Notes on the Troubleshooting and Repair of Video Cassette Recorders


Chapter 18) Video Heads and Upper Cylinders

  18.1) What is a Video head?

The flying video heads in a VCR or camcorder are the actual transducers which
scan the tape during REC and PLAY.  The head drum or upper cylinder,
as it is often called, spins at 1800 RPM (for NTSC, actually 29.97 Hz)
with one complete rotation representing a video frame (525 lines in the
US consisting of 2 fields which are interlaced).  The result of the spinning
head is to provide an effective head-tape speed of over 24 feet/second needed
to achieve the required video bandwidth.

The actual video heads are the nearly microscopic transducers that contact
the tape and magnetically record or playback the video information.  The
upper cylinder is the entire rotating assembly including the video heads.

The heads are aligned and locked in place on the upper cylinder at the time
of manufacture and this alignment should never be touched.

(Note that the terms 'video heads' and 'upper cylinder' are often used
interchangeably but strictly speaking this is not correct.)

The heads themselves are made from ferrite which is an extremely hard
ceramic magnetic material which is also very fragile.  The head chips
can be seen at the very bottom of the rotating upper cylinder.  The actual
construction is of a 'C' shape with a very small gap between the arms of
the 'C' - about 1 um or so.  This is filled with with a non-magnetic
material to force the magnetic field out of the head into the tape and
to prevent material from collecting in the gap.  A few turns
of fine wire form the coil of an electromagnet for recording and as a pickup
coil for playback.  If you look at a head chip from below (on a cylinder that
has been removed) you can see the coil and the shape of the core, though
you will not be able to tell if a head is bad or worn by this inspection
unless there is obvious damage).  A powerful microscope is needed to even
see the gap.

VCRs are described as having '2 heads' or '4 heads' or whatever.  This
actually refers to the number of head gaps and not actual head chips though
usually this is the same number.  However, two head chips may be placed
very close together and thus appear to be a single head when in fact there
are a pair of head gaps.  Therefore, without a close examination, there may
only appear to be 2 heads when in fact there are 4 - in 2 pairs.  You are
not being short changed.

Two heads are required for any play, record, or search function.  Usually,
these are exactly 180 degrees apart - directly opposing one another on the
upper cylinder.  With 4 head (or 3 head or 5 head) VCRs, various combinations
of heads are used for each mode to optimize record or playback video quality
by selecting a pair of heads with optimal widths and other characteristics.
These may end up not being exactly 180 degrees opposed requiring video delay
line to line up the two video fields in a video frame properly.  This
complicates head testing as it is not always obvious even which set of
heads is used in any given mode.

An additional pair of opposing heads is required for HiFi VHS audio and
another one is present if the VCR has flying erase head.  Usually, there
is only a single flying erase head - it is double width and clears a pair
of tracks (fields) on each pass.  So, there may be up to 7 (or even more)
heads competing for space on the upper cylinder!

Also, see the section: "Video head construction".

  18.2) Video head construction

The actual video head chips themselves are mounted just about flush with the
lower edge of the spinning assembly called the head drum or upper cylinder.
They are made of ferrite - an extremely hard but fragile material.  In terms
of physical strength, its properties are similar to glass.  The head actually
consists of the core, pole pieces, and gap filler molded as a single unit and
fired at high temperature along with the coil wound on the core after firing.
This 'chip' is then glued to a metal support which is screwed to the bottom of
the drum.  A screw presses against this support from above and is used at the
factory for final head height adjustment on the drum.

CAUTION:  Do not touch these mounting screws or the height adjustment screw
accessible from above the drum.  It is virtually impossible for these to
become loose or misadjusted on their own and alignment in the field is not
possible except by trial and error.

These structures may be viewed under a strong (e.g., 10X) magnifier though
the actual record/play gap between the pole pieces will not be visible except
under a powerful microscope.  It is filled with a hard non-magnetic material
in any case.

The thickness of the ferrite chip is about .1 to .15 mm but the width of the
active part of the pole tips narrows down to around .03 mm.  This is one of
the dimensions that is optimized for various special effects in VCRs with more
than 2 video heads.

