12.1) Video playback problems
If the VCR works in all respects when tuning broadcast or cable channels but
playing a tape results in no picture, a very snowy picture, or just a blue
screen, there may be problems with the video heads, the lower cylinder, head
preamps, or other video electronics. Testing most of these is beyond the scope
of this document and will require a service manual and test equipment. However,
you can do a decent job of determining if the video heads are likely to be at
fault.
12.2) Video record problems
If attempting to record results in unexpected behavior, there could be a variety
of causes depending on what you get for playback:
Attempts to record are ignored by the VCR or cause the cassette to be
ejected. This may mean the the record protect tab on the cassette is broken
off or the record tab sense switch is dirty or bad.
Playback results in video snow. This means that the old recording is being
erased (if there was one) but nothing or too weak a signal is being written
by the video heads. This could be dues a variety of electronic faults as
well as marginal or bad video heads.
Playback results in a picture but it has a wiggling rainbow pattern running
through it. This is normal at the start of a recording made on top of an old
recording if your VCR does not have a flying erase head. However, it should
wipe down the screen in a few seconds and disappear. If it does not, then
your full width erase head is not working.
Playback results in a flickering picture alternating between good video and
snow at the frame rate (about 30 Hz for NTSC). This could mean that one of
the heads used for record is dirty or defective.
Playback results in proper video but the previously recorded or no audio.
The audio dub switch (if any) may be in the wrong position or the audio
circuitry may be defective.
12.3) Snow on one or more speeds
Did the problem happen suddenly? Or develop over time? If suddenly, what were
you watching at the time? A (literally) dirty rental movie?
If this VCR has 4 or more heads, SP and EP may use a different set of heads, so
certain heads may still be dirty or bad. If the machine tracks perfectly in EP,
then alignment is probably fine - EP is more critical as to alignment as the EP
track is 1/3 the width of the SP track.
Have the video heads been cleaned using the proper procedure (not just a
cleaning tape - see section on video head cleaning).
New video heads may fix this, though it can be caused by other problems such as
weak read electronics. See the section on video head problems.
You should also check the backtension adjustment - if too loose, head to tape
contact will be compromised. Try increasing it momentarily by pushing the
backtension lever slightly to the left while the tape is playing. The usual way
to adjust backtension without a backtension meter and service manual is to look
at the image just before vertical retrace at the bottom of the screen - this is
normally not visible unless you can reduce vertical size or play with vertical
hold to get the vertical blanking bar to appear. Of course, most modern TVs
don't have any such controls! This is the head switching point and when the
backtension is properly set the image above and the bit of image below this
break will be approximately aligned.
If increasing backtension helps, either the heads are marginal or the
backtension was low. However, low backtension will usually show up as a waving
or flagging effect at the top of the picture.
12.4) Jumpy picture in play
You have a VCR with known good heads that produces jumpy (vertically) video in
play that cannot be stabilized with the tracking control. Perhaps you have
attempted to adjust the mechanical tracking and maybe some other stuff. Some
questions:
Did you replace the heads? Could you have gotten them 180 degree rotated
from the correct position? I don't know what the implications would be on
your model VCR, but there is a definite right and wrong on this. It would
certainly show up as tracking being way out when attempting to play back
tapes recorded on this VCR on another machine.
Exactly what adjustments did you touch?
Have you verified that the roller guides are fully engaged against the
stops?
Have you checked backtension?
Did you touch roller guide height?
This is probably a mechanical problem, most likely an adjustment or fault
related to tape path alignment. However, it could also be due to electronic
problems with the video or servo circuitry. The vertical sync could be corrupted
or the head switching point not set correctly.
The head switching point is 6.5 lines before vertical sync. If this ends up
moving into vertical sync for some reason, you will get unstable video. The
supply side roller guide height adjustment is also critical and would be the
first thing to check mechanical alignment problems are suspected.
However, don't overlook the obvious: your TV is marginal or misadjusted or you
are attempting to play a bad tape.
12.5) Picture shakes or jumps or has snow in PAUSE/CUE/REV
Note that on a 2 head VCR, it is not possible to display a noise-free picture on
a tape recorded at the SP or LP speeds. Therefore, for rental or prerecorded
tapes, what you are seeing may be normal. A 2 head machine should execute these
special effects perfectly fine with EP(SLP) recorded tapes, however.
VCRs with 4 or more heads will usually have a V-Lock adjustment - either a knob
on the front or rear panel, or sometimes 'conveniently' accessible from under
the VCR. Sometimes, a special tool is needed to adjust this control. There may
be separate adjustments for SP and EP(SLP) speeds as well. In any case, these
settings are made while viewing a tape recorded at the appropriate speed in
PAUSE mode.
For LP speed - which is being phased out by many manufacturers, at least for
record - these special effects usually do not work well if at all. This is
basically due to the nature of the sync signal alignment on tapes recorded at LP
speed and would require complex circuitry to handle properly at anything other
than normal LP play speed. (If you care, the sync tips between adjacent tracks
align on the tape in SP and EP recorded tapes but are off by 1/2 line with LP
recorded tapes. This results in the tearing seen in search modes with LP
recorded tapes.) Since this tape speed is of little true value - it is a
compromise anyhow - the added expense has been found not to be justified except
on professional machines.
(From: Rob-L (rob-l@superlink.net)).
