Notes on the Troubleshooting and Repair of Video Cassette Recorders


  18.18) Where to obtain replacement video heads

Once you have concluded that a replacement head is required, you need to
decide whether you will undertake this yourself or take the VCR to a shop.
Video head replacement is relatively straightforward and low risk as long
as you are comfortable working on mechanical devices and take your time.
A little unsoldering and soldering is usually required.

Electronics suppliers such as MCM Electronics, Premium Parts, and Dalbani
stock a wide range of video heads for VCRs that are more than a couple of years
old. (They may not have heads for the latest models.)  In some cases,
they will offer two kinds of heads for the same model - a generic version
and a 'name brand'.  Unless you are extremely critical, there is probably
no need to spend the extra on the 'name brand' head.  There is also no need
to pay the premium charged by the original manufacturer of your VCR - it is
often priced 2:1 or more over what a generic head will cost with no
substantial difference in performance, if any.  You may even end up with
exactly the same head manufactured on the same assembly line!

Note that currently, the price of many upper cylinders (video heads) for
2 head VCRs is well under $25 so ordering a replacement may be a better
investment of time and effort than a long diagnostic procedure especially
if the old head has high mileage and video quality has been steadily

  18.19) Upper cylinder not replaceable as a separate assembly?

On some newer VCRs, it seems that in the manufacturer's infinite wisdom (or
cost crunching), the normal video head drum or upper cylinder cannot be
replaced by itself.  Only the entire expensive cylinder unit is available.

Unless you can find a junk VCR (try Allbrand, see the section: "Used VCR parts"), repair may be too expensive.  Just buy a different brand next time,
which, of course, may not matter :-(.

  18.20) Video head replacement technique

1. Do not touch the actual video head chips themselves.  Handle the head as
   little possible.  You can touch the upper part of the head cylinder
   if necessary.  One thumb through the center hole with fingers resting
   on the upper edges works pretty well.

2. Before you unmount the old one, mark or make a note as to its position -
   sometimes it is possible to mount the new head 180 degrees off from
   way it is supposed to be oriented causing tracking problems at the least
   as the opposing heads are not identical.  (The azimith angles are +/- 6
   degrees for VHS, +/- 30 degrees for VHS HiFi audio).  Also make a note of
   the wiring if there is any possibility of confusion (i.e., there are
   individual wires, not connections from below to a printed circuit board).

3. Unsolder the connections between the head and the upper cylinder.  There
   will be 2n solder connections for an n head VCR. (Sometimes there is
   some kind of connector rather than solder connections, but this is rare.)
   Examining the new head should reveal exactly where to unsolder.
   For pins through the printed wiring board type, you should use some
   kind of desoldering tool - solder pump, SolderWick, or a vacuum rework

   In rare cases where there are individual wires, a generic replacement
   head may not be color coded the same or have the wires originating from
   different places than the original.  In this case, you will have to try
   to determine which physical head chip the wires originally connect to.  You
   must get each of the connections from the lower cylinder to the head in the
   same physical head as before (though the polarity or phase of the pair of
   connections to each head should not matter).

4. Unscrew the 2 or 4 philips head screws holding the old head in place.
   It should be obvious from the new head which screws need to be removed.
   You may need to remove the static brush if your VCR has one or some other
   usually obvious stuff to get at it.  DO NOT touch any other screws
   on the head drum as these are critical adjustments one should not mess with.

5. Lift the old head straight up and off.  You should not need to use
   any drastic measures though a little jiggling may help.  I have never
   actually needed a head puller.

6. Replace in reverse order, solder the connections, replace any
   other hardware that was removed.  Refer to your notes on the position
   of the old head and/or the color codes (wire colors, dabs of paint,
   etc.) as to orientation on the drum.

7. Carefully clean any fingerprints from parts of the head drum you touched.
   Again, do not touch the video head chips themselves.  You may use 91%
   medicinal alcohol, though pure isopropyl is preferred.  Avoid
   rubbing alcohol especially if it contains any additives.  Let the
   machine dry completely.

8. Unless you tweaked any mechanical adjustments, the VCR will very likely
   work fine assuming the video head was the problem.  Try recording and
   playing back at all speeds as well as playing pre-recorded tapes as well.
   Carefully examine the video for excessive snow, jittering, or tracking
   problems.  For HiFi VCRs, also confirm that the HiFi audio is solid and
   stable - that the HiFi light is not flickering in addition to audible
   dropouts or muting.

   If the tracking is now way off or you experience serious video noise,
   lack of or erratic color, or bad or missing HiFi sound, refer to your
   diagram and double check that you didn't replace the head rotated 180
   degrees from the proper position by accident.  Make sure the drum is seated
   properly - not on a bit of dirt on one side.  DO NOT be tempted to adjust
   tape path alignment - if the heads were the problem, it should be fine.
   Also see the section: "HiFi/video tracking problems after upper cylinder replacement".

   It is a good idea, however, to perform what is known as the 'Tape 
   Interchangeability Adjustment' (this terminology is used in Panasonic
   VCR service manuals, meaning is self evident) in any case.  This procedure
   consists of adjustments to the roller guides, other guide posts, and the
   A/C head.  See the chapter "Tape Path Alignment and Backtension Adjustment"
   or follow the set of steps in your service manual.  On rare occasions, some
   electronic adjustments will also be required to obtain optimum video
   quality but this is the exception rather than the rule.  Tapes recorded
   at EP speed will almost always be more finicky and may require these
   adjustments more so than those recorded at SP speed.

  18.21) How about when the head drum refuses to budge?

(From: Raymond Carlsen (rrcc@u.washington.edu)).

If it's a Samsung based machine, you might need a head puller. I got one at
a service seminar many years ago and need it once in a great while to pull
a stubborn one, usually a Samsung.  Avoid the temptation to pry up with
screwdrivers. I've seen the results of such abuse... not pretty. Look for
threaded holes in the head drum. That's a clue it needs a puller, which
attaches to the head drum and presses downward on the center spindle with a
few twists of the handle. The new head (also a tight fit) is seated by
alternately tightening the two mounting screws.

You could make a puller with a bar of metal and some long screws. Drill two
holes in the bar to fit the spacing of the threaded holes in the head wheel
and one in the center between the other two. Tap the center hole (8-32 is big
enough). Use screws (small metric) long enough to thread into the drum to
attach the bar. Run a screw down the center hole until it contacts the center
shaft of the drum.  Keep rotating until the drum pulls off.

  18.22) What if the replacement upper cylinder doesn't work?

There are a number of possibilities and one of the more common particularly
with generic replacements is, guess what?  A defective replacement!

When replacing the upper cylinder, the orientation and wiring must be exactly
the same as the original.  For many VCRs, this is automatic since the mounting
is keyed and the wiring is via direct printed circuit board connections.
However, there are also many where it is possible to screw up either the
orientation or wiring or both:

* For lack of color, erratic color, excessive video noise, and tracking
  problems double check its orientation.  Accidentally replacing the head drum
  180 degrees from its correct orientation will result in a variety of video
  quality problems.

* Where individual wires are used rather than soldered connection to a printed
  circuit board from below, make sure you have attached the lower cylinder
  terminals to the proper heads.  Some generic heads apparently do not have
  the identical layout.  It may be necessary to visually trace the wiring on
  the old and new upper cylinders to determine which actual video/HiFi audio
  head chips are attached to which wires.  Also don't assume that the wiring
  color codes are same!  However, you are lucky in one respect: The polarity
  or phase of the pair of connections to each head should not matter!

  18.23) HiFi/video tracking problems after upper cylinder replacement

Unless you had such symptoms originally where best settings of the tracking
control for the HiFi audio and video are at grossly different positions, the
problem is with the video head drum itself or its installation.

(From: John R. Hepburn (jhepburn@recorder.ca)).

If you have an oscilloscope and service manual, check the envelopes for
maximum output.  Audio and video should max at roughly the same tracking
position. If they do, forget about heads or any mechanical problems, it is
electronic (but probably existed before the replacement - possibly masked by
an originally defective set of heads!).

If they do not, then it is probably a drum problem.  DO NOT adjust your
roller guide height or any other posts!  They were in the right position
before (unless you have already moved them) so they are in the right position
now.  Minor post adjustments are OK after heads are replaced, but that is just
to peak it out.  No serious problem has ever been solved on a VCR by adjusting
posts that are in their original position. Remove the heads and check for
proper seating. You would hate to make a big deal out of this, only to find one
side of the drum was seated on a small piece of residue.

