CAPSTAN WEAR AND FAILURE
The capstan motor and pinch roller pulls the tape over guides one through six, around the video drum, past guide seven, over the ACE head assembly and past guide eight. For a review of the tape path see the CLEANING CHART by clicking here. It must run at a very consistent speed and, in many cases, be capable of running in reverse. To do this properly it has to very precisely aligned, straight up and down. Early machines used capstans that had a flywheel driven by a motor using a belt, but these were abandoned early on. Most manufacturers adapted the familiar flat pancake-style motor similar to the ones used in the reel tables seen earlier. What you see above are the two most routine problems found with the shafts of the motors (and a new one shown for comparison). The motor shaft on the right is an example of oxide and dirt build up. It can be removed with solvent in all but the most extreme cases. Sometimes the oxide can literally weld itself to the shaft. If this occurs replacement of the motor is required. When the shaft gets greatly worn and polished, as in the center example, it must also be replaced because the slick surface begins to lose its ability to evenly pull the tape. This produces tracking and perpendicularity problems, especially in reverse play. If you click on the picture you'll see the motor opened up for examination. Notice the surface mounted capacitors. They are used to absorb the electrical shock or pulse generated when the flat coils (brightly copper colored) are turned on and off when moving the armature (the round gray disc with the shaft in the center). It is a permanent magnet with six fields. Because it has six defined north and south poles radiating around it it can be driven by the coils, just as with a regular motor, except that these are laid out flat. A common problem with these design motors is that over time the capacitors leak electrolyte, a corrosive liquid in the presence of moisture, which eats away the printed conductive traces on the circuit board. When the circuits start failing to conduct, strange things begin to happen, such as no forward or reverse play, no rewind, or uncontrolled capstan speed. Also as the capacitors fail an annoying, repeating burst or white line shows up in the picture information. All of the capstans are refurbished and the surface mounted capacitors replaced during the refurbish and renewing process. Now on to what presses against the capstan to pull the tape through the machine. To examine the PINCH ROLLER click here. To go back to the panel for the THREADING RING AND BLOCK click here
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