Adjusting & Repairing a Rheostat

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by Les Godfrey

A ceramic resistance wire rheostat
Concealed resistance wire

There are two main types of Rheostat used in vintage sewing machines, Resistance Wire and Carbon Pile. Rheostats work by limiting the amount of current going to the motor.

Coil resistance wire
A ceramic carbon pile rheostat, single type

A ceramic carbon pile rheostat, double type

A Resistance Wire rheostat has resistance wire wrapped around a ceramic core, either visible or concealed within the ceramic core. Yet another has resistance wire that is coiled and looks very much like a long spring, several rows of the coiled wire is strung out from end to end of the controller housing. This is generally suspended freely inside the controller helping to dissipate heat.

The Carbon Pile type is made up of individual thin carbon disks which are stacked on top of each other thus forming the pile, this is held together within a sleeve contained in the ceramic holder, there are both single and double pile configurations.

Please note for your safety when working on electrical equipment disconnect from power supply before proceeding.

One of the most common faults encountered with all types of foot control and knee lever controllers is a problem known as running on. This causes the motor to run flat out with no control whatsoever, the only way to stop the motor is to disconnect it from the power supply.

To diagnose this problem first check the controller for any mechanical binding or an obstruction which might be jamming the unit, deteriorated wiring can also cause this fault. If everything looks to be ok and working normally check to see if there is a capacitor wired across the terminals. If you have a multi meter and are competent in its use, simply test the capacitor to see if it has a short circuit. If you do not have a meter disconnect one side of the capacitor from its terminal, test by plugging back into the power supply, if the motor no longer runs then you have established the capacitor has shorted out, after disconnecting from the power supply remove the capacitor.

Various Capacitors

Usually the capacitor will appear to be burnt see photo examples. If the capacitor is burnt or showing signs of overheating it should be removed, if a replacement is readily obtainable then it can be replaced. However it will run fine with the capacitor removed. The capacitor is used to suppress electrical arcing across the contacts thereby protecting them from burning, this arcing is also the cause of electrical interference on older valve radios and televisions.

It should be noted that all rheostat controller systems will get hot if the machine is used for long periods at slow speed, this is a normal operating condition and is not a fault.

Resistance Wire Type: If the controller is not operating correctly look for broken or burnt through resistance wire, if this is the cause a replacement rheostat is require. Sometimes however in the case of a foot control it is more cost effective to replace the complete unit with a generic type. Some other causes of malfunction are broken, burnt or worn through contacts. See also mechanical checks and testing capacitors above. The concealed wire ceramic rheostat usually long and tubular in shape will have several connection points protruding from the ceramic tube along its length, these are wired up to the activating mechanism of which there are many and varied designs. Most faults encountered are broken wires from the contacts or burnt contacts. Like the concealed ceramic rheostat a coil wire type does not usually burn through as there is no actual contact with the coiled resistance wire itself. The activating mechanism makes contact with brass segments throughout the resistance range, the resistance wire is connected to these segments, again look for burnt or broken contacts in the activating mechanism.

Carbon Pile: On a double carbon pile type there is usually an adjustment where the two screws fix the rheostat to the foot control base. The holes that the screws go through are elongated and it’s simply a matter of moving the rheostat further away or closer to the activating mechanism. Move closer if the foot control does not respond until say half depressed, move away if machine starts too fast, for fine tuning it may be is necessary to bend the contacts slightly.

A single pile rheostat has a retaining bracket which is also adjustable and usually an adjustable contact of some type, adjustments are carried out as above, moving closer or further away from the activating mechanism.

Should a carbon pile rheostat not operate correctly after checking and eliminating the above then it’s possible that some of the carbon disks have been crushed. You will need a supply of carbon disks, generally they all tend to be the same size and so any scrap foot control using a carbon pile rheostat will be a good source. Before proceeding place some old newspaper down on your work surface, this repair is going to cause some black carbon dust. First remove the rheostat completely from the foot control.

Retaining Clip holding Carbon Block in place
Sprung retaining Clip, Rounded Carbon Block, Single Disk
Single Carbon Disks

If this is a single pile type there is generally an internal sprung retaining clip at one end and a protruding thick round carbon block with a rounded end at the other, also held in by a sprung clip. Remove the sprung clip from the block end only, simply hook it out, now tip up the rheostat slowly and pour out the carbon disks gently to keep the dust down.

There will be broken and damaged ones, starting with the block gently clean each disk separately on each side, gently removing loose dust, a dry soft rag is sufficient for this. Keep the disks dry, any moisture will reduce the effectiveness of the rheostat, discard all broken disks or any that shows signs of being burnt (badly pitted) as you go. Once all the disks have been cleaned start putting them back into the ceramic sleeve, you should have noticed that the last disk that came out was about twice the thickness of the others, this one of course goes back first. As you drop them in one after the other make sure they lay flat within the sleeve, depending how many were broken you will need to top up with ones from your scrap rheostat. Fill the sleeve up to almost the top, now sit the rounded block on top of the other disks, there must be enough room for the block to be able to be retained by the sprung clip. Looking at the rounded block it will have a wider flange, this is the same diameter as the disks, the flange goes against the other disks, now replace the sprung clip over the flange locking it into place. Once in place check that the block is not held tight against the other disks, there should be some play. Refit the rheostat and test the foot control, by means of the mechanical adjustment mentioned earlier and by either adding or removing disks it should be possible to get the machine to start slowly and run up to full speed.

The double pile configuration is dismantled in the same way but usually have contacts that clip on or are screwed in place at the end of the ceramic sleeve opposite the thicker rounded block. They are usually dismantled from this end opposite the block, although this is not always the case. Commonsense should guide your decision, whichever way requires the least amount of work is the way to go.

Remember the carbon pile rheostat works by compressing the disks together thereby reducing the resistance causing the machine to speed up. If you have too many disks in place the motor will run or you will hear a buzzing sound, without the foot control being depressed. Back off the rheostat by moving it away from the activating mechanism. If you find that you cannot stop it from buzzing by this means then you will need to remove the rheostat and then remove one or two disks. On a double pile rheostat an equal amount of disks must be added or removed from each side, by not packing the disks too tightly to start with you can hopefully avoid this possibility.

Keep in mind that you are working on electrical equipment take great care, remove unit from power supply when undertaking repairs or making any adjustments.