The basic patient chair hasn't really changed in almost 50 years. All chairs share the same basic design and functions. As discussed previously, patient chairs have some basic inputs (control pads or switches) a circuit board and micro-switches that will limit total chair movement.
The chair movement is operated by either two electric motors or two hydraulic cylinders. One motor (or hydraulic cylinder) is used to raise and lower the chair (this is typically referred to as the "base" motor or cylinder) and the other is used to recline the back or tilt it forward (referred to equally as the "back" motor/cylinder or "tilt" motor/cylinder). Some switches (e.g. auto exit) will cause both motors (or cylinders) to operate simultaneously. Nevertheless, each motive device is only working on one part of the chair (base or back). If you have trouble with chair movement, odd noises, or other signs of trouble, it is helpful to use the individual movement inputs (base up alone, or back recline alone, for example) in addition to programmed functions involving operation of both motors/cylinders to help isolate any problems.
The back motor is usually located beneath the seat of the chair and is accessed by simply lifting the seat by grabbing the "toe" and pulling up. The seat is usually hinged and not locked down in any way so you can just grab and lift. Naturally, there are exceptions, if in doubt, do not force it. Subscribers can also reach out to me directly for additional assistance.
The base motor is usually under a cover at the bottom of the chair. This is typically a large plastic housing with the name brand of the chair written on the side in some fashion. This housing is usually secured with a few screws at the front and back (3-4 is typical).
A handy tool for locating the source of noise from a patient chair is a stethoscope. I don't recommend keeping one in your tool box, but you should certainly have one in the office. Listening directly to each component with a stethoscope can help you identify if it is the source of an unusual noise so you'll have a better idea of how to proceed from there.