Understanding Sensors

SENSORS

Ever since the first VCR was built, VCR sensors were a major problem. All brands had sensor failures. Because VCRs have so many sensors, there is a possibility that one may give you trouble someday.

Some of the common sensors found in a VCR are:

Reel sensor - all VCRs sense the rotation of the take-up reel. During all tape transport modes the CPU must sense rotation or it will send a shutdown command. This type of sensor rarely failed in VCRs manufactured after 1990.
Tape end sensors - all VCRs have two, one for supply and one for take-up. Tells CPU it either has tape to use, shutoff or REWIND when it gets to the end. These sensors rarely failed in VCRs manufactured after 1990.
Mode switch - every VCR has something to sense the position of the moving mechanism. The CPU needs to know if everything is in the correct position before executing functions. This sensor commonly fails in most VCRs ever built, including many late models.
Cassette up/down switch - senses if cassette is up, down or moving.
dew sensor - senses condensation inside the VCR. This a problem especially in cold winter climates. If you take a VCR out of a freezing car and bring it into the house, moisture in the air will condense all over inside of it. Trying to play something with wet video heads isn't good for the video heads or the tape. When moisture is detected, the CPU goes into a shutdown mode and displays an indication of some kind until the moisture disappears. Many low end VCRs did away with this sensor after 1990.
Record safety tab switch - senses the record safety tab on the back of cassettes. If gone, the record circuits will be disabled to prevent accidentally recording over what's on the tape.
Video drum rotation sensing. If the drum isn't spinning, the CPU will send a shutdown command.
Capstan rotation sensing. If capstan isn't spinning, the CPU will send a shutdown command.

All of these sensors are connected to the CPU. The CPU is part of a VCRs most complex circuitry. Fortunately, CPU failures are rare. There are, however, a few components that keep failing again and again. Remember, knowing what commonly fails is the key to successful servicing. In this circuitry, sensors are the biggest offenders. Reel sensors, tape end sensors, mode switch, cassette up-down switches and record safety tab switches are famous trouble makers. On the other hand, problems with dew sensers, capstan rotation sensors and drum rotation sensors are very rare.