Electronic sewing machine question

[scrubba]

My machine won't power up. I've inspected all the circuit boards and don't see any obvious suspects. When I press the on/off switch, and snoop around with an inductive voltage tester, it indicates there's voltage present everywhere. But, it doesn't turn on. What is this newbie missing? Any tips would be appreciated. Thanks for reading.

[Jay Kominek]

Exactly what does it mean for it to not be "on", in this case? That's perhaps obvious to someone looking at it (or, who even knows what model you've got), but is hard for us on the other end of a forum to discern.

No power light? Nothing on the LCD? No motor in response to the pedal?

I think my mom's 80's sewing machine didn't have a light, and certainly didn't have an LCD. So the only way to know it wasn't "on" would be the that nothing happened when the pedal was pressed. Which could be the result of a fault in the pedal, and have nothing to do with the power.

[scrubba]

All of the above ... no lights, LED's, no motor response to pedal. Normally, this machine lights up when it is turned on. FYI, machine is a Bernina 1260. The pedal has been tested and works on another machine. Thanks for your response.

[doktori]

A quick check I use on 'dead' electronics.  Use a small portable AM BAND (not FM) radio tuned to a quiet portion of the band.  Hold it near the display of the sewing machine and turn on the machine.  If you hear a whine then there is some life somewhere inside.  If there's no noise - start checking the outlet, power cord, switch, fuse (if any) up to the power supply.  If the power supply is receiving power.... well it could be so many different things.  Good Luck!

[scrubba]

Thank you doktori for your suggestions ... I do seem to have current flowing through all the circuitry as indicated by my inductive voltage tester, but for some reason the machine does not power up, cord, switch and fuse are all okay.

[james_s]

An inductive voltage tester is useless for this sort of thing. You're going to need either a multimeter and a little basic electronics knowledge or an experienced tech who has the tools and knowledge to diagnose the problem. Most faults have no visible symptoms, without a mutlimeter you are flying blind and so is anyone who might try to help you. Keep in mind that if you don't know what you're doing it's not hard to turn something that was easily repairable into a write-off.

[andy3055]

Here is a link to the 1230 service manual. It might help you to get an idea as to how the 1260 works.

https://www.manualslib.com/manual/1069892/Bernina-1230.html

[scrubba]

Thanks james_s for your thoughts ... your point is well taken. I'm a mr. fixit kind of guy who's in over his head this time. The machine had been given up for dead,  I was just about to throw in the towel anyway.

[SilverSolder]

Maybe you can buy a broken one from eBay (one that has a different fault) and make a good machine out of the two?   

That kind of approach has worked for me with many "in over my head" projects -  it is usually pretty easy to swap entire modules / boards around.

[james_s]

Thanks james_s for your thoughts ... your point is well taken. I'm a mr. fixit kind of guy who's in over his head this time. The machine had been given up for dead,  I was just about to throw in the towel anyway.

Well you could buy a multimeter, if you're a "mr. fixit kind of guy" then it's a sound investment that you will find other uses for it. You don't need a $400 Fluke, there are meters in the $20-$40 range that will do everything a typical handyman/shadetree mechanic/casual hobbyist needs. My multimeter is one of the most used tools I own, after a screwdriver it's the first thing I grab when anything electrical has a problem.

[scrubba]

james_s, thanks for your suggestions. Actually, I do own a multimeter and a limited amount of electronic test equipment ... my hobby is restoring old tube amplifiers. But, those are all point-to-point wiring, troubleshooting is pretty straightforward, and there are many sources of helpful information. The circuitry on the sewing machine is not so simple, and there isn't much info available online, especially for the model I'm working on. Even a used circuit board for my model has been impossible to find ... 'll keep an eye out for a used machine .. thanks again.
Cheers!

[james_s]

Well if you can troubleshoot a tube amplifier then you can troubleshoot this, I assume it's going to be a switchmode power supply so I would start by checking for voltage on the main bulk filter capacitor, then check for voltage on any of the outputs, and if there is power on the capacitor and the outputs are dead then I would look for the resistor that supplies startup power to the switcher IC. It's common for these to go open and/or a capacitor involved in startup to fail. Most small switchmode supplies use roughly the same topology. Can you post a picture of the board?

[scrubba]

wow, thanks for your diagnosis ... I'll try to identify the components you've mentioned and do the checks ... I'll also post a pic ...

[Kleinstein]

This looks like a SMPS and likely a little more, like a motor controller an the primary side (the brown, black and white cable).

So it is not that easy, but at least it is not all potted of covered in a thick layer of conformal coating.

The obvious part to test is the fuse. If blown check the rectifier diodes for a short (can be done in circuit).
A point to test would be if there are voltage on the output side large capacitors (e.g. yellow axial one). Chances are there is no voltage and thus no power to much of the machine.

[scrubba]

Thank you for your input ... will try the tests you suggest.

[james_s]

What's that IC there? Is that a UC384x? A trick you can use when you can't find a schematic for the equipment is look for a datasheet for the IC, in the case of a power supply it will usually be similar to the example in the datasheet.

[scrubba]

Thanks again james_s.  The IC is a UC3844P, I'll look for a datasheet on it. There seem to be insufficient voltages on the capacitors and outputs, but I'm not sure what they're supposed to be without a correct schematic. But, they do seem low compared to the specs of a similar machine ... e.g. 1.5V where it should be 5V or 30V.
Thanks for your advice and patience.

[james_s]

Yeah that's a UC3844, all of the UC384x series are current-mode SMPS controllers, there are minor variations across the series but they are all roughly the same thing and they are extremely common in small power supplies. You should be able to find a datasheet easily that will have an example circuit that is likely very similar to what you have in your sewing machine. You need a voltage of around 18V or so to bootstrap the IC and get it running so that it can power itself from its own output. Usually this bootstrap supply is just a resistor from the input DC bus.

[shakalnokturn]

Some things you may want to consider:

Removing power supply PCB to check solders.

Disconnecting connectors to other PCB's and rechecking voltages on power supply. (A dead [ULN2003?] stepper motor driver could bring the power down.
I think the correct voltages are given in the service manual.