Power supply & air switch on sewing machine

[pintglass]

Hi

I'm trying to repair my mothers 30+ years old sewing machine which she's got quite used to over the years.

At the moment I've only done some basic tests as i think there could be numerous faults.

Model is a Elna Stella electronic air.

The machine is controlled via an air foot peddle which as can be seen in the image is connected to an air switch, sorry don't know the correct term for this device. One of the contacts on this is a bit loose which i will try to fix back down somehow although I'm not sure how I'll achieve this at the moment. But when i tested this i do get varying resistance from the device when operating the foot peddle.

I have about 150 ohms resistance across the motor so assume this is OK for now.

I have tested all the resistors and all but one seem ok, apart from the 3k9 which can be seen in the schematic, this was obtained from the following website.

http://www.diceyhome.free-online.co....lla_refurb.htm

My main question for now is am I Ok to connect my multimeter to the motor spring terminals on the pcb to test or will this be an issue without the motor being connected?

[redwire]

Re: Power supply & air switch on sewing machine

That other site's Elna Stella TSP Air Electronic schematic has many errors. If I had a pic of the backside I can draw a correct schematic. The 3k9 snubber resistor is OK in your pic, not burned.

I would first find out why the other resistor burnt up. Some Stella have a pcb air-flow cutout for more cooling for that resistor. I think it's for current-sense feature for constant-torque to keep up speed level with changing load.
It has a brown stripe so value starts with "1", and "2" in that other website's pics. We'd have to figure out where in the circuit it is.

It looks like a progressive (air pressure) switch that engages more contacts with pressure against the spring. It could be (decimal) 4-levels, or 32-levels (binary). Just pull off the brown terminal and measure ohms across the terminals (brown) should have about 220k steps, starting with 270k perhaps.

[stj]

Re: Power supply & air switch on sewing machine

that burned resistor probably indicates the 0.1uf capacitor is short circuit.

[petehall347]

Re: Power supply & air switch on sewing machine

wouldn't harm to test the diodes

[redwire]

Re: Power supply & air switch on sewing machine

OP if you can look for numbers or a pic of the two small diodes next to the blue cap at the board edge. I think it's a zener diode, not 1N4007. Just need pic of the traces around the blue cap and can finish the schematic.

You can use a light-bulb in place of the motor for testing. The other two pins go to a switch or something.
I found a pic with the original resistor, wow these boards are rare.

[pintglass]

Re: Power supply & air switch on sewing machine

Thanks for all the info

There's a trace layout on the website that i linked to, if your not happy with using that for a reference I can remove the pcb and take photo's.

I will take a look at the board in more detail after work.

[redwire]

Re: Power supply & air switch on sewing machine

It's an action photo with his cutters and soldering iron blocking traces
I couldn't make it out. The right side of the blue cap, no pics on that diode.

[pintglass]

Re: Power supply & air switch on sewing machine

This image was a bit further down his web site.

[redwire]

Re: Power supply & air switch on sewing machine

That helps alot. He mixed up the triac MT1, MT2 and not sure how there are 7 switches in the air control. Two are redundant for safety perhaps. It's a sophisticated speed controller.

The burnt resistor is in series with the motor and triac, and does torque limiting.
If the machine jams and you are heavy on the throttle, it would burn up.

I would say it is a 1k part (use 3W) because I saw that at http://blog.bambersew.com/sewing-mac...machine-repair and you have a brown stripe intact. It is 2k7 in the other website but it must be a different model Elna.

Any other shorted parts (triac, cap, diodes, etc) would just leave the motor stuck running all the time I believe. I'd change the four film caps due to age. The one diode next to the blue cap is a zener I think.

[pintglass]

Re: Power supply & air switch on sewing machine

Could you tell me what caps to get as replacements?

[pintglass]

Re: Power supply & air switch on sewing machine

Could i use a 2w resistor as the 3w resistors are quite long, I only have 15mm between the pads.

[redwire]

Re: Power supply & air switch on sewing machine

For the capacitors, three are across the line (mains), there is no fuse, so they should be X1 safety rated. I don't know all the dimensions but took a guess. New ones are thick maybe.

0.1uF/100nF 275VAC X1, LS=20mm?
The old part lead spacing looks like around 20mm, but the standard is 22.5mm so they will need a little bend.
Do not use paper caps like Rifa/Kemet PME271 series or Wima MP3-X2, MPX21 (clear epoxy) they are known to fail and burn.
Suggest Kemet R49 R49AN31004001K CAP PP FILM 0.1uF ±10% 1000VDC/310VAC X1 RAD, LS=22.5mm, long leads
Kemet R47 R474N310050A1K CAP PP FILM 0.1uF ±10% 1000VDC/440VAC X1 RAD, LS=22.5mm, long leads
--------------------------------------------
0.047uF/47nF 275VAC X1, LS=15mm
Suggest Kemet R46 R474I247050A2K CAP PP FILM 0.047uF ±10% 1000VDC/440VAC X1 RAD, LS=15mm, long leads
--------------------------------------------
0.033uF/33nF 63VDC ±5% LS=10mm? (7.5mm and 5mm are more common) timing cap for speed control
Suggest EPCOS B32621A4333J000 CAP PP FILM 0.033uF ±5% 200VDC RAD
Wima FKP3 FKP3C023303D00KSSD CAP PP FILM 0.033uF ±10% 63VDC RAD

Many other Kemet/Wima/EPCOS parts would be fine.
--------------------------------------------
1k ohm 1/2W carbon, LS=15mm, it should be flameproof safety rated.
Suggest Panasonic ERG-2W
ERG-2SJ102V 1kOhm ±5% 2W 12mm Through Hole Resistor Axial Flame Proof, Safety Metal Oxide Film
-----------------------------------------------
Let me know if the outside diode next to the blue cap is a zener, I think it is.

[pintglass]

Re: Power supply & air switch on sewing machine

Will these be ok

https://uk.farnell.com/kemet/r463i24...ial/dp/2446240

https://uk.farnell.com/neohm-te-conn...ide/dp/1738638

[redwire]

Re: Power supply & air switch on sewing machine

Yes, X2 (2500V) can be used instead of X1 (4000V). They have a smaller size with lower transient overvoltage rating like what you'd could see in a home. I'm used to spec'ing industrial parts.
The 2W resistor is a bit long at 16mm+bend, but should fit. It's almost 10x higher power than the original part.

[pintglass]

Re: Power supply & air switch on sewing machine

Is the diode that i have highlighted in the image the one your asking about, if so its a 1N4148.

[redwire]

Re: Power supply & air switch on sewing machine

Thanks for the pic, hmmm hard to tell it's covered in belt dust. Needs a wipe with q-tip and IPA. 1N4148 is usually reddish/pink body and a zener is orange.

[pintglass]

Re: Power supply & air switch on sewing machine

I've cleaned the board now, the image was taken before, 1n4148 is whats printed on it, its a yellow colour. I can take a new photo if you like.

[redwire]

Re: Power supply & air switch on sewing machine

ok, 1N4148 it is. I thought I saw an orange one (zener) on another board which made more sense as the circuit looks at motor back EMF to keep speed constant.

[pintglass]

Re: Power supply & air switch on sewing machine

All fixed now so thanks for all the help again.

Did you say that the schematic that I linked to is wrong, did you think the triac was in the wrong place, I may try to redraw this during the holidays.