Wednesday, 2 November 2016

Fixing a vintage Singer sewing machine motor controller

    With my work writing for Hackaday, this blog has been rather neglected of late. However there are still a few things to write about on a personal basis, and this is one of them.
    I have more than one 1950s and 1960s era Singer sewing machine in my life. These machines are about as good as it gets in terms of straight line sewing, they don't have fancy stitches like a modern machine but they are tough as old boots and will outlast us all.
    Sadly there are a few parts even on a '50s Singer that won't outlast us, in fact they're failing here and now. I'm talking about the suppression capacitor in the motor controller, which after sixty years of service is now in most cases reaching the end of its life and failing. You plug in the machine one day and it starts up all of its own with no foot on the button, there is a horrible burning smell, and smoke comes from the foot controller. You have two choices if you want to keep the machine running, buy a modern electronic controller from a specialist Singer spares supplier, or fix the original. It's the latter course I'll now describe.
Cover off, dead capacitor with brown gunge.
    In that foot controller is a rheostat, a contactor switch, and a capacitor. The capacitor suppresses sparks on the switch, and when it fails it goes short-circuit. Some people say you don't need the capacitor, but while the machine will run without it the life of the switch is reduced.
    The capacitor is a pretty standard mains suppression capacitor, a 0.1 microfarad item with a 250VAC rating. It's the voltage rating that's the important bit rather than the capacitance, use a lesser rated one and expect fire. Happily these are easy to order and don't cost much, you are looking for one described as "Class Y", or sometimes Y1 or Y2. Mine came from Farnell, but you can buy the same from almost any other electronic supplier. Make sure you pick one with decent lead length though, as you'll see you need those wires.
    If you open up the Singer foot control by undoing the four screws on its bottom and gently prising it apart, you'll see the problem immediately. On top of the ceramic rheostat housing is the silver can of the capacitor, and if it's failed you'll see plenty of brown gunge spilling from one end. Be careful of this gunge, it's probably quite harmless but you don't know quite what chemicals went in to '50s components.
New capacitor in position.
    If you undo the two screw contacts and lift up the two mains wires without losing their orientation, you'll see the wires from the capacitor have just been bent round where the screw would go. Using a bit of tissue to keep the gunge from your fingers, lift the dead capacitor away and eventually discard it. Give the ceramic a wipe too, best to take away any residue.
    Then take your new capacitor and carefully bend its wires to fit round the screw holes in the same way as the previous one. My capacitor fitted neatly in the recess in the ceramic, as you can see in the picture. Be careful to ensure it doesn't foul the mechanism beneath it.
All done.
    Then reassembly is just the reverse of disassembly. Put the wires back in place, and do up the screws with the top half of the contact. Check everything looks as it does in the final picture, then ensure that each of the four rubber feet is in place on the corners and screw the cover back on. Give it a final check, then test the machine. You should have a working machine again, with no smoke.
    I've done this with all my machines, even those whose capacitor hadn't failed. Preventative maintenance makes sense, in this case.