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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 pre-recorded 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 pre-recorded 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.
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. If the start and end sensor assemblies are interchangeable, swapping them may be instructive. For example, this may shift the symptoms from play to rewind or vice versa.
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. The only disadvantages of real-time counters are that: * They do not operate with a new or bulk erased tape since there is no control track. Thus, it is not possible to leave a specific length section of such a tape unrecorded by using the counter to space over it. You must lay down a control track first by recording something - anything - for the time you want. However, it is advisable that this be a valid video source so that the sync pulses occur with the proper timing. * The tape must be in contact with the control head for all operations. In principle, this results in more head (and tape) wear though I know of no cases of the A/C head stack requiring replacement because of this design. 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.
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 - even on a VCR with a real-time counter. 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. * Some VCRs will have a similar sensor on the supply reel. The output from this sensor can 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. Exactly when each is used will vary by design. If your VCR has identical sensors monitoring both reels, swapping the sensor assemblies may be instructive: the behavior will change if one is bad. For example, a VCR that would shut down in a couple of seconds in play mode may continue to operate correctly but now have problems with rewind. * 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.
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.
(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.
There may be anywhere from 2 to 6 or more motors in your VCR. Some designs use a single motor to power all functions except the video head drum. Others have separate motors for each function. Most typical are 3 or 4 motors. Motors perform the following functions: 1. Cassette loading (front loaders only). 2. Tape loading (position tape around video head drum, etc.). 3. Video head drum rotation (servo controlled). 4. Capstan rotation (servo controlled). 5. Takeup reel rotation (PLAY, REC, FF, CUE). 6. Supply reel rotation (REW, REV). The video head drum (3) always has its own motor. It is internal to the lower cylinder or above the upper cylinder (except in the very oldest VCRs) and directly drives the spinning upper cylinder. Most consumer VCRs use a single motor for the capstan and the takeup and supply reels. Some also use this same motor for cassette and/or tape loading. Several possible types of small motors are typically used in VCRs: 1. Small brush-type permanent magnet (PM) DC motors similar to those found in small battery operated appliances, CD and tape players, and toys may be used for cassette loading and/or tape loading. 2. A similar but larger PM motor may provide power for the capstan and reel rotation and possibly multiple other functions (older VCRs). 3. A single low profile or 'pancake' brushless DC motor may provide power for a direct drive capstan, reel rotation, and possibly multiple other functions. 4. Brushless DC or 3 phase direct drive motors are usually used for the video head drum. Some of the very earliest VCRs used a belt drive for the video head drum.
Aside from obvious mechanical problems and lubrication if needed, you usually cannot do much to repair defective motors. If you enjoy a challenge, it is sometimes possible to disassemble, clean, and lubricate a motor to restore it to good health. However, without the circuit diagram, even knowing what the proper voltages and signals should be on (2) or (3) type motors would prove challenging. The following are some of the possible problems that can occur with the basic permanent magnet motors: * Open or shorted windings or windings shorted to case. * Partial short caused by dirt/muck or carbon buildup on commutator. * Burnt out armature due to defective driver, power supply, controller, or mechanical overload. * Dry/worn bearings. An open or shorted winding may result in a 'bad spot' - a position at which the motor may get stuck. Rotate the motor by hand a quarter turn and try it again. If it runs now either for a fraction of a turn or behaves normally, then replacement will probably be needed since it will get stuck at the same point at some point in the future. Check it with an ohmmeter. There should be a periodic variation in resistance as the rotor is turned having several cycles per revolution determined by the number of commutator segments used. Any extremely low reading may indicate a shorted winding. An unusually high reading may indicate an open winding or dirty commutator. Cleaning may help a motor with an open or short or dead spot but most likely it will need to be replaced. Note that unlike a CD player which uses some motors constantly, the small PM motors in VCRs are only used for loading operations and are generally quite reliable unless damaged by other problems. For more information on small PM motors, see the chapter: "Motors 101" in the document: "Notes on the Troubleshooting and Repair of Small Household Appliances and Power Tools".
Capstans are expensive especially if they are integral with the capstan motor, but unless it is bent (very unlikely), or the bearings are totally shot, or it is direct drive and the motor is bad, the capstan should not be a problem as long as you **carefully** clean off all of the black tape oxide buildup with alcohol and a lint free cloth or Q-tips. Don't get impatient and use anything sharp! The black stuff will come off. A fingernail may help. A dry bearing may need a drop or two of light oil (electric motor or sewing machine oil). Sometimes, there is a bearing cover washer that works its way up and interferes with the tape movement. Push it back down. Some Sony VCRs have had problems with defective capstan motors resulting in intermittent pausing or stopping of video playback when hot. The entire motor or just the bearing assembly needs to be replaced in this case.
From: whitmore@jila.colorado.edu (Mike Whitmore) VCR capstan motors are servo-controlled to allow precise speed and phase control. Typical signals are: VCC - power to chip/motor- probably 9-12V FG - frequency generator output from motor to servo loop CTL - control track pulse from Audio/Control head F/R - forward/reverse (one high, one low) There will probably be other connections for a variety of servo voltages, braking, grounds, etc. - You may need to find service literature for this VCR or the datasheet for the particular driver chip to get more info. Data will also tell if motor is 3-phase. This is common for many capstan motors and would require this IC to run it.