  18.24) Tape interchangeability

Since video heads are not all manufactured exactly the same, there is a slight
chance that you will experience problems of playing tapes recorded on other
VCRs in yours.  However, before adjusting the roller guides or other settings,
make sure that the *other* VCR is aligned properly.

(From: John F. Reeves (jreeves@uwf.edu)).

Take an alignment video-cassette tape and verify that the P2 and P3 posts
are adjusted properly. you should use a scope and monitor the RF envelope
while adjusting the above mentioned posts. Once this procedure is done, 
make a recording and play it back in another VCR. If it still does not
track properly, it may necessary to perform the tape interchange ability
adjustment. This adjustment in more critical and more in depth, should be
performed be a qualified technician.

  18.25) What are the symptoms of a video that is on backwards?

Suppose I screwed up and installed a video head 180 degrees rotated from
what is correct.  What will happen?

First confirm that it is even possible to do this - some are keyed in such
a way that a hammer would be needed.

The effects will depend very strongly on the particular VCR but the following
are among the possible symptoms:

* Tracking that is way off for tapes recorded on another (properly adjusted)
  VCR.  It may be so bad as to be beyond the correctable range with the user
  tracking control.  The azimith angles of the head pairs are opposite of
  what is expected and this directly affects tracking.

* Noisy picture or no/erratic color in some or all play and search modes.
  On 4 or higher head VCRs, even opposing pairs of heads have different
  characteristics so these will not be matched to the electronics with the
  head on incorrectly.

* Flying erase will not work where only a single flying erase head is
  used.  To test for this (assuming your VCR has a flying erase head),
  record over an old recording.  If flying erase is not working, you will
  get a rainbow pattern (assuming you get any color at all) which will
  wipe down the screen over a 10 second or so period (just like a VCR
  that lacks flying erase).  The 7th head in a 7 head VCR is likely a
  flying erase head.

  18.26) Can I substitute a video head from another VCR?

The quick (and long) answer is: NO.  The heads themselves are in
no way standardized.  You can substitute a video head drum (upper cylinder)
if it is identical - VCRs sold under different labels are often manufactured
by the same few companies.  Check a cross reference if you have a dead VCR
with a good set of heads but not the same model as the one you are
trying to repair.  As far as the heads themselves, don't even think
about attempting to interchange the actual head chips - even if
your replacement were physically and electrically compatible, you would never
be able to get the alignment within tolerance since you do not have the
factory jigs.  Not to mention that the head chips themselves are really
really tiny and really really fragile and their specifications
all vary - head width, azimith angle, etc.  Forget it.

  18.27) Disassembling the lower cylinder

It is rarely necessary to do this but if you should - from curiosity or
anything else - beware that the reference for the #1 head may be a magnet
attached to the motor shaft.  This may not be keyed and unless you carefully
mark everything beforehand, will have no way other than trial and error to
get if back at the proper angle.

Chapter 19) Tape Path Alignment and Backtension Adjustment

  19.1) General tape path alignment problems

There are separate descriptions of the procedures for adjusting the
various components of the tape path - in particular, A/C head azimith,
tilt, and height; and roller guide height.  Before you attempt these,
you need to determine whether either of these are likely to be your

For really major tracking problems with all tapes, check for broken or
missing parts or for problems which prevent proper positioning of the roller
guide assemblies during tape loading.  Types of symptoms include: broken
up picture, snow across part of picture, multiple breaks (sort of like the
VCR is in a search mode such as CUE or REV but is not) in picture, totally
unstable picture, or multiple of the above.  Of course, someone before you
may have messed with various mechanical (or heaven forbid, electrical)
adjustments without having a clue of what they were doing.

  19.2) Symptoms of bad A/C head alignment

The following are some symptoms you may experience indicating the
need for A/C head adjustments:

* Weak, muddy, or wavering sound. (Azimith, height, or tilt adjustment).

* Tracking incompatibility between this VCR and tapes recorded on
  other VCRs - you always need to adjust tracking or keep the tracking
  control way off center when playing tapes from other VCRs.  However,
  if it is only one other VCR, that VCR may be misadjusted.  (Mechanical
  tracking adjustment).

* Erratic loss of synchronization or frame lock, or speed changes.
  (Height or tilt adjustment).

Before you try to adjust the A/C head, make sure that there is not some
obvious mechanical problem that has shifted its position.  There may be a
bit of something stuck in the mechanism.  If this appeared after you did
some work on the VCR, you may have accidentally caught a cable or something
else preventing the A/C head assembly from returning to its proper position.
This is particularly likely if the problem happened suddenly.

Once you change its settings, any tapes recorded on your VCR prior to these
adjustments may not play back properly.  For example, if you touch the A/C
azimuth screw to correct a muddy weak sound problem when playing tapes from
other VCRs, any tapes previously recorded on your VCR will now sound muddy
and weak.  You need to decide which is more important - your recorded tape
library or compatibility with other VCRs.

  19.3) Symptoms of bad roller guide height adjustment

The following are some of the symptoms you may experience indicating
the need for roller guide height adjustments:

* Video noise at top (supply/left side roller guide) or bottom (takeup/right
  side roller guide) of picture that cannot be removed with the user tracking

* Video noise in various areas of picture that comes and goes in a few second
  cycle.  For example, a few lines of video noise may travel up or down the
  screen or start at the edges and meet in the middle.  (Misadjustment of
  either roller can cause these symptoms.)

* A jumpy picture - as though the vertical hold control (which most TVs
  no longer have) is misadjusted.  (The supply/left side roller guide is
  probably misadjusted.)

Before you try to adjust the roller guide height, make sure that there
is not some obvious mechanical problem which is preventing the roller guides
from seating properly.  This is particularly likely if the problems
happened suddenly.  See the section below on: "Likely causes for sudden change in tracking behavior".

Gently check each roller guide to see if one is loose in its threaded mount.
If one turns with finger force, that one is likely the problem AND YOU SHOULD
NOT TOUCH THE OTHER ONE!  Where both are loose or have been adjusted, it may
take quite a bit of trial and error to get them both set correctly again.
Try not to make this an issue!

  19.4) A/C alignment adjustment locations

While there are many variations on the exact locations of each of the A/C
head alignment adjustments, the following description is for one of the most
common layouts.  See the appropriate sections elsewhere in this chapter for
the adjustment procedures for the A/C head.

* A/C mechanical tracking.  This is a conical nut on a small shaft fastened to
  the transport base.  It is wide on top and tapers down below.  A slot across
  the top allows the nut to be turned and thus raised or lowered.  Its angled
  side presses against a projection on the A/C assembly base plate.  Thus
  raising or lowering the nut moves the A/C head assembly from side-to-side.

  - When raised (counterclockwise), the A/C head assembly moves away from the
    video heads increasing the delay between the video and sound.

  - When lowered (clockwise), the A/C head assembly moves toward the video
    heads decreasing the delay between the video and sound.

  You may need a special screwdriver with a cutout in the middle of its blade
  (or modify one of your own) to easily adjust this nut.
* A/C head height - A hexagonal nut on a large shaft behind the A/C head.  This
  moves the entire A/C head assembly up and down.  A spring underneath provides
  both the upwards pressure to keep the A/C head assembly against this nut and
  torque to keep it against the conical A/C mechanical tracking nut.

* A/C head tilt - A Philips screw is directly behind the A/C head on the plate 
  that the head sits on.  This adjusts the A/C head forward and backward (with
  respect to the tape).
* A/C head azimuth - Another Philips screw is to the right or left of the head
  on the same plate just mentioned. It probably has (had) some red (or other
  color) paint on it to lock its position.  This adjusts the A/C head azimuth
  angle (side-to-side) with respect to the tape.

* A/C head plate pressure.  A third Phillips screw (on the opposite side of
  the A/C head from the azimuth screw) with a spring under it.  This should
  just be left alone as its only function is to provide downward pressure to
  keep the A/C head assembly in place as determined by the tilt and azimuth
  screws and a pivot point underneath near the front.  However, if it seems
  loose, tighten it a few turns clockwise.  This should not affect any of
  the other settings.

  19.5) Adjustment of A/C head - problems with tracking or sound (linear audio)

If the problems happened suddenly, it is probably not a misadjusted
audio/control head but some other mechanical fault - eliminate
this possibility before considering A/C head adjustments.

The following will attempt to get your mechanical settings back to something
approaching normal even if the audio/control head was tweaked:

I assume that you have cleaned it and replaced any dead rubber parts.