The gap azimuth angle of + or - 6 degrees (for VHS) is implemented by actually
twisting the pole pieces during the molding process.  This is actually visible
if you look carefully with the magnifier even though you cannot actually see
the gap.  (The azimuth angle has obviously been exaggerated in the diagrams
below due to the limitations of ASCII art.)

The coil used to generate the magnetic field during recording and to sense the
magnetic field for playback consists of a dozen or so turns of fine insulated
wire with a typical resistance of 1 to 1.5 ohms.

HiFi audio head construction is generally similar except that the gap azimuth
angles are +/- 15 degrees instead of +/- 6 degrees.

Damage to the core, pole pieces, or coil, and oxide on the surface or clogging
inside of the core can be seen with the magnifier in many cases.

  18.3) Diagram of single video head

The diagram below shows a typical single video head as would be found on
a 2 head VCR.  Two of these (with opposite gap azimuth angles) would be
mounted exactly 180 degrees apart on the upper cylinder.  This is a view
as would be seen from the bottom of the upper cylinder:

               |                            |
               |           _______          |
     Coil+ o---------------+      |         |---o Coil-
              +------------+     +-----------+
           .  +------------+     +-----------+  .
           .  +------------------/          |   .
           .   |            \  /            |   . Side of upper cylinder v

                          -->||<-- Record/Play gap, width about 1 um

The same head viewed from the edge:
                ______________ _____________      Bottom of upper cylinder v
     ------    |_____________//_____________|    ----------------------------
           |                                    |
               |<---------- 1/8" ---------->|

  18.4) Diagram of double video head

The diagram below shows a typical double video head as would be found on a VCR
with more than 2 video heads if video heads are grouped together.  The quite
visible space in between the two head chips should not be confused with the
actual microscopic record/play gaps in the pole pieces even though the total
width of the two head chips (1/8") is about the same.  For a 4 head VCR, there
would be two such assemblies (with opposite gap azimuth angles for each head)
mounted 180 degrees apart on the upper cylinder.

This is a view as would be seen from the bottom of the upper cylinder:

                           Coil1+ o o Coil2+
              ___________________ | | ___________________
             |                   || ||                   |
             |          ______   || ||   ______          |
  Coil1- o---|         |     +----+ +----+     |         |---o Coil2-
            +-----------+    +----+ +----+    +-----------+
         .  +-----------+    +----+ +----+    +-----------+  .
         .   |          \---------+ +---------/          |   . Side of upper
         .   |            \  /   |   |   \  /            |   . cylinder v

                        -->||<--       -->||<-- R/P gaps, widths about 1 um

The same head viewed from the edge:                            Bottom of upper
              ______________ ____     ____ ______________      cylinder v
      ---    |_____________//____|   |____\\_____________|    ----------------
         |                                                   |
             |<------------------ 1/8" ----------------->|

  18.5) Photos of typical video heads (upper cylinders)

The following site has photos of a variety of typical 2, 4, and 6 head
assemblies as well as 8mm and VHS-C camcorder heads.

        * http://www.shadow.net/~gury/vh1.html

They also sell video heads and will quote prices by return email.  I have
not purchased anything from him so I cannot say anything about his prices
or service.

  18.6) How do the signals go to/from the upper cylinder

The rotating upper cylinder and stationary cylinder form a transformer - the
space between them is very small and coupled the signals between the primary
and secondary ferrite cores.  Each of the heads for R/P video, HiFi audio, and
flying erase, are electrically independent.  The cores are arranged coaxially
which should get to be pretty tight for a 6 or more head VCR!

  18.7) Are your video heads really bad?

No picture (total snow or a blue/black screen depending on model) or a snowy
picture in play modes and/or failure to produce a good recording may indicate
dirty or bad video heads.  First, make sure that the VCR's tuner and RF
modulator are working by viewing a broadcast or cable channel.  Next, refer to
the section: "Video head cleaning technique" and follow the instructions
carefully.  If there is no change even after a couple of cleanings, then your
video heads may have problems.  Of course, if your inspection reveals any
physical damage, you will need a new set of heads (new upper cylinder).

Indications of a bad video head include:

* Any visible damage to the ferrite chips.  Heads nearly always appear in
  opposing pairs on the upper cylinder (head drum).  Any visible discrepancy
  between the chips in a pair is probably damage. Sometimes 1/2 of the core
  breaks off leaving the windings dangling.  Common causes for this damage are
  improper cleaning techniques or the use of damaged or spliced tapes.  Use
  a magnifying glass and bright light to examine the heads but do not touch!

