Switches are used to open circuits to stop flow of current. A non pressed working controller, opens motor circuit. For safety, machine cord should be pulled from electrical outlets when not in use.
Early 6 step wire wound resistors:
Six step controllers have six contacts selected by a sliding lever. The the first step (contact) is “off”, the last step is full “on”, between steps are four series resistor of about 35-45 Ohms each for 110V operation. The control with sliding contact is “off, 160, 120, 80, 40, on” assuming 40 Ohms each. While it seems the steps might be course, they work well for speed regulation. Step technology is found in early singer BT7 foot controls, early knee operated bentwood controls, and portable Singer 101 portables. The resistors are hair diameter nichrome (nickel-chromium alloy) wire wound on a ceramic substrate. Contacts posts serve as junctions for joining of resistor segments. Sand and glaze mixture bonds wire to ceramic, and sometimes masks bottom terminals.
The resistor above is defective.
Patent US1409950A,Application 1919-7-18, Patent 1922-3-21
Patent US1507734A, Application 1919-10-25 Patent 1924-9–9
Nichrome wire expands when heated by electrical flow, sand provides abrasive means, with use resistors may fail. With power off, resistor values can be measured between terminals.When resistors fail, the machine will still run, but without low speed regulation ability.
If controller passes Ohm test it is usable, but might need wiring replaced. To replace wiring, resistor needs to be removed, for access of terminals on bottom. The top steel control lever needs to be removed by loosening set screw on pin that holds it in place. The pin is removed, hold on to spring and roller that come out with lever. The plastic roller is an electrical insulator that isolates electric potential of selector blade from lever. Three screws with fiber washers top and bottom, mount resistor to housing. If washers stick to resistor or housing, leave them stuck, it helps in reassembly. Reassembly requires location adjustment of resistor in housing. It is very important that rest location, is on first terminal “off”, and wiper does not contact second terminal or housing (+1/8” gap. When sliding top lever to full “on” at limit slot, selector lever should be at last terminal, and not extend to contact housing (+1/8” gap). There is slop in the mounting locations, accommodating small position changes. Adjustment may require a bit of trial, the lever must be installed to test (without return spring), but conflicts some with mount screw access. Do not over tighten screws or leave out fiber washers, resistor might stress fracture. When assembly work is complete, verify controller electrical terminals are isolated from housing.
Early step controls were used about a decade, then replaced with carbon disk piles. Nichrome replacement wire is readily available. Removing old glaze, rewinding, re glazing, and expecting long term fix is questionable. I have tried simple fix, re-flow of solder connections, without resistance change.
Early machines are easily upgraded with used of later bentwood style controller, or addition of later foot control, enabling controller section of bentwood to be used for accessories. There were a few variations in bentwood internal controllers over several decades of production. The changes involved controller body, controller, terminals, wiring details, cover plates, and knee control bars.
Carbon Pile Controllers:
The Featherweight shop has an interesting information on single button controllers here:
Single button, Bakelite case. Often need foot bumpers, cleaned inside of lint, wiring replacements. Less often mechanical adjustments.
Carbon pile controllers can be adjusted if stacks of carbon washers are complete and unbroken. There is an off adjustment, where stack has zero pressure, with no conductivity. Very important! If stack is conducting and left plugged in, long time undetected, it may over heat, motor too! Other adjustment is high end bypass, it provides full on, zero resistance. These adjustments are checked with an Ohm meter at two terminals, that connect to cable, without AC on of course. The Featherweight shop has a nice videos on adjustments.
Carbon pile resistors were also incorporated in bentwood bases, and some Singer cabinets with knee bar operation.
Patent US1792818A Application 1927-12-16 Patent 1931-2-17
Electronic controls use electronic switches called triacs to switch selected portions of power signals, regulating motor speed. Triacs are used in light dimmers. The selected power portion, is what your foot desires by depressing pedal. The unused portion is not taken from power line.
My wife likes electronic controls. They are light weight, easy to press, and smooth linear operation. A control without cables under $20 from Sew Classic. Ngosew on ebay has electronic controls with cables for $23.50. Electronic controls are energy efficient and run cool. Additional information is in a post about RPM and power measurements.