I also assume that someone (we won't name anyone) has tweaked just about
every mechanical adjustment.

I would adjust the audio/control (A/C) head as best you can (don't touch this
unless you know it was messed up by someone):

* Play a tape that you know was recorded on a good machine.  It may be easier
  to start with a tape recorded in SP mode since this is less critical.
  Once the basic alignment is complete, go back and fine tune with a tape
  recorded in EP.

* Adjust the A/C tilt as vertical as you can by eye.   If necessary, fine
  tune it for most stable tape movement - the tape should be at the same angle
  moving over takeup roller guide, A/C head, and adjacent fixed I guide.

* Adjust the A/C height for loudest sound.  At each end of the range of this
  adjustment, you will lose tracking/sync and tape speed may fluctuate (in
  addition to the sound becoming weak).

* Adjust the A/C azimith for best treble (high frequency) sound.  This is
  a precise adjustment - a 1/16 of a turn is significant.  There will be
  a very small range over which the sounds will be clear and natural.
  A tape with music is best for making this adjustment.

* With electronic tracking control centered, adjust A/C mechanical tracking
  (usually, a conical nut that moves the entire A/C assembly) until
  you get the least snow (if you have a picture at this point).
  Satisfactory tracking may be obtained at several positions of this control.
  However, only one will produce current video-audio sync.  For the others,
  the words and the picture will be off by some multiple of 1/30th of second.

* You may need to go back and touch up some of these again.

There can still be other problems in the tape path including the height
and angle of the roller guides and the height of the impedance roller
assembly (on the left before or after the full erase head.)

  19.6) Roller guide height adjustment

You can do this by eye.  Sophisticated test equipment and expensive test tapes
are not needed.  One trick is of course not to mess with both guide posts at
the same time - but even if you do it isn't the end of the world.
This doesn't even require a scope - the video picture is an excellent alignment
tool!  It does take patience and a steady hand.

Also, have you touched any other mechanical adjustments - other guideposts,
etc?  Hope not.  Also, I assume that any repairs to the guideposts have left
them perfectly vertical - if they are tilted, then other tape path
instabilities can result.

The following checks and adjustments are made in PLAY mode.

There is a ridge on the lower (stationary cylinder) on which the tape should
ride - not above and not below.  Play a tape that is in good condition
and look closely at its bottom edge to see if it is sitting precisely on
this ridge.  If it is not, first verify that both roller guides
are snug against the 'V-Stoppers' - the brackets at the end of the
tracks where the roller guides stop in PLAY and REC.  If they are not,
then you need to determine what is binding or what has fallen off of the
tape loading mechanism.  See the section: "General tape path alignment problems".  Assuming that the roller guides are correctly positioned on the
tracks, the first step is to visually adjust the roller guides so that the tape
just rides on that ridge on the lower cylinder.  That ridge is a very
critical part of the guide mechanism.

There will be a set screw to lock each of the roller guideposts from turning.
The appropriate one(s) will need to be loosened slightly - just enough to
that the post is snug but can be turned by hand.  The set screws may require
a miniature metric hex wrench.  Some just have a square head screw which
can be loosened with a pair of needlenose pliers.  Adjust each guidepost so
that the tape just rides on top of the ridge.

Now, for the fine adjustments.  Which part of the picture is bad?

* Left guide -> mostly problems with top of picture.

* Right guide -> mostly problems with bottom of picture.

Misadjustment can also cause a periodic loss of sync on a several second cycle.

Make careful **small** adjustments of each one - then wait for a few seconds
for any results to become apparent.  Since the tape moves so slowly,
it takes several seconds for the tape motion to stabilize to the new
guide position.  The left guide will affect the top part of the picture
(mostly) and the right guide will affect the bottom.

Once you are happy with SP, get a tape recorded on a known good deck in
EP (SLP) mode since the tracks are narrower and fine tune it.

Tape path alignment comments:

1. An EP recording requires the best tracking, and will thus make the best
   test source. (But it must have been recorded on a unit that was aligned

2. Using forward and reverse search modes helps to narrow
   the adjustment.  The guide height on the "feed" side for whichever
   direction you're going will have more affect.  In other words, tweak
   one while searching forward, and the other while searching in reverse.

3. You could have the tape centered at the middle of the contact path, but
   too low at one end and too high at the other.

4. You could have the entire contact path too high or too low, and be in-
   advertently "correcting" by misadjusting the tracking control.  You could
   be off by an entire track getting a good but very unstable picture since
   the ridge is not providing any guidance.

Roller guide tilt:  The roller guides (but not the fixed guide posts next to
them) should be perfectly vertical.  Sometimes there is an adjustment for
this but usually not.  Roller guide assemblies that have tilt due to wear
will need to be replaced.

  19.7) Likely causes for sudden change in tracking behavior

If it is impossible to find a position of the user tracking control that
results in a stable picture, this section is for you.  Some amount of the
picture may be noisy - top or bottom - or the tracking may be fluctuating
with a few second cycle.

Mostly, these symptoms are related to problems with the roller guide
assemblies.  (though electronic causes are also possible).  The roller
guides are on the assemblies that move on curved tracks to wrap the
tape around the video head drum in play and record modes (and on newer
instant start VCRs, other times as well).  Each roller guide assembly
includes a white cylindrical roller which should turn freely on a metal
guidepost, and a fixed guidepost at approximately a 20 degree angle.

1. Roller guides not fully engaged against 'V-Stoppers' (the metal brackets
   at the end of the track on which the roller guide assemblies move when
   entering PLAY or RECORD modes.  Common causes:

   * Obstruction or ridge on track preventing guides from completing their
     movement.  Visually inspect and observe behavior while entering and
     leaving PLAY mode.  Sometimes with use, an edge develops and the guide
     gets hung up.  A fine file can sometimes remove this.

   * Parts have fallen off (don't laugh - JVCs tend to do this).  Various parts
     of the mechanical linkage that move the roller guides may loosen with
     use and either fall off entirely or change position enough to prevent
     full engagement.  Compare left and right roller guide assemblies, they
     are usually nearly identical in their operation and you should be able
     to identify parts missing or out of position.  These are usually on the
     underside of the deck and will necessitate removing the bottom cover
     (unplug the unit!).  To gain access to critical parts of the linkage
     which may be obscured by circuit boards or other components, you may
     need to power the VCR, turn it on, press PLAY, and then pull the plug
     just as the roller guides are in the middle of the track and accessible.

     For the JVC problems, the parts are usually either a brass post or a
     plastic link.  The brass post can be glued back in place using a drop
     of Epoxy.  Make sure its shoulder is fully flush with the body of the
     roller guide casting.  For the plastic link, I have used a very small
     screw to secure it in place from above.  Some plastic cement may
     work as well.

   * Tracks on which roller guide assemblies slide are dirty and/or need
     lubrication.  Clean and grease.

   * Obstructions such as toys or Cheerios blocking tracks.

Check the roller guides while the machine is playing a tape.  They should be
firming pressed against the V-Stoppers.  Any looseness indicates a problem
preventing full engagement.  If pushing the offending guide into position
fixes the tracking problem, this confirms the diagnosis.

Note that in modes where the roller guides are retracted, the roller guide
assemblies are relatively loose and free to move.  However, the amount
of movement possible should be similar for the left and right roller guides
and you should not be able to lift either entirely off of the track - the
ability to do so means missing parts underneath the deck.  If the missing parts
can be located, they can usually be glued back into position.

Warning: if you find a roller guide assembly that can be lifted off the
track DO NOT attempt to load a tape - the floppy roller guide assembly
can smash into the spinning video heads ruining them - and your entire day.

2. One of the fixed guide posts next to roller guides (the ones that are
   tilted about 20 degrees) have worked loose and fallen off.  There should be
   a tilted guide post next to each roller guide.  If one is missing, it has
   probably fallen into the machine.  Immediately unplug (to avoid the
   possibility of it jamming something and/or shorting components in the
   electronics).  Locate the escaped post - turn the unit upside down,
   sideways, shake it, whatever until the loose post falls to the table
   or floor.  Glue it back into position with a drop of Epoxy or other
   household cement.

3. The backtension band has come loose or broken.  The backtension band
   provides the force needed to keep the tape pressed against the video
   and audio head.  A backtension lever on the left side just as the tape
   leaves the cassette is connected to a felt lined metal band that presses
   against the edge of supply reel.  The position of the level determines
   the  tension and is set up with mechanical feedback so that the tape tends
   to move it against spring force just enough to provide the correct amount.
   Test by moving the backtension lever a bit in each direction - you should
   be able to observe the tension change.  Backtension bands are easily
   replaced.  See section: "Backtension adjustment".

4. Mechanical damage due to trauma such as VCR falling off of TV.  Cure, if
   possible, will depend in extent and type of damage.

  19.8) Backtension adjustment

Most VCRs use a backtension band - a thin metal band with a felt liner -
to apply a carefully controlled torque to the supply reel during forward
tape motion in play, record, and CUE.  A backtension lever or arm contacts
the tape as it leaves the supply side of the cassette and provides feedback
to control the tension on the backtension band and thus how much it resists
the rotation of the supply reel.

If the backtension is too low, poor tape-head drum contact results and
you get a noisy intermittent picture.

If the backtension is too high, there will be excessive head wear and in
extreme cases, the drum will slow or stop entirely.

Backtension is normally set using a special backtension gauge which you
most likely do not have.  If you own a TV with a vertical hold control,
you can adjust backtension by setting the vertical hold so that you can
view the head switching point - just above the vertical blanking bar.
Above this point, you see the video from one head and below you see it from
the other.  When properly adjusted, these two segments should more-or-less
line up.

There are two adjustments for backtension: a spring position and the effective
length of the band.

To set the length, there is a setscrew which allows the end of the band to be
moved back and forth.  It is unlikely that you would need to set this unless
you have just replaced a band or unmucked someone else's repair attempt.
I usually consider the length to be correct when the angle that the tape
makes going around the lever post is about 90-120 degrees.  In other words,
the tape should not be so tight as to not be deflected by the arm but
should not be so loose as to be near or at the end of its possible travel.

Then, set the spring force to align the picture above and below the head
switching point.  If you do not have access to vertical hold, you may be able
to set backtension in the middle of the range where flag waving (see the
section: "Flag waving - top portion of picture wiggling back and forth") is
absent or minimized. 

  19.9) No, you don't need a fancy back tension meter

(From: Alan McKinnon (alan.mck@pixie.co.za)).

Well, I'm about to open myself up to all sorts of scathing comments, but here
goes: You can get by without a back tension meter. You will notice that just
about every VCR ever made puts the back tension pole between a post and the
impedance roller. Adjust the pole landing position so that it lines up with
the middle of the impedance roller.  Check your picture. If you have flagging
at the top, or wavy lines, adjust the position. Fiddle it both ways to get the
feel of it. Once you have experience, you can gauge the back tension by
holding a screwdriver against the tape after it has passed over the full erase
head. Your fingers are probably more accurate than most gauges anyway - I've
never seen two give the same readings. My meter lies unused most of the time.
I've lost count of the number of times I have chased around the VCR only to
find my backtension meter was leading me astray.

  19.10) More on adjusting backtension

(From: Paul Weber (webpa@aol.com)).

The objective of the back tension adjustment is to prevent "flagging"
which is horizontal displacement of scan lines at the top of the picture. 
You can use either B&W or color TV (or video monitor), provided that the
unit has an accessible  vertical hold (vertical sync) adjustment.  You
mis-adjust it until the picture rolls half-way and you can see the
horizontal sync bar.  This lets you see the very top of the picture (just
below the bar).

To make the adjustment, you need a known-good reference tape.  You might
trust a commercially-produced movie, but I'd recommend a real vcr
alignment tape if you can find one.  If you use a movie, then try four or
five different ones to help insure you don't have one made on a defective
machine.  Adjust your machine so that vertical lines in the very top of
the picture are as straight as possible. 

As to the specifics of what to adjust on your machine:  You didn't mention
the make or format of your machine, but I'll wager that the moving arm
nearest the feed reel is attached to a felt-covered metal band (the brake)
that wraps partially around the feed reel table.  With tape loaded and
moving, the arm balances tape tension applied by the drive system against
tension supplied by a spring.  If the tape tension becomes excessive, the
brake is applied more; if the tape loosens, the brake is relaxed.  Look at
the attachment points for the spring attached to the arm. Usually, the
back tension adjustment is at the chassis end of the spring.  It may be an
eccentric post than can be turned with a screw driver or a special tool,
or it may be that you have to gently bend the tab.  Either way, adjust the
spring tension in very small increments, then observe the effect on the

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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]