed control.

I was kept from plotting the input of the '393 by not being able to see the left connector interfacings.
Currently . . . I'm suspicioning it to be the three little solder dots*** just below the massive black AC POWER connector at the board center left.
*** With them having a mating foot control plug going to the active 3 / or / 2 pins on the other side.

Another thing which could make the unit " dead in the water" would be loss of the sub power supply for the single end configured '393 and the 2 discrete small signal transisters . . . . . .
Those parts . . . . . I will just point out now.. . . textually.

BOTTOM PHOTO . . .

Going back to the now CONFIRMED BLUE disc ceramic capacitor which I had initially, optionally dual classified as also possibly ? being a MOV, instead.
That was just by my having read it at a sloped downward perpective, along with its marking being upside down. Which I then read as being . . . . .
Chinese/Korean/Japanese symbol followed by ? + ? + ? and then a final Chinese/Korean/Japanese symbol.
Or, if I now read again, I am now properly seeing a 4.7 nanofarad. So the unit is NOT being a Metal Oxide Varistor. But being the C element of a snubber network associated with the FET.

OK . . . now just above that cap is being a black diode which takes voltage dropped AC to convert to DC to feed to the BLUE cased E-cap at the top, which is regulated with the ORANGISH /YELLOW zener diode placed just below it.
That supply then feeds your single ended power for the '393 and the 2 discrete driver transistors.
Read across that zener and expect the DC voltage rated spec of the zener to be across it, and that being the sub power supply level / value.

If you want to get a jump on failure analysis, before your parts arrive, you can plug in the unit and reconfirm that it is still running at breakneck speed and then remove AC power and desolder and pull the power FET from the board and then try the board again and confirm that there is SILENCE . . . . with your having opened up the power loop, with no FET, now being in circuit.

Thaaaaaaaaaaaaaaasit . . . .

TECHNO REFERENCING ILLUSTRATION . . . ..




73's de Edd . . . . .


From my wifes perspective . . . IF YOU WANT BREAKFAST IN BED . . . then sleep in the kitchen.

 

Sir MidAtlantian . . . . .

In following relevant circitry traces, had those 3 unconnected "dud" solder domes been electrically interfaced, that would have been my guess for the inputting of the speed control , as the left side of the '393 is where the time constant parts of the circuitry are located .
You gots the unit in hand, for examination . . . . here I am 1,473 miles away and olefactorily challenged, deaf , senseless, blind . . . . and DUMB.
Can you filll me in on specific connections where the speed control wiring ties into circuitry ?
At least you now know that you might have a FET that is breaking down D-S on voltage application or might have a constant HIGH drive level present on its GATE.
To somewhat confirm that, replace the original FET into the pcb and lift the two right side leads of the two carbon film resistors located just above the FET GATE feed trace, that feeds it drive..
Then Retest.
If no wild motor action, the fault is suggesting of a gate drive circuitry problem.
If the motor runs wild . . . a possibly of voltage induced breakdown of the FET D to S.
Also, there is some possibility of the FET electrostatically self triggering, when being with a floating GATE connection.
To confirm . . . . power down and temporarily solder tack in an ~10k resistor between the FETS GATE to SOURCE .
Re test.

RE . . .
I don't really understand how the comparitor (sic) (comparator) fits into the design, except that it must be the source of the pulsing of the voltage applied to the motor, and probably controls the pulse width ?
YOU got it ! . . . . The speed control resistance tells it to create a constant stream of narrow width pulses for slow speed . . .stream of medium width pulses for medium speed . . ya da ya da ya . . . . . . a la . . .

https://www.google.com/search?newwi...73.339.5......0....1..gws-wiz-img.Lig0eE29XqM

73's de Edd . . . . .

If they don't have chocolate in Heaven . . . . . . I ain't uh goin'.