In order for the video to be read off of the tape properly, the spinning video heads must be centered on the very narrow diagonal tracks. The width of these tracks is as small as .019 mm. The actual reference point is not on the video heads but the A/C head - several inches away. The control pulses put down during record are used to phase lock the capstan to the spinning video heads. The distance between the control head and the video heads determines whether the required centering will be achieved. In the ideal world, the distance would be identical for all VHS VCRs - that is the goal. It is part of the VHS specification. However, whether from wear and tear, or even if the technician doing the setup in the VCR factory had an off day, this distance may not be quite identical on the VCR that the tape was recorded on and the machine being used for playback. Therefore, a way is needed to adjust the effective distance. A mechanical control would be possible but not very elegant. Therefore, an electronic tracking control is provided. This basically allows adjustment of the time delay or phase of the control pulses from the control head during playback. Record tracking is fixed. Actually, there may be as many as three tracking controls: (1) the user tracking knob or buttons, (2) an internal master tracking adjustment, and in fancier models, (3) an autotracking servo system. (Note: tracking is always automatically reset to the default when a cassette is inserted.)
A single pair (2) of heads is needed for basic record and playback. With more heads, various aspects of these functions can be optimized to improve picture quality - usually for the special effects like CUE and REV. For example, a 4 heads are usually needed to produce decent quality playback in CUE and REV modes for SP recorded tapes. Another set of heads is required for HiFi audio. The only possible difference for record or at normal playback speeds is in picture quality since with 4 or more heads, head widths/gaps can be better optimized for each speed. For example, a wide track width can be used at SP speed and a narrower one for EP speed. Which VCRs do this, I have no idea. In fact, such differences might only be visible to the average viewer in an A/B comparison under controlled conditions. The stability of the video playback has nothing to do with the number of heads. A jumping picture during playback is due to a servo system problem. With problems of this type particularly on a new VCR commercially recorded or rental tapes, it is more likely that the VCR is having problems with some kind of copy protection scheme.
The quick answer is: "almost any combination which includes at least one head of each azimuth angle on each side of the video drum" :-). For a 4 head VCR, this may even include all 4 at once. In this case, signals from both heads of the pair on each side of the drum are monitored and the one with the greatest amplitude is sent to the video circuitry. This provides clearer special effects for SP recorded tapes in particular - CUE, REV, SLOW, and PAUSE - where the video heads may be crossing tracks of both azimuth angles. Such an approach may be called a 'double azimuth' design by the manufacturer. For record and play modes, an opposing pair will be used but which pair will depend on speed - EP, LP, SP. Thus, almost anything is possible and it gets to be confusing very quickly! Don't count on finding this information in the service manual either.
(From: Paul Weber (webpa@aol.com)). A six head (VHS) vcr has 4 video heads and 2 audio heads on its rotating upper cylinder. The 2 audio heads record VHS Hi-Fi. They are about 1/3 the width of the most narrow video heads (about 6 microns). A four head machine lacks the audio heads and is therefore incapable of playing or recording Hi-Fi. There are also 2 head machines on the market. They use the same pair of video heads for all tape speeds. 4 and 6 head machines use the 28 micron wide heads for SP (highest speed), and the 19 micron heads for LP (middle) and EP or SLP (slowest) speed. Some machines have a 7th head: the flying erase head. It is about 40 microns wide, and when activated, can erase the recorded tracks of both video fields that make up a frame. Most vcrs use all 4 video heads to smooth out the picture when scanning in fast forward and rewind. This is why 2 head machines have much more noise in the picture when scanning. Machines that have a switch to turn off the 19 micron heads do so in an attempt to improve the playback of tapes made on old 2-speed (SP and LP) machines. These machine had video heads that were something like 22 microns wide, and scanning them the narrower heads of a modern machine sometimes produces unacceptable results. Compatibility problems between machines are a fact of life because of mechanical differences. Recording at the highest possible speed minimizes problems, but hi-fi audio tracking problems can happen even then because the tracks are so narrow. If you have video tracking problems between 4 and 6 head machines on tapes recorded at the highest speed, it is because of mechanical differences, not because of the number of heads on the machine; the number of video heads is identical. The differences are in the alignment of the audio/control head that controls synchronization of the video upper cylinder, and in alignment of the video heads themselves.
SVHS won't be better than a good 4 head (+2 HiFi) unless: 1. You use high quality (read: expensive) SVHS type tapes (usually, there may be exceptions and some people claim that premium VHS tapes will work for SVHS recordings if the proper hole is drilled in the case but don't count on it). 2. The recordings are actually made in SVHS mode. 3. They are played back on another SVHS deck. Since few people have SVHS decks, there is probably little benefit if the objective to to make high quality recordings to share. I would probably go with a good 6 head (including 2 HiFi) since it will be compatible with everyone. However, just saying it has 4 or 6 or 25 heads doesn't mean it will produce a high quality result - there is a lot of variation in video and to some extent HiFi audio quality.
