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Notes on VCR Troubleshooting and Repair INDEX V.
Chapter 20) VCR SENSORS AND TAPE COUNTERS

20.1) Tape start/end sensors

VHS cassettes use a clear leader and trailer for the purposes of detecting beginning or end of tape. A light source that pokes up in the center of the cassette illuminates photodetectors on either side of the cassette through passages in the plastic passing through the tape as it leaves and enters the cassette.
The light source can fail - this is common on older VCRs where this was an incandescent lamp but rare on modern VCRs which use a special IR LED. The failure of this light source can produce a number of symptoms: The VCR may simply shut down and refuse to do anything. VCRs with incandescent lamps often were able to figure out that the light bulb was burnt out since it was drawing no current and then shut down or flash an error code.

The VCR may go through the motions of playing a prerecorded tape thinking that a tape is present because the sensors return signals indistinguishable from what it would see if a tape were present. Eventually, it may give up and probably shut off power.

The VCR may do strange things when you attempt to load a cassette since the microcontroller is receiving conflicting signals - the cassette is out but the sensors think otherwise.
If your VCR uses an incandescent lamp and it is not lit when power is on, then the bulb is most likely burnt out.
If either sensor fails open, then similar symptoms may result. If the sensor on the supply side fails shorted, then it will appear as though the tape is at the end. The VCR may refuse to play or FF or will attempt to rewind as soon as a cassette is inserted.
If the sensor on the takeup side fails shorted, then it will appear as though the tape is at the beginning. The VCR may refuse to REW. In both cases, sometimes you can trick the VCR into cooperating and confirming that there is a sensor problem by pulling the connector for the appropriate sensor once the cassette is loaded.
If you can get at the connectors, you can test the sensors by monitoring the voltage on the outputs.
One test you might try if the VCR attempts to play an imaginary prerecorded tape as soon as power is turned on is to locate the microswitch for record lockout protection - it will be located near the front (where the record protect tab would be once the cassette is loaded). Press this in while you turn power on. If the VCR now just initializes and displays cassette-in without trying to play, then it really thinks there is a cassette in place most likely due to a faulty sensor.
In some cases, there could be other problems like a faulty mode switch or microcontroller producing symptoms that might be mistaken for faulty start/end sensors.

20.2) Start/end sensor testing

The start and end sensors are usually a combination of a light source (IR LED) and IR photodiode. With a little effort, these can be tested for functionality. For an incandescent lamp (older VCRs), if it is not lit with VCR power on, it is most likely burnt out. Test with an ohmmeter.
For an IR emitter, an IR detector like the circuit provided elsewhere in this document or an IR detector card can be used to determine if the LED is operating.
You can also try powering the LED with a low voltage supply and 500 ohm or so resistor using the IR detector to see if it works. Disconnect it from the circuitry first! Try both polarities to be sure you got it right. The sensors themselves can be tested by disconnecting them from the circuitry and shining an IR source on them (a remote control or incandescent bulb) while monitoring the resistance with a VOM or DMM. Use the polarity which give the higher reading (reverse bias). This resistance should drop dramatically if they are functional.

20.3) Tape counters

There are two kinds of tape position counters: reference and real-time. What I call a reference counter is what all VCRs used up until a few years ago. A sensor counts revolutions of the takeup reel (usually) either directly or via a belt drive. A mechanical or electronic counter displays an arbitrary number which provides some idea of location. Since the rotation rate of the reel is not constant with respect to the actual time of the tape, it is not possible to use this for anything other than a reference. In addition, the tape may slip a bit and be wound tighter or looser depending on whether it was wound in play, FF, or REW. Thus, even the reference is not accurately repeatable.
Failures can be caused by a broken or weak belt for the mechanically operated counter or defective circuitry for the electronic display. A failed sensor would most likely also cause the VCR to shut down and unload the tape as this is what is used to confirm that the takeup reel is rotating and that tape is not spilling into the bowels of the VCR.
Real-time counters - which really are a vast improvement - operate off of the control track pulses from the control head. Tape location is measured in hours, minutes, and seconds though it is still relative and must be reset at the beginning of the tape if an absolute location is to be determined.
One characteristic of the real-time counter is that it will not keep track of location on a tape that has not been recorded (no control track). Therefore, if you want to leave a specific length section of tape blank, you will still need to lay down the control track by recording a blank channel or just ignoring what you have recorded later when you go to fill it in.
Failure of the real-time counter on a VCR that otherwise works normally is quite unlikely and is probably an electronic problem since the control head must be functional for all record/play modes to work properly. However, it is possible that a failure of a half loading arm to fully extract the tape would result in problems in (non-search) FF or REW.

