Troubleshooting the Transport Mechanism

TROUBLESHOOTING THE TRANSPORT MECHANISM - (VHS)

Many mechanical defects can be solved by simply observing. First, it may be helpful to understand what happens in the transport normally. With this knowledge, it's easy to tell when something is not working the way it should.

With over 9,000 different VCR model numbers in use today, it's nearly impossible for a page like this to explain all of them. However, the following should be at least 90 percent accurate for most. The basic information on the Database Page for your VCR will give you some information on what major assemblies it uses and shed some light on which of the following applies in your case.

Loading Operations

There are two kinds of loading operations; cassette loading and tape loading. Tape loading is sometimes used interchangeably for cassette loading, but never on this website. Cassette loading refers to the loading and unloading of the VHS cassette while tape loading refers to the operation which pulls the tape out of the cassette and wraps it around the video drum.

CASSETTE LOADING - cassette loading must be successful before tape loading can begin. If cassette loading does not complete see Trouble Shooting the Cassette Carriage

TAPE LOADING - the CPU energizes the moving mechanism motor which turns the Master Cam Gear. As this gear turns, it pushes and pulls on linkages that move things. The supply reel brake is released while the Roller Guides pull the tape out of the cassette and continue on until they hit the V-stops at the end of the tracks. There is a spring loaded mechanism in each Roller Guide linkage to firmly hold the Roller Guides against the V-stops. Check to make sure the Roller Guides are tight against the V-stops. It's OK if they are loose while travelling or retracted.

As soon as the left Roller Guide advances far enough, the spring loaded tension arm springs into the correct position to regulate tape tension. The pinch roller is moved into contact with the capstan and the take-up break is released. As the reel drive mechanism comes to life (usually a belt from the capstan), the idler swings to the right and makes contact with the take-up reel table. Both cassette reels should now be turning smoothly. The video drum and the capstan should be also turning at this point and we should have a viewable picture.

Trouble Shooting the Tape Path

Starting at the supply reel table and going in the direction of tape flow:

Before loading a cassette, inspect the tape tension band to make sure it isn't wrinkled or the felt isn't coming off. The tape tension arm should be perpendicular and not bent. With a cassette loaded and in PLAY mode, the tension arm should be stable and not jumping.
Inspect the brakes for any signs of breakage.
Impedance Rollers on older VCRs have a tendency to fall apart and jam the works. Check to make sure yours in one piece and spins freely.
Check both Roller Guides for tight height adjustments by attempting to rotate the tops of them with your finger; they should not move. Inspect each Roller Guide assembly for the presence of angle pins; sometimes they fall out. Check the Roller Guides to make sure they retract fully. Load a cassette and lift up the lid to verify the Roller Guides are sufficiently behind the magnetic tape. Press PLAY and verify that the Roller Guides hit the V-stops; they should also be snug against the V-stops. Examine the tape as it flows across the drum; it should flow smoothly on the shelf with no edge wrinkling.
The tape should be centered as it crosses the ACE head and pinch roller. Examine the tape carefully before and after the pinch roller; if it's exiting the pinch roller with wrinkles, it may need replacement.
Observe the takeup reel inside the cassette; it should be rotating smoothly. Any jerking or hesitation indicates a worn idler tire. If you have a dummie cassette, grab the takeup reel table and see if the idler slips against it; any slippage at all indicates a worn tire.

Idler And Reel Tables

There is a lot to be said about idlers and reel tables for transporting the tape. The reel tables actually make the connection to the tape reels and the idler is what drives them. The capstan is the only other tape drive component in the transport system.

Reel tables rarely ever fail. Idlers are a little more famous for trouble. Newer gear driven idlers are having problems with warpage and cracked spring retainers, while the older rubber tire idlers are having problems with the tires themselves. The rubber on them eventually hardens causing a loss of traction. This will eat tape.

Capstan and Drum Servos (motors)

A servo is any motor that can send feedback to a micro-processor for analysis. The analysis results in voltage correction for the motors power source. The analysis and correction cycles occur faster than most people can think, possibly hundreds of times per second. The result is a motor with very accurately controlled speed.

Every VCR has two servos, one for the video drum and one for the capstan. Servos make it possible to line up the diagonal tracks on the tape with the rotating heads. The speed is so accurate they can actually be synchronized.

CAPSTAN SERVO - servos are an important part of the transport system. In fact the capstan servo is responsible for transporting everything, because it's the main power source behind the tape flow. One can easily check the health of the tape transport by monitoring the capstan servo.

The CPU listens to the control track on the tape with the ACE head (stands for Audio Control Erase, basically it's a stationary head that records and plays audio as well as the control signal) to determine which speed the tape was recorded at. Once determined, this information is passed on to the capstan circuit in the form of a speed assignment. As soon as this circuit has it's speed assignment, it maintains it by listening to pulses from the capstan rotation sensor and the ACE head. It times every pulse and determines by the time intervals exactly how fast to go.

The easiest way for a beginner to "performance test" the capstan servo for tape speed, is to play a cassette with your favorite music on it. That's it. Even slight speed variations can be detected easily. You can use non-music audio for this also, but many defects can get past the ear undetected. Wavering sound (tape is speeding up and slowing down rapidly) is usually caused by an egg shaped belt, worn capstan bearings or contaminated DC voltage from the power supply.

DRUM SERVO - the drum servo is the motor that spins the video heads. The drum servo is far less likely to give you trouble because it spins much faster than the capstan and only has one speed to remember; 1800 RPM. Usually, if there is trouble with the drum servo circuits, it's speed will be way off; like above 5,000 RPM. You can tell by looking at the picture. A video drum running at the wrong speed looks exactly like a TV with the horizontal not locked.