Notes on the Troubleshooting and Repair of Video Cassette Recorders


  10.14) VCR has gone whacko

You may think you are on the set of the latest sci-fi movie.  The VCR
displays are counting at random, pushing buttons produce unexpected results,
motors may be spinning, or the VCR may be repeatedly loading and unloading
a non-existent tape.  I may be attempting to play a tape even without
you pressing any buttons.

While these could be symptoms of a actual problem, first try unplugging the
VCR from the wall outlet (don't just turn it off) for a minute or so.

If this does not help, try unplugging for a couple of hours - this will
usually drain the backup battery and reset many other functions of the VCR.

If one of these techniques results in the universe returning to normal,
there may have been a power surge or lightning strike nearby
which threw the microcontroller into a confused state.  It may never happen
again.  However, power surges can be the result of heavy appliances like
air conditioners on the same circuit.  If this is the case, you should
consider using a different circuit for your electronic equipment.

If this behavior started when the VCR was just plugged in or following
some other action requiring the mechanism to move or initialize, check for
mechanical problems like a broken belt or one that has popped off its
pulleys or an obstruction like a rock or toy that is preventing the VCR
from completing the required motions.  Also see the section: "VCR is failing the power-up sequence".

Once you have ruled out mechanical problems, it is likely that the VCR
has a microcontroller, power supply, or other electronic problem which
may require professional service.

  10.15) VCR forgets settings following power failure

Normally, the AC line provides power to retain the clock, active channels,
and programming settings.  During a power failure, the clock and programming
is usually powered using a supercap or battery (usually rechargeable).
Channel settings for older style varactor type tuners were often stored in
some kind of non-volatile memory while active channels for quartz tuners
generally use battery backup.

The clock and programming backup may be a supercap - a very high value
special electrolytic capacitor - as much a 1 F (1,000,000 uF) at 5-12 V.
Alternatively, it may use a rechargeable NiCd battery.  In either case,
these are easily replaceable with standard parts.  A NiCd battery pack of
similar ratings should be readily available.  Supercaps are available from
large electronics distributors.

NiCd batteries fail in two ways - loss of capacity or shorted cells.  If
memory is retained for a much shorter time than it used to, then the battery
has probably lost most of its capacity.  If you measure less than n x 1.2 V 
for an n cell NiCd battery pack after it has been charging for awhile,
there is likely a shorted cell.  In either case, the best solution is a
replacement though the various common techniques for rejuvenating NiCd
battery packs can be attempted (remove from VCR first!).

The non-volatile memory could use a special chip like EEPROM which does not
require power or a battery backed SRAM or be internal to one of the
VCR's microcontrollers.  Channel memory may use a separate power source
from the clock and programming, possibly a Lithium battery since it is
undesirable for the channel settings to be forgotten even if the VCR is
unplugged for a month or more as it is such a pain to reinitialize them.
Rechargeable batteries have too high a self discharge rate.

  10.16) Display is dead but everything else works

This usually means that one or more of the voltages to the vacuum fluorescent
display (VFD) are missing or that the display controller is bad.  If the front
panel suffered physical damage, the display tube, circuit board, or other
components could be damaged.

The VFD requires around +30 VDC for the tube anode and 4 to 6 VAC or DC for
the filament.  Its cathodes in the form of character segments or formed words
or symbols will likely be driven directly from one of the controller chips.

Remove the front panel and with the VCR plugged in, turn out the lights and
inspect the filament, several very fine wires running the length of the
display.  They should be glowing a very faint red-orange.  If you see nothing,
the filament voltage is likely missing.  Filament voltage may come directly
from the power transformer (if a non-switching type power supply) or be one
of the DC outputs of the supply.

Check around the VFD for the +30 VDC (approximately).  If this is missing,
there will be nothing displayed.  In some VCRs like those manufactured by
Hitachi, a separate DC-DC converter module provides power for the display
only.  See the section: "Dead clock in Hitachi manufactured VCR".

Look for bad connections, open resistors, blown IC protectors or fuses, etc.

Of course, if the VCR has an on-screen display, you will be no worse off than
many newer models that have done away with the front panel VFD entirely!

(From: Paul Grohe (grohe@galaxy.nsc.com)).