  By the way - improper splicing of broken video tapes is a good way to break
  video heads.  Any kind of splicing should be avoided if at all possible.
  (See the section: "Recovering damaged or broken tapes".)

* Excessive video snow which cannot be eliminated by the tracking controls.
  The appearance may also be of trailing lines or bullet shaped streaks
  particularly following highlights.

  Note: in rare instances, similar symptoms are the result of a static brush
  not making proper contact with the shaft of the spinning drum.  See the
  section: "Firing (static) lines in picture during playback".

  An image where more or less good video alternates with snow at a 30 Hz rate
  means that one of the 2 heads in a pair is probably either dirty or bad.
  If your TV has a wide range vertical hold control (yeh, right, give me a
  time machine), then you may be able to display both fields on the screen
  at the same time.

* Excessive video snow or no picture (total snow or blue/black screen
  depending on model) for some playback speeds (SP, LP, EP, X2, still, slow,
  etc.) since different sets of heads (in 4 head or more) machines are often
  used for different speeds.  If this is due to wear, then it would probably
  gradually deteriorate and not happen suddenly.

* Inability of certain internal adjustments such as backtension to eliminate
  erratic tracking problems may indicate a worn video head.  Horizontal
  bands of video noise may come and go at various places in the picture
  depending on what speed is being used or the playback location on the tape
  (beginning, middle, end).  These may come and go in a periodic cycle.

* Need to frequently clean the video heads even if you are only using new
  good quality (name brand) tapes.  Video heads are normally self cleaning but
  very worn heads can tend to collect tape oxide resulting in a noisy, snowy,
  or totally missing picture.

* You have just been playing a rental, damaged, or spliced tape and you notice
  any of the above symptoms.

  The video heads may have picked up some oxide and are no longer making
  proper contact with the tape.  Letting the VCR play a newer tape for a few
  minutes may clear this if it is minor.  Otherwise, video head cleaning
  (using the proper technique!) will be needed.  However, seriously damaged or
  improperly spliced tapes can result in serious damage requiring video head
  (upper cylinder) replacement.

If your VCR has HiFi audio, similar symptoms may apply to the HiFi audio
heads on the rotating drum.  Noisy or loss of HiFi audio or erratic
switching between linear and HiFi audio may be due to bad HiFi audio
heads (but could also be a tracking problem since HiFi audio tracking
can be even more critical than video tracking).

However, many other problems can result in similar symptoms - video head
diagnosis is one of the most difficult to make (except for physical damage).

Some pros claim to be able to determine if a video head is worn by feeling it
with a finger.  I can guarantee that you will not be able to do this, so the
set of guidelines given above is the best to go on.

  18.8) More on evaluating video degradation

From: Frank Fendley (frank.fendley@datacom.iglou.com)).

It depends on what's wrong with the picture.   If you are getting "highlight 
streaking in high luminance areas" (meaning that white objects in the picture 
seem to have "tails" trailing off to the right of the object), then a new
head would help immensely.   If there are random lines in the picture 
(especially on tapes you have recently recorded on that machine), then a new 
head will most likely help.  

If the picture just isn't as sharp as the VCR next door, then a new head 
probably won't help much.   Technology has improved picture quality 
considerably since your VCR was manufactured.

One recommendation - if you want the best picture quality from *any* VCR, 
forget about recording programs in SLP or LP.   The SLP (or EP) speed should 
be banned and made illegal - the picture and audio quality are terrible.   LP 
should only be used on programs which exceed 2 hours.   You should use SP 
speed on everything you record if at all possible.

  18.9) Need for video head cleaning

When should you clean a video head?  Only when symptoms point to a problem
with the head.  See the section: "Are your video heads really bad?".  Periodic
cleaning is not necessary and may cause excessive wear if done with a head
cleaning tape, especially the dry kind which may be excessively abrasive.
Frequent cleaning by hand, while not damaging, still represents a slight risk
since you never can tell when you might do something you will regret!

VCRs should be cleaned periodically, but video heads usually do not need
periodic cleaning as the spinning heads performs a self cleaning function.
If it ain't broke, don't fix it.

