Mechanical Operations

VCR MECHANICAL OPERATIONS

Many will say that a VCR is a electronic gadjet. But from a repairman's point of view, it is a mechanical machine. Thats because most of a VCRs fuctions are mechanical in nature and most defects are caused by mechanical malfunctions. Thit means a mechanic is more qualified to fix it than an electronics specialist.

A VCR needs to do many mechanical functions. It needs to load the cassette, wrap the tape around the heads, EJECT, FAST FORWARD and REWIND, apply brakes, lock and unlock the clutch plates, spin the reels, spin the heads, move the pinch roller, move the tension arm and transport the tape. And that is just for openers.

In order to do all this, a VCR needs to convert electrical energy into mechanical motion. Everyone is familiar with the electric motor. Apply the correct voltage and the motor shaft spins. Motors do a major portion of the work inside a VCR. They operate the cassette carriage, load the tape, spin the video drum and rotate capstan.

Figure 1. - Loading and Capstan Motors

Solenoids

A solenoid is another thing some VCRs use to convert electricity into motion. A solenoid is an electromagnet that moves a mechanical linkage in a straight line. Solenoids were common on older VCRs. They were used to release the latch on most of the top loaders. They were also used to perform some of the moving mechanism functions like the brakes and pinch roller. Solenoids are used on newer machines as well. They were used in nearly half of all VCRs made until the early 90's. Sharp was still using solenoids in their VCRs until 1994.

Figure 2. - Typical Solenoid

Master Cam Gear

Another method a VCR uses to convert electricity into motion is the cam gear. This gear rotates on a shaft just like any other gear, but the difference is this device has a linkage track. See figure 3. A linkage rides in this track. Figure 4 is the same as figure 3 except the gear has rotated 180 degrees. Compare figures 3 and 4 and notice how the linkage has moved.

Figure 3. - Cam Gear

Figure 4. - Cam Gear
Rotated 180 Degrees

This setup is used in virtually every VCR ever built - mostly in moving mechanisms. Figures 3 and 4 are simple illustrations. But in actual use, the linkage tracks are closer to 360 degrees (one full turn) and more complex. It is common to find tracks on both the top and bottom of the gear and often there is more than one track on a side. So one gear can do quite a few different things. This device is known as the Master Cam Gear and is usually the largest gear in a VCR. On most VCRs, one master cam gear is used to operate the roller guides, pinch roller, brakes, clutch plates and the tension arm.

The master cam gear becomes automated when you put a sensor in the mechanism to tell the CPU (VCR's brain) exactly where it is. This sensor is called the Mode Switch. The mode switch is a sealed unit that contains a series of switches (usually three) that open and close as the cam gear rotates. The CPU sees this as three channels of pulsed data and can determine from this exactly where the master cam gear is at all times.

To perform different functions such as PLAY, REWIND, STOP etc., the master cam needs to rotate to precise positions so the engineered tracks can precisely push or pull on all the linkages to do the correct thing. This is all done with the CPU programming and mode switch.

Technical Note - the mode switch is one of the biggest trouble makers of all VCRs in general. Even on many late models, it is still a big offender. Many times it can be intermittent. A VCR can play for weeks at a time with no trouble, suddenly go zonkers, and then play trouble free for several weeks again. Because of this intermittent temperament, repair shops are often forced to guess at the customers expense. Replacing a mode switch usually is not difficult, but determining if it is defective or not can be a challenge. If you check the Database Page for your VCR to see if it is an offender, you can solve your own problem quickly and easily. Most Database Pages also have layout drawings that show where the mode switch is located.