The fluorescent display in most VCR's require three voltages:

1. The filament requires a floating 3 VAC.

2. The filament has to be biased at -12 V to -15 V

3. The segments need -20 V to -30 V to light.

The DC-DC converter usually provides a "floating" 3 VAC winding, a
low current -12 VDC tap connected to one of the filament leads,
and a -20 VDC to -30 VDC segment drive voltage.

If you look really, really closely at the display, you will see
the faintly glowing filaments stretching across the length of the

Chapter 11) Play and Record Control Problems

  11.1) VCR randomly switches speeds, tracking problems, and muddy sound

First, don't ignore the possibility that you are attempting to play an
old, worn, or defective tape.  This is especially true of rental tapes
which have been through who knows what kind of VCR hell.  The control
and audio tracks - along the edges of the tape - are the first to wear.
Weak muddy sound and erratic tracking are also common symptoms caused
by old worn tapes.  There have even been instances of new name brand tapes
which were cut too wide - though this would be extremely rare.

To confirm that it is your VCR doing the dastardly deed, play or record for
at least a minute on a tape known to be in good condition.  The use REV to
back of the tape for about 15 seconds.  Eject and open the cassette door by
releasing the latch and inspect for edge crinkling.

Any rippling along either edge of the tape is an symptom of a possible
problem.  It isn't only that the tape does not make good contact
with the audio or control head (depending on which edge is damaged)
but just an indication that the tape may not be moving through the
transport precisely positioned.

Assuming you are having the same problem on multiple tapes and that
using a known good (new) tape results in damage:

This is an indication that your tape path alignment is off or your rubber
parts (probably the pinch roller) need replacing.  The tape is wandering
up and down as a result of unequal pull from the capstan due to a glazed/worn
pinch roller.  There could also be other aspects of tape path alignment like
roller guide tilt (which is probably not adjustable), A/C head tilt, dirt,
roller guide height (don't mess with it), etc.  See the chapter: "Tape Path
Alignment and Backtension Adjustment.  It could also be worn feet on the
roller guide assemblies causing the guides to not be perfectly vertical.
Replacement of these parts may be the only cure.  Other much less likely
possibilities: excessive or varying backtension, tight idler clutch,
electronic problems.

For a VCR with very high mileage, it is also possible that there has been a
ridge worn in the surface of the control head preventing consistent contact
between it and the tape:

(From: Phil Reed (100555.244@compuserve.com)).

"One thing that can happen is that the control track head gets a ridge on it
 (due to wear) which prevents the tape making good contact with it.  This can
 make the tracking go mad and sometimes even mute the video.  Pausing the VCR
 overrides any muting, resulting in a clean still picture.  Another clue is
 that some tapes will do it worse than others, this is due to slight
 variations in tape width or condition."

Other related symptoms include:

* Sound does not always appear at full volume or normal quality for a few
  seconds after the VCR starts playing.  It may vary in loudness during play
  as well.  Slightly changing backtension may make a big difference in audio.

* If your VCR has autotracking, its indicator may be flickering as the logic
  attempts to solve an impossible problem.

* On HiFi VCRs, there will likely be no HiFi sound as its tracking is even
  more critical than video tracking.

* Tape speed may be changing resulting in wavering sound or even running
  (usually) faster than normal.  This may be due to the control head not
  reliably reading the control track.

If you look carefully, you should be able to see the tape wandering
slightly producing the muddy sound and erratic tracking.  The tape
may not be perfectly smooth in passing over the various guides and rollers.
Normally, you will almost not be able to tell the tape is moving at
all except by examining the reel rotation - it is that mirror smooth.

First, clean the tape path properly, especially the capstan and pinch roller,
tape guides, A/C head.  Inspect the pinch roller for glazing, cracking,
etc. and replace if necessary.  See the sections: "General guide to VCR cleaning and rubber parts replacement" and if necessary, the chapter: "Tape Path Alignment and Backtension Adjustment".

Another possibility is that the control portion of the A/C head stack is dirty
or defective or there are problems in the wiring or its circuitry.  Double
check that the tape is in solid contact with the bottom of the A/C head stack
(where the control track is located), that the head is clean, its connector is
clean and seated properly, and look for any broken wires or bad connections.

  11.2) VCR plays but at fast forward speed (or beyond)

Normally, speed is controlled via phase locking the capstan to the 30 Hz
control pulses read off of the tape via the stationary audio/control head.

On a VCR with autotracking, the autotracking light may be flickering as well.

Possible causes for loss of lock:

* Dirt or bits of tape or oxide on control head - clean and inspect.