  18.10) Automatic head cleaners

I do not see any advantage in buying a VCR which claims to have automatic
video head cleaning.  Healthy vVideo heads are basically self cleaning in any
case.  The automatic head cleaner is a foam roller that contacts the rotating
heads for a couple of seconds when the tape is loaded.  In my opinion, this is
worse than useless as any crud collected by this foam may just be redeposited
on the heads during the next cleaning cycle.  So, if your VCR has this
'feature' and you experience symptoms of dirty video heads after each tape,
remove the 'feature' and performance will improve :-).

As noted below, there is a slight risk that at some point they may actually
destroy the video heads - no doubt timed to be 1 day after your warranty runs
out :-(.  In addition, they do nothing to clean those portions of the VCR that
really may need periodic maintenance like the rubber parts, A/C head, and tape

The only benefit of an automatic head cleaner is to the manufacturer of the
VCR as it increases their profit margins!

(Portions from: Joseph E. Fealkovich (jef812@ix.netcom.com)).

I would pull those 'automatic head cleaners' out and let the customer know
about it. I hate those things, they do more trouble than good. The least they
can do is redistribute the garbage back onto the heads, the worst, actually
catch on a head and tear it right out of the drum, (I've seen this happen on
a Goldstar VCR). When new, they do a fairly good job 'dusting' off the video
heads, but they wear out quickly, (they work every tape loading and unloading
cycle), and can seriously ruin the heads.

Furthermore, they clean only the video heads when they work; they do absolutely
nothing for the grooves cut into the drum, I'll say that's the most important
part, as the grooves form an 'air bearing', where the tape floats on the drum,
without that that effect, the heads can wear out prematurely due to the tape
sticking to the drum. My opinion on 'self cleaning' VCR's is, they've done just
about everything dumb and automatic on these VCR's.

  18.11) How NOT to clean your video heads or a very expensive lesson

The following is a true story.  Don't let it happen to your VCR!

Read the section: "Video head cleaning technique" before breaking out the
pliers and sandpaper (well almost).

(The following is from someone who not surprisingly would rather
remain anonymous).
"Thanks a lot for the FAQ on VCR repair (unfortunately too late --- sam).
I now realize that I have made a boo-boo of quite unprecedented magnitude!!
You're not going to believe this...  (I'm almost too ashamed to admit to
My friend asked me to clean the heads on his VCR.  I got out
my isopropyl alcohol spray (good) and my cotton buds (not good!).

I proceeded to scrub the upper drum furiously, getting lots of lovely
black deposits on the cotton buds.  I, ahem... also scrubbed vigorously
around the four little 'recesses' positioned around the bottom of the
drum.  When I spotted a little bit of metal sticking out of one of the
recesses, I got out my tweezers.....
(You can stop reading here if it's too painful!)
...and poked around, thinking 'this will cut the tape to ribbons if I
don't get it out'.  After succeeding in removing the offending piece, I
noticed a very fine copper wire emerging from the hole too.... OUT IT
CAME (with the tweezers of course).  I checked the other three holes
and succeeded in removing some more shrapnel from one other.
Guess what?  It wouldn't play properly after this.... but the sound was OK.  
To be fair, I was fairly sure I'd screwed up big time when I saw the copper
wire... but of course it was far too late by then.
I admitted to my friend that I thought I'd broken his video head and that
I would replace it for him.  Thinking that heads cost around the $30 mark,
according to most of my catalogs, I was horrified when I got a price for
his particular model....  $162.00 + 17.5% tax and postage..... However, this
was much less than what the original manufacturer wanted: #350!!  Well,
I finally found a replacement for $85.  Still, an expensive lesson."

  18.12) Video head cleaning technique

Caution: Read the following in its entirely to avoid an expensive lesson.

As noted, improper cleaning can destroy your video heads.  The head chips
are very fragile and just rubbing them in the wrong direction (NEVER use an
up-and-down motion) can break the chips off requiring replacement of the
entire upper cylinder assembly - one of the most expensive parts in your VCR!

Manual cleaning using the proper head cleaning sticks is best but requires
that you gain access to the interior of your VCR - i.e., take off the cover.

