Technical Advice - Door Chime Solenoids

Solenoids are just a component of the mechanism, but as they are most frequently the cause of trouble with of old chimes, I devote a separate page to them.

The most common problem is a dirty solenoid cylinder and plunger. On many chimes, there’s a label warning NOT to oil the solenoids That’s because the oil will in time gum up or attract dirt which will easily overcome the weak power of the solenoid and render it non functional. If some of the solenoids are working but others are not, chances are that some are stuck due to gummed up oil, decades of accumulated dust, or a little corrosion. Easy to fix. Most solenoids have a rubber or metal cap on the back. Very gently remove the cap, and slide the plunger out of its cylinder. Keep track of the spring which might try to get away. Polish the plunger and inside the cylinder with metal polish and reassemble. A little powdered graphite can serve as a suitable lubricant, though nothing is needed if the parts are well cleaned.

Solenoids can have more serious problems. Most plungers are made of two parts: a steel piston and a strike pin. Very early strike pins are typically made of wood or hard leather, later ones generally made of plastic. In chimes with plastic pins, it’s common to find strike pins broken or gone missing. The effect of a missing strike pin is that the plunger is too short, which makes it entirely ineffective. With a little finesse, new pins can be made from plastic rod or a hardwood dowel. New plunger sets from NuTone are theoretically available though can literally take months to receive. For other brands, NuTone parts might work, but mostly you’re on your own.

Beyond being gummed up a bit with old oil and dust or debris, I have also seen older units where the solenoids were locked in place with corrosion. I’m not sure what sort of conditions would cause the problem but I suspect that a long period of storage in a damp location could do it. Stuck plungers can be removed with a few smart taps, though be careful not to damage the strike pins if approaching from the front . Some brands have removable tube caps at the front end, which allows removal of the plunger through the front, which is less risky to the strike pins.

Another common problem with solenoids is a hardened glob of gunk just inside the rear cap, effectively sealing the back end of the solenoid tube and contributing to the immobile state of the plunger. This gunk had once been a shock absorber that cushioned the plunger on its return stroke and made for quiet operation. Apparently, early models used some foam rubber that petrifies after 50 years or so. The residue can be removed with a solvent like lacquer thinner. Later models used a wad of cotton thread. Current plunger replacement kits include a spongy foam rubber shock absorber, presumably made of a stable modern synthetic.

Here’s a nice solution if you need to replace end caps. Any hardware store that sells Ace brand products will have something called 3/8” Tips, which I believe are made of polypropylene and intended for dressing off the ends of various metal legs. They are a perfect fit for the 3/8” solenoid tubes used on NuTone chimes, and perhaps other brands. You will need to drill a hole in the rear of the cap to allow air to pass through the tube as the plunger travels.

A much more serious problem than stuck or corroded plungers is a missing plunger. As with all parts for vintage chimes, almost all replacement parts are extinct, which means that in most cases the options are to scavenge from a similar chime or have parts made by a machinist. Making a new one is further complicated by the fact that a plunger should really be made from a special alloy intended for the task which is extra sensitive to magnetic fields. All doable, just pricey. Local machinists in my town charge $70/hr, and making a solenoid is definitely not less than an hour job.

If a plunger is gone, the return spring is probably also AWOL, and springs are even more trouble to replace than plungers. The ultra light-load springs that are used in chimes are not something you are going to get at any hardware store. I recommend Century Spring, which has a catalog of a few hundred thousand springs, and among that vast inventory there are a handful that are suitable for use in chime solenoids. Also they have a $50 minimum order. Lesson here - keep track of your plungers and springs, and if you are planning on replacing lost parts, be prepared for a surprisingly high price.

A rare but possible problem with solenoids is that the coil can deteriorate. The coil is made of up a spool wound with a specific number of "turns" of magnet wire of a specific gauge, which when charged, creates a specific amount of magnetic field to drive the plunger. Magnet wire is copper with an an insulating coating, usually enamel. The insulation keeps the coil winding from shorting the electrical path through the coil circuit.. If the insulation becomes damaged enough to cause shorts in the coil, the magnetic field will be weakened , reducing the strength of the plunger's action. The solution is to unwind the old worn magnet wire, noting the number or turns, and then rewinding using the same gauge and the same number of turns... and of course terminating the ends patterned after the original set up. After consulting with a solenoid expert, I have learned a few important facts about solenoid coils. The number of turns, not the gauge of the wire determines the magnetic strength. Heavier gauge wire only serves to make the coil more rugged and more resistant to being burned out by electrical overload. A coil can be rewound with heavier gauge wire and the same amount of turns to result in the same magnet strength, though the physical volume will be different and perhaps not fit on the spool or in the allocated space in the device.

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