* Defective control head.  Try making a recording.  If recording plays
  normally on another VCR, then control head is probably ok.

* Tape wandering up and down so that control track is not sensed properly
  (how is the sound - this would also cause fluctuating or missing sound.)
  See the section "VCR randomly switches speeds, tracking problems, and muddy sound".

* Mechanical fault preventing firm tape-control head contact such as a stuck
  movable guide post.

* Mechanical or mode switch problem preventing firm capstan-pinch roller
  contact.  Under certain conditions - possibly at the beginning of a tape
  when takeup tension is greatest - the takeup reel may have enough torque to
  pull the tape past the video heads without the capstan controlling the speed
  as it should.

* Defect in servo or control circuitry or power supply (voltage out of

* Bad tape.  Don't overlook this possibility especially if it is a old or
  rental tape.  The control track may have gotten erased or warn off - it is
  at the edge of the tape.  Try another tape.

Inspect the tape path really really carefully to determine if there is some
obstruction preventing tape-control head contact or other mechanical problems.
Try cleaning the tape path and checking the rubber parts.  Check power supply
voltages if you can determine what they should be (see the section: "VCR power supplies".  If these procedures to not reveal anything amiss, you will need a
service manual to pursue electronic faults.

  11.3) Tape edge gets creased and/or random switching between speeds

As always, rule out the possibility that this is just a bad tape.  There have
even been instances of new name brand tapes which were cut too wide - though
this would be extremely rare.  It could have been creased by someone else's
VCR.  Try a tape you can afford to sacrifice (though it will still be safely
usable) and run it through the VCR.  Sometimes, there will be a problem only
near one end so you will need to try it at various sections of tape.  Record a
few minutes and then back it up a bit and inspect for damage by opening the
cassette door (press the release on the side).  Both edges should be perfectly
flat and smooth.  If you get similar playback symptoms with this cassette
and/or find that the tape is being creased along one or both edges, then it is
your VCR doing the dirty work.

When the bottom of the tape gets creased, the control head may no longer
align with the control track and you loose servo lock on the sync signal.
Your audio may be fluctuating in intensity as well since the audio track
is wandering also and the tape may be intermittently going in and out of
correct tracking and/or changing speeds.  Since the tape can no longer
seat stably on the lower drum guide ridge, there could be other problems
such as noise bars along the top or bottom of the picture, jumping, etc.

It could be the guide posts or other tape path components, but before you turn
every screw you can find and make the problems hopelessly worse, replace all
of the rubber parts - belts, idler tire, pinch roller. And while you are at
it, give the machine a good cleaning.

A dirty, worn, hard, dried out pinch roller in particular can result in
the tape wandering up and down causing tracking problems and creasing the
tape in the process.  This is probably the most common cause of tape damage
assuming the VCR itself has not been abused (i.e., jammed cassette removed
using a pair of Vice-Grips(tm)).

With a thorough cleaning of everything before buying the new rubber (which BTW
should not be more than a total $10-$15 from a place like MCM Electronics),
you may at least see a temporary improvement in performance - and confirmation
of the diagnosis.

You really need to determine exactly where the tape is being creased.
Once you do this, you may be able to determine the cause and visually
verify whether the problem is affected by any of your adjustments or probing.

Some other possibilities include:

* Worn feet on the roller guides causing them to not be precisely vertical.
  Sometimes there are adjustments for tilt; usually there are none.  Sometimes
  replacements are readily available (especially if this is a common problem
  with your model).

* Cassette not seating properly.  Press down on cassette while playing
  a known good tape.  If it moves, then check for obstructions or foreign
  objects such as toys or peanut butter and jelly sandwiches!  A dirty, oily,
  or just tired belt may not grip well enough for the mechanism to complete
  the cassette load cycle.

* Oil seal washer on bottom of capstan has worked its way up out of place.
  Carefully push it back down and then clean the capstan shaft.

* Various guides too high or too low but this is pretty unlikely unless they
  have loosened somehow.  Don't adjust unless you have a service manual or
  are absolutely sure that they have changed height.

* Backtension misadjusted (usually too great).  If the tape passes around the
  backtension lever at too straight an angle (it doesn't bend enough), in
  addition to the possible incorrect (excessive) backtension, it may simply
  not seat properly when passing around the subsequent guidepost or impedance
  roller (that white plastic wheel that doesn't seem to serve any purpose).