If you do not want to do this, you can try a commercial wet cleaning tape.
These is some slight risk, however.  The material used in some of these
may have an excessively coarse fiber structure which can catch a video head
and break it off.  I have not seen this happen nor could I recommend a specific
brand as there is no way of knowing what their current product uses.  I do
not recommend the dry type at all as these are almost always much more abrasive
and may cause premature wear of your video heads especially if used regularly.
When using the wet type cleaning tapes, follow the directions and - very
important - wait sufficient time for everything to dry out or else you will
have a tangled mess inside your VCR.

Regular video head cleaning should not be needed!  Therefore, the regular
use of a cleaning tape is not recommended.  As noted, some cleaning tapes
will cause excessive wear to the video heads and no cleaning type can
adequately deal with other parts of the tape path anyhow.  If you find
yourself needing to clean your video heads frequently, the video heads may
be worn, the backtension may be set too high, or you may be playing old
or dirty (literally) rental tapes.

To clean by hand, you will need what are called 'head cleaning sticks'.
These are covered by chamois and are safest.  DO NOT USE Q-TIPS (COTTON
SWABS). These can catch on the ferrite cores and damage them or leave
fibers stuck in the heads.  Q-tips can be used for cleaning the other
parts like the rollers and audio/control head as described above but
not video heads.

To use the cleaning stick, moisten it with head cleaner or alcohol.
Pure isopropyl is best, however, the 91% medicinal stuff is ok as long
as you dry everything pretty quickly.  Don't flood it as it will take
a long time to dry and you run the risk of any water in the alcohol
sitting on surfaces and resulting in rust (very unlikely, but don't
take the chance).

Gently hold the flat portion of the chamois against the upper cylinder
where it is joined to the lower (non-rotating) cylinder.  Rotate the upper
cylinder be hand so that the heads brush up against the moist chamois.  DO
NOT MOVE THE HEAD CLEANING STICK UP-AND-DOWN - you will break the fragile
ferrite of the heads - $$$$.  Side-to-side is ok as long as you are gentle.
(The following tip from: Steve (sreed@amsupply.com)):

  A good quality automobile chamois (the real thing, not the fake 
  stuff), cut up into 1 to 2" squares, is far cheaper and easier to 
  manipulate than the sticks.  After cleaning the heads, the chamois square 
  can be re-moistened with cleaning fluid and used to clean the A/C head, 
  rollers, guides, etc.

  There is another advantage to this approach.  The chamois sticks can on some
  occasions "catch" on a video head, because the chamois area is small and 
  the edges are rigid.  Using a larger cut-up square of chamois eliminates 
  this problem because the edges of the chamois are away from the rotating 
  head and you're simply holding it against the drum with your index finger."

I know people who use a piece of moistened typing paper or a business card, or
even their Mark-1 thumb for video head cleaning but I would not recommend
these for a general service procedure!

(I suppose the only real requirement to prevent damage is that the material
have a fine enough structure and not have fibers that can get stuck in the
heads.  So, the short list of acceptable materials is quite long - some more
effective than others.  My concern for a general recommendation is that
people's interpretation of these requirements can vary quite a lot.  If a
novice comes to me and asks what to use, I will say 'cleaning sticks'.  Once
they understand the characteristics of the heads and their mounting, they are
free to use whatever works.)

Depending on how dirty your heads are, a couple of passes may be enough.  Let
everything dry out for at least 1/2 hour.  This may need to be repeated for
stubborn clogs.  However, one pass will often do it.

In addition, inspect and clean the drum itself staying safely away from the
video head chips.  The five fine grooves in the drum help control the air
bearing that the tape rides on and helps to stabilize tape motion.  These
should be clear of dirt and tape oxide (DO NOT use anything sharp - the
moistened head cleaning sticks will work).

  18.13) Rob's alternative video head cleaning techniques - use with care

(From: Rob-L (rob-l@superlink.net)).

As an alternative, I'd use a *dry* bit of paper.  Moderate finger pressure
against the whole side of the drum, overlapping to the motor assembly.  Then
twirl the drum in each direction a few times.  Look at the paper and you'll
see exactly where the dirt is coming off.  Once you can do this and get no
marks, you're heads are relatively clean.  A tiny, soft, short brush and a
puