The Moving Mechanism

THE MOVING MECHANISM

Just as the name implies, the moving mechanism in a VCR is just about everything that moves. It includes mechanisms for cassette loading, tape loading, tension arm and brakes. The electronic brain, the CPU, is programed to control all of it automatically. You just press something like PLAY, REWIND or EJECT and it does the rest.

Most problems in Moving Mechanisms are mechanical in nature, not electronic. Things like roller guides not loading all the way, cassettes that do not eject, erratic mode switch or the brakes not working correctly are all mechanical Moving Mechanism problems. Sometimes a careful examination of the mechanism will reveal what is not right.

The heart of the system is the CPU controlled master cam gear. The CPU knows what position this gear is in by sensing data from the mode switch (sensor). By using the load motor, the CPU can rotate the gear to any position. It's total travel is less than one turn and is usually the largest gear in a VCR. When you compare the above images, you will notice the linkage has moved considerably. This explains how the CPU uses the mode switch to sense position of this gear to control everything mechanically inside a VCR.

The master cam gear is mechanically connected to and controls things like the roller guides, pinch roller and brakes. As it rotates, it pushes or pulls on these linkages to physically move things. The example in the table below, shows STOP at the 20% rotation point of the master cam gear (it varies from one VCR to another). For complex reasons we won't go into here, most master cam systems have their home positions in the middle somewhere with tape loading on one side and FAST FORWARD and REWIND on the other. The home position is usually where the mechanism is when the power is off. Notice what the brakes and other things are doing at the various positions of the master cam gear.

Moving mechanisms vary from one VCR to another, but all contain the basic components. Our example here uses a simple mechanism without instant play or half load arms.

Master Cam
Percentage                Supply         Takeup         Pinch         Roller
of Rotation  Function     Reel Brake     Reel Brake     Roller        Guides 

5%           FF, REWIND   off             off           disengaged    unloaded  
20% HOME     STOP         on              on            disengaged    unloaded
30%                       off             on            disengaged    loading
40%                       off             on            disengaged    loading
60%                       off             on            engaging      loading
90%          PLAY, REC    off             off           engaged       loaded

When we press PLAY, the gear rotates to the 90% rotation point. As it begins to rotate, it pushes or pulls on various linkages to actuate various things. The supply reel brake releases while the takeup side remains on. The roller guides pull the tape out of the cassette's supply side and wraps it around the video head drum. When the gear arrives at the 90% point the pinch roller is fulley engaged and the takeup brake is released. There are also points along this travel where the mode switch tells the CPU to turn on the drum and capstan motors (servos). We are now in the PLAY mode and there should be a watchable picture on the TV screen. Once again, when we press STOP, everything is undone in reverse order as the master cam gear returns to the home position at the 20% rotation point. The exact same process occurs when we press RECORD.

Now let's say we want to REWIND the tape. As soon as we press REWIND, the master cam rotates to the REWIND position, which is at the 5% rotation point. The CPU knows when it gets there by listening to the data from the Mode Switch. Both brakes are released while there is no change in the pinch roller or roller guides. The CPU, also at this point, automatically turns on the reel motor to complete the process. When we press STOP, the CPU rotates the gear back to the home position at the 20% rotation point.

One reason why the home position is between the PLAY/RECORD and the REWIND/FAST FORWARD positions is so that it can have two seperate functions engineered into the same gear. Or in other words, think of it as two seperate cam gears with the tracks on one for PLAY/RECORD and the tracks on the other for REWIND/FAST FORWARD.

In addition to the above, all moving mechanisms control the tension arm. When unloaded, it's behind the left roller guide. As the tape loads, the roller guide gets out of the way and the spring tension on the arm puts it in the correct position without any other help. But on the unload part, something needs to pull it back slightly so the roller guide can catch it and pull it back in. Most moving mechanisms have a cam on a slide bar that does just that.

Also for FAST FORWARD and REWIND, the clutch plates need to be locked together for increased torque. The clutch bypass mechanism does that and it's usually activated by another slide bar cam.

This is pretty much in a nutshell what the moving mechanism is, what it does and how it works. If you can understand this page, you have grasped the basic concept of VCR mechanical operation. From a troubleshooting standpoint, it's an excellent advantage to understand how this works. If something is not doing what it's supposed to, you now have the basic knowledge to figure it out.