  11.4) Recording stops at random times on previously used tapes

Symptoms may be that the tape counter stops moving and/or the VCR
enters stop mode and shuts down.  Assuming this is not a mechanical
problem - bad idler, belt, etc., make sure you don't accidentally
have an 'insert editing' mode enabled.  Insert editing uses the
previously laid down control track as the timing reference.  This
provides clean glitch-free transitions between scenes. Insert editing
will not work at all on a new or bulk erased tape.  If you routinely
use your cassettes over and over, there will be varying amounts of
previously recorded material - with control tracks - on the tapes.
At some point your recording may start to use tape beyond the recorded
sections and - presto, no more control track.  Poor VCR is confused
and aborts.

  11.5) Record (or play) stops after 15 minutes (or 30 minutes, etc.)

Make sure you are using the proper record button.  Most VCRs have a
OTR (One Time Record) or 'quick record' feature which starts just like normal 
record stops after a multiple of (usually) 15 minutes depending on how
many times you press the button.  The (normally) red button should be
used for unrestricted untimed recordings.

Some VCRs also have other timed modes - sort of like the timed off function
of a clock radio.  Pressing the 'Off-T' button adds time to record or play
in 15 minute increments and then the VCR shuts off.

  11.6) Tape counter is erratic

The result may be inconsistent positioning of the tape if you use the
counts to locate programs.  It might also result in the VCR aborting
PLAY, REC, FF, REW, or search modes if it thinks that the counter is
not changing as expected - missing pulses or skipped counts.

For real-time counters, this may mean a problem deep in the electronics
requiring a service manual.  However, if you are attempting to play a tape
that has nothing on it, the real-time counter will not change.  This is
normal as there are no control pulses on the tape.

For non-real-time counters, if the display skips counts or 'free runs' -
counts very quickly at certain times, this could be due to a defective
sensor or hysteresis circuit.  If it counts in the wrong direction, a logic
problem is indicated as direction is determined by the microcontroller being
aware of what mode the VCR is in - there is likely no actual direction
sensing on the reel.

See the section: "Reel rotation sensor testing" for further information.

Chapter 12) Video Play and Record Problems

  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.

Sometimes, when snow or serious video noise suddenly occurs while playing a
rental, old, or damaged tape, it means the video heads have picked up some
oxide and are no longer making good contact with the tape.  Letting the VCR
play a newer tape for a few minutes may clear this if it is minor.  However,
video head cleaning (using a cleaning tape or the manual procedure described
in the section: "Video head cleaning technique") will probably be needed.
But, first start with the section: "Snow on one or more speeds" and NEVER
NEVER attempt to clean the video heads without using one of the recommended
techniques - you can easily destroy the heart of your VCR!  Also, never
attempt to play or record on a spliced or seriously damaged tape as this can
also result in destruction of the video heads.

  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

  This may mean that the record protect tab on the cassette is broken off or
  the record tab sense switch is dirty or bad.

* Record (either manual or timer) stops at random times - possibly with
  flashing display and/or ejects cassette.

  This could be the result of a dirty or defective record sense switch or
  misalignment preventing proper engagement with it.  Some VCRs check for the
  record tab constantly while others just check when the REC button is pressed
  or the timer initiates record.

  It could also be a defective reel or tape end sensor halting record though
  these would likely affect playback as well.

* Playback results in video snow and whatever was on the tape, if anything,
  is gone.

  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 due to 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 go away, 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 two heads used for record is dirty or

* 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.

* Playback results in a picture which is cycling in brightness or flashing.

  This likely means that you are attempting to record (copy) a Macrovision(tm)
  (see the info at: http://www.repairfaq.org/filipg/LINK/F_MacroVision.html)
  or some other copy-protected tape or your cable or satellite company is
  transmitting copy-protected video.

  Some of the new digital DBS satellite receivers output a Macrovision copy
  protected TV signal so you can't tape the movies from them either.

  Newer VCRs will generally not record successfully.  Some older VCRs will
  record without problems.  See the section: "Why VCRs will not copy (Macrovision) copy protected tapes".  (8mm VCRs may record the entire
  signal and therefore be able to playback successfully.  However, attempting
  to copy the 8mm tape onto a VHS tape will result in the same problem.)

  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 the section: "Video head cleaning technique").

New video heads may fix this, though it can be caused by other problems such
as weak read electronics.  See the chapter: "Video Heads and Upper Cylinders".

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 back-
tension was low.  However, low backtension will usually show up as a waving
or flagging effect at the top of the picture.

  12.4) One or more lines at fixed locations in picture

This means that there is one or more horizontal lines during playback that are
at fixed locations on the screen.  These could be the result of electronic
problems or marginal video heads but the possibility that should be explored
first is that of tape damage.

If a prerecorded tape that plays properly on another VCR, shows the effect on
the suspect VCR - AND - then shows the same thing on the other VCR, it is
being damaged by something in the tape path.

Open the door of the cassette by releasing the catch on the side.  Look
carefully at the surface of the tape - it should be mirror smooth all
across.  If you see any evidence of hair fine (or larger) scratches running
the length of the tape these are what are causing the line.  This is likely a
result of a bit of debris or a rough edge on one of the guide posts in the VCR.

Get a brand new tape or a known good tape (that you can afford to mess up)
and test it on another VCR (at the tape speed that is worst, if this matters).
Assuming playback is fine, play it on the suspect VCR for a couple of minutes.
Pull the plug (DON'T hit STOP) so the transport remains in the fully loaded
position.  Now, carefully examine the surface of the tape all along the tape
path (disturbing its position as little as possible) to identify the location
where the damage begins.  It may just be a bit of something stuck to a guide
post.  Has the VCR been cleaned in the last 10 years?

Note: This sort of damage to the tape does not represent a risk to your VCR's
video heads so you can continue to use the tape if desired.

  12.5) 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.

  12.6) 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 pre-recorded 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.
Where tracking is adjusted with a set of +/- buttons, these may also be used
in PAUSE mode.  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.

  12.7) Video search blanks out or doesn't work on recordings made at certain speeds

This may be a 'feature' of your VCR.  On some older models, the designers
in their infinite wisdom (or that of their marketing departments) decided
that no picture or no search capability at all was preferable to a picture
with serious noise bars or one which didn't sync properly.  This was usually
before the days of 4 head VCRs which directly addressed at least some of
these issues.

Most 2 head VCRs will work fairly well on EP recordings but show noise bars
over about 50 percent of the picture with SP recordings.  For those made at
LP speed, tearing will occur in addition to noise bars if they sync at all.
Few VCRs deal properly with LP search as substantial additional circuitry is

In my opinion (IMO), any picture is better than a blank screen or no search

  12.8) VCR plays pre-recorded tapes but its own recordings are noisy or jumpy

Problems will be similar to the following:

"I have a General Electric VCR model VG4217 that's displaying the most
 unusual problems. When I play back a pre-recorded tape from a video store
 it plays fine. When I play back a tape recorded on the machine I get video
 noise for 4 seconds then clear pix, then video noise for 4 seconds, then
 clear pix and so on.
 I also noticed that if I have the tape counter displayed on the screen, and
 when the counter progresses its count, the tape plays properly. Then all of
 a sudden the counter stops counting and the problem continues again.
 I have cleaned heads well, cleaned tape path, and even cleaned the underside
 of the takeup reel, all to no avail?"

First, make sure the tapes are in good condition.  They may have been damaged
(edge crinkled) before you serviced the VCR.  This is now causing your erratic
behavior and there is nothing wrong with the VCR.

Before considering drastic action, record on a brand new tape - from end-to-end
if the initial results seem promising.  You may have a non-problem.

Try recording you your VCR and playing back on another one.  If this works,
then bad tapes are the most likely explanation.

If this does not work, there could be electronic problems:

(From: Stephen Isaacs (stephen@myna.com)).

The normal playback of a pre-recorded tape suggests most things are working
fine.  The self recorded problems point to a faulty control track recording
system. bad oscillator, or amp.  It is also possible the erase head is not
doing its job making it difficult to record a new control track over an old

(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)

  12.9) 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
entire 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.

The assembly on which the rotating magnet(s) are mounted and the upper
cylinder may be secured with one or two set screws.  If these loosen, the
the precise relationship may be lost resulting in a shifted head switching
point.  It may even be random - changing location each time the drum starts
up due to the inertia of the upper cylinder.  If this is the case, you will
need a service manual to properly adjust the angular location of the magnet
assembly unless there are obvious 'timing' marks to guide you.

Beyond confirming that the pickup coil is in close proximity to the drum,
the rotating magnet and sensor are secure, 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 For Your Information (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.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]