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.