20.4) Reel rotation sesors

Reel rotation is detected most often using optical sensors under the reels though some older VCRs may use mechanical or optical interrupters driven off of belts from the reel spindles.
There will always be a takeup reel sensor. It has two functions: to (1) confirm that the reel is rotating and that tape is not spilling into the bowels of the machine and (2) to operate the (non-real time) tape counter. Failure of this sensor will cause the machine to shutdown almost immediately and will result in a stuck tape counter.
Many VCRs will have a similar sensor on the supply reel. The output from this sensor will be used to confirm proper rotation of both reels both during modes involving tape motion as well as during the tape load and unload operations. Some fancier VCRs will display an estimate of tape remaining using the difference in rotation rates of the supply and takeup reels based on assumptions about tape thickness, hub size, and total length (which you may have to tell it).
Sometimes, reel rotation sensor problems are simply due to accumulated dirt on the reflective surfaces - clean them. In other cases, replacement sensors will be needed. While you are at it, replace both sides where appropriate - most of the cost to you is in your time, the cost of the sensors themselves is modest. Note that on VCRs with real-time counters, the real-time display as well as possibly the tape movement sensing operates off of the A/C head control pulses. Failure here could be due to dirt, a bad A/C head, tape path alignment problems, or failure of a half loading arm to properly extract the tape so that it contacts the A/C head.

20.5) Reel rotation sensor testing

The counters on some VCRs are active at all times - rotate the appropriate reel and the counter will change (count up or down depending on its default mode - the direction of rotation probably will not matter). If your VCR is of this type, testing is particularly easy. Slowly rotate the takeup (usually) reel by hand. The numbers should change several times - probably 4 - per revolution. There should be no missed counts and there should be no positions where the counter free runs - the display increments or decrements on its own very quickly. Any of these could indicate a problem with the sensor or LED, a buffer amplifier, bad connection, or the microcontroller or other IC that actually drives the counter and display.
For electrical tests, first, locate the LED and photodiode. You can tell the difference by testing with a DMM on its diode test scale - the LED will have the higher forward voltage drop. Sometimes, the connections are even marked. What a concept!
Momentarily touch and remove a resistor (1K ohms or so should work) across the sensor leads (while the VCR is in PLAY mode before it quits if needed). This should make the counter change if the the LED is bad or the photodiode is open. Alternately, a remote control may be able to activate it providing pulses that will look to the counter exactly like reel rotation.
If this has no effect, unsolder the sensor (or unplug the sensor assembly from the main board if there is a connector) and try the resistor across the terminals where it was connected. If you now get a response, the sensor was shorted (or the connection was bad).
If you do not get the counter to change in either case, there is a problem with an intermediate buffer amplifier, the electronics on the main board, or a bad connection leading to the main board. You will need to obtain the service manual or trace the circuit leading to where the sensor signal is detected. It is possible that the counter will only change when the microcomputer expects the reel to be moving, so a test while in STOP mode may not be valid.
An alternative test is to use an ohmmeter across the photodiode on a high ohms scale. Use the polarity which gives the higher resistance and shine a light on the sensor. The resistance should drop dramatically with a bright incandescent light (these put out a good amount of IR). If it is infinite in both directions, the photodiode is open. If it is low in both directions, it is shorted. You may be able to make a measurement while the sensor is still in circuit, though other components may mask the resistance change. As noted, the IR sensor/LED combination is often a pluggable assembly. Using my VOM on a photosensor, I read infinite ohms with no light and 200 ohms with a bright light. However, your mileage may vary.
If you have an oscilloscope, monitor the sensor output. If it is a voltage signal at this point (likely), then you should see it go high and low as you rotate the reel or shine light on it. With the reel rotation, the low and high periods should be roughly equal. There may be a buffer amplifier driven by the sensor - check its output as well. The signal there should be a cleaned up version (low pass filtered and possibly inverted) of the sensor output. In all cases, the signal should be a DC value without noticeable ripple or noise (block external light as fluorescent lamps in particular may add a 120 Hz ripple to your detected signal). Even at transitions between low and high or high and low, the level should change smoothly. You may be able to trace the signal to its final destination, the microcontroller or other large multilegged part, and monitor it there as well.
Play a T120 tape recorded at EP speed near the end of the tape. This will result in the slowest takeup reel rotation. Or, if your VCR has the counter active in stop mode with the cassette out, rotate the takeup reel by hand very slowly. If the counter skips or 'free runs' at certain positions of the reel, there may be a problem with the hysteresis circuit. If this is external to the microcontroller, a resistor may have opened or there may be some other easily identified bad component. If it is internal to the microcontroller - either an actual circuit or firmware - then replacing the microcontroller may be the best solution unless you want to add your own circuit - I have done this to repair a Sears VCR with an erratic counter problem. It is a simple 1 or 2 transistor circuit (depending on what external circuits are already present).
Monitor the sensor output when rewinding a T120 tape to the very end - this will be the worst case test as the pulses will be at the highest rate. There should be no missing pulses and the high and low times should still be similar. A bad sensor might result in unequal high and low times and dropped pulses at high speed.

20.6) Stan's tips on reel sensors

(From: Stan Cramer (stvcrm@Gramercy.ios.com)).
Try removing the take-up reel disk. Look on the bottom surface to see if there are a series of pie-shaped vanes - shiny, dark, shiny, dark, etc. If the shiny vanes get misted with smoke or general grak, the symptom is the same as if the sensor itself is faulty. Use some Windex or some such mild cleaner on the vanes and test the machine again.
On some earlier machines, the take-reel disk might have a series of evenly spaced slots - blank,solid,blank,solid etc.-that interrupt the flow of IR light creating an electronic pulse stream. If your machine has this type of motion sensor, you can try brushing or blowing out the dust that may have accumulated in the small recesses surrounding the IR emitter and receiver devices on the sensor assembly.
If these attempts don't do the trick, you probably have a faulty sensor.


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