Some VCRs with +/- tracking buttons also use those buttons for adjusting the
still-frame mode. If your VCR has them, try them. Other units may even have a
little stem/knob you can turn for this. This assumes that you are talking about
vertical jitter, which is caused by the two heads reading the same field on the
paused tape and not lining the signals up. You actually get half as many lines
as a moving picture. If the fields are off a lot, you get jitter.
12.6) VCR plays prerecorded tapes but its own recordings are noisy or jumpy
(From: Richard (vcrtips@mail.vii.com)).
Almost all pre-recorded tapes are recorded at the SP speed. If you are like most
people, you probably do your recordings at the EP speed (to get as much on the
tape as possible). Do you have the same problem if you record at SP? Your VCR
probably uses different heads for SP and EP. You may have dirty EP heads,
defective EP heads, a head amp problem. Or, there could be a tape tension or
other mechanical problem.
(From: Frank D. Ralston (fdr@continet.com)).
Check the following:
Dirty (or worn) heads
Low back tension (common problem)
Tape path alignment (particularly input tape guide)
12.7) Incorrect frame alignment or bad video for part of frame
Symptoms like a picture which has a portion that is noisy or missing, or where
the picture is split between top and bottom with the vertical blanking somewhere
in between may indicate a problem with the PG sensor.
The rotational position reference for the video head drum is usually supplied by
a pickup in close proximity to the edge of the lower cylinder (probably) which
has a small magnet fastened to it. This generates the so called 'PG' pulse and
is used by the servo circuitry to properly control the drum rotation and the
head switching point. If this sensor is moved or if there is a fault in the PG
circuitry, a variety of record or playback problems can result. Without this
reference, the servo circuitry has no way of knowing where the A and B heads are
at any given time. During record, this may result in recording video which is
not properly lined up with the video tape - a track may consist of the end of
one field and the beginning of the next rather than an entier fields as it
should. During playback, the head switching point may occur at the wrong time
resulting in a partially snowy or missing picture since a head that is not even
in contact with the tape may be active. Similar problems may make look like your
TV's vertical hold control is set incorrectly with the vertical blanking bar
visible at an arbitrary point on the screen.
Beyond confirming that the pickup coil is in close proximity to the drum and
that there are no bad connections or loose connectors, there is not much to be
done for these problems without a service manual.
The definitions below are just FYI:
PG - pulse generator
The pulse is derived from the rotation of a magnet on the video head drum
past a sensing coil. I suppose this could be done optically as well.
FG - frequency generator
This is a signal (sine or square) derived from the rotation of the video
head drum. This may be phase locked to the PG pulse but can be a multiple of
the frame rate. This could also refer to the capstan or reel rotation rather
than the head drum.
12.8) Rainbow pattern in recordings made over previously recorded tapes
Unless your VCR has a flying erase head - located along with the normal video
heads on the rotating drum - you will see a faint rainbow pattern near the start
when recording over a previously used tape. The reason is that there is a
separation of a few inches in the tape path between the video heads and the full
width erase head. When you start recording at an arbitrary point, it takes
several seconds (actual time depends on recording speed) totally erased tape to
make it to the video heads. You are seeing an interference pattern between the
old and new video signals. The pattern will slowly wipe from top to bottom as
the diagonal tracks of new video intersect more and more of the erased tape.
This effect will not occur (except possibly at the very beginning of the tape)
as long as you record from start to end without backing up the tape at any time.
If the rainbow pattern is present whenever recording over previously recorded
tapes and does not go away, then your full width erase head is not working. This
could be due to an electronic failure or simply a bad connection to the full
width erase head. Alternatively, a mechanical problem such as a broken or popped
spring or gummed up lubrication might prevent the pivoting full width erase head
from contacting the tape properly.
12.9) Flag waving
You have just loaded a videotape sent to you from your long lost cousin and you
notice that the top of the picture is wiggling back and forth.
First, if this wasn't the original complaint, make sure the flag waving problem
exists with the TV that will actually be used with the VCR - it may just be your
test TV or monitor that is unhappy.
(Parts of the following from: Andrew Morphitis, Andrew@andrewsm.demon.co.uk).
This fault is sometimes known as flag-waving when associated with video
recorders. If the tape back-tension provided by the tension arm and supply
reel-table is not the same as the back-tension provided when the tape was
recorded (possibly on another machine) then the field timing of the video tracks
being played back will be inconsistent . Your back- tension can be checked using
a back-tension cassette gauge (a typical reading would be about 35g-cm for VHS)
or you could adjust the back tension using a known good test tape (or reliable
pre-recorded tape) until the waving disappears. If your back-tension does turn
out to be incorrect and you adjust it according to the manufacturers spec. then
all of the tapes you have previously recorded will probably still exhibit this
waving problem - adjust to spec. or to your tape library - take your pick.
Thats the theory - now the practice. Back-tension refers to the tension of the
tape over the head drum, this is provided by the felt covered metal band
(tension band) which is wrapped around the supply reel (left-hand reel from the
front), the friction providing the tension. There are usually two adjustments
associated with back tension and these can be found near the opposite ends of of
this tension band, the tension arm operating position and the anchor point of
the band. Adjusting the latter position will increase or decrease back tension
(you will want to increase your back tension which has dropped due to excessive
wear on the belt). If you do give it a bash then be aware that poorly adjusted
back-tension can, at worst, give rise to premature head wear. Because of the
differences between the back tensions of different machines, all modern TV's
have a dedicated video channel button (usually channel 0) which has a shorter
flywheel line-timing duration allowing the TV timebase to lock up more
effectively to unstable video sources such as video machines. Are you using the
video channel? - try playing the video through different channels on your TV.