I assume VISS stands for "VHS Index Search System" or something similar. In any case, VISS and VASS provide the means to mark the start (usually of a video segment so that it can be accessed quickly later on. "How standard is this system? My Goldstar VCR has VISS and now I see reference to an LXI brand with the same system. I've heard of other VCRs with functionally similar features, but never had the opportunity to try exchanging tapes. Do they use the same marks? When my Goldstar finally bites the bullet (beyond my powers to resuscitate it), will the collection of indexed tapes I've built up be useless, or will another VCR with indexing features find the marks that Goldstar put on them?" (From: Ed Ellers (kd4awq@iname.com)). VISS is a real standard, issued by JVC in 1986. There is also a VASS -- VHS Address Search System -- using the same techniques, but it was taken off the market in 1988 after a patent infringement suit by a German company; JVC settled that case, but VASS never came back (at least not in North America); JVC later developed a more sophisticated system called CTL Coding, but it's not used on consumer VCRs over here either. VASS records four-digit codes instead of a single index mark; CTL coding records an actual time code on the control track, and also provides for VISS and VASS use. Incidentally, VISS and VASS work by altering the duty cycle of the 29.97 Hz square wave recorded on the control track; the servos still work on the average phase of the signal, but the changes in duty cycle are decoded into a slow bit stream. Before VISS was developed a number of VHS VCRs had a different indexing system that recorded a low-frequency signal across the entire tape, using a special head on an arm that contacted the tape while it was rewound into the cassette; these index codes could only be placed at the beginning of a recording and couldn't (then) be read except during rewind and fast forward modes. Theoretically a modern VCR could be made to read this signal using the control head, but this would require a special circuit to be added; I don't know of any VISS-capable decks that can do this, and given the small number of recordings likely to still exist with the old index signal it wouldn't seem to be worth the trouble.
(From: Matt Kruckeberg (sackmans@ndak.net)). My understanding of commercial advance is that the program is monitored during recording for fade to black and silent audio between programming and commercials and between commercials. The microprocessor stores these events in memory until the recording session is over. It then analyzes these events to determine whether an event was part of a group of commercials or just a dark silent passage of programming. The tape is then rewound and the beginning and end of the commercial groups are marked with special start and end signals recorded on the control track, similar to index search marks. During playback with the feature activated the unit will automatically forward search when a start signal is detected and resume normal play when an end signal is detected.
It always amuses me to listen to comments about how anything older than 6 months (or 30 minutes) should be tossed in favor of some newer, more cheaply made piece of crap. Yes, convenience features and HiFi audio have made newer VCRs a lot nicer in many ways. But for time shifting and the kids, that old clunker will do just fine, thank you. Some of the older VCRs will just keep going and going and going and going with a cleaning and a few rubber parts from time-to-time. On the other hand, I had to repair my high-end (for 1990) moderately used Mitsubishi VCR because a 10 cent plastic part broke (their cost, my cost - $10) - clearly an exercise in design-to-fail engineering. For about .5 cents more, it could have been built never to fail. The replacement part was identical to the original, so I give it about 4 years.
(From: Stan Cramer (stvcrm@Gramercy.ios.com)). In recent years, the rapid decline in the quality of construction of VCR's has been widely chronicled here and in other forums. Through all of this criticism, I have staunchly defended JVC as the last bastion of construction integrity! Alas - no more! Tonight, I had the occasion to open up a JVC HR-J620U and was shocked at what I saw! I am sad to announce that even the once venerable JVC has sold out to the concept of making machines really light and really, really crummy! This new JVC transport is the epitome of designing "throw away" machines - even worse than the transports offered by Matsushita or Funai! Glaringly absent is the modular power supply. You may no longer fall back on the last resort - replace the power supply! Folks, this is just an unmitigated piece of unadultered crap! As both a consumer and a VCR technician, I am truly offended by the shoddy construction of all new VCR transports and, in particular, by the caving in of JVC to make machines geared to the lowest common denominator. All of us should be outraged! (From: Greg Monbourquette (gregm@globalserve.net)). I too am concerned about the lack of care that the engineers who put these things together take when considering the amount disassembly required in order to only clean a lousy belt. (I'm talking mostly about the RCA/GE models with a plastic plate covering the bottom of the VCR) And yes there was once a time when you could buy/sell a vcr and KNOW that the customer won't have any problems for at least 5 years . I tell my customers all the time when they finally decide that the 15 year old TV that finally died ( for the first time) will be replaced by a new one, " don't expect 15 years out of any TV you buy today. Oh well we've (I've) ranted long enough. I now know my feelings aren't only mine.
Possibly, but why bother? You will most likely be limited by the VCR's circuitry anyhow. All S-Video means is (1) a special connector and (2) separate luminance (Y) and chrominance (C) rather than composite video. In a VCR, you will need to bypass the input circuitry and get to the place where Y and C are separate. This may or may not be possible depending on its design. It is probably not worth it as you will likely not gain much in picture quality but if you really are determined, a schematic will be essential. If all you want to do is allow for an S-video input, there are single chips which will combine the Y and C into a normal composite video signal.Go to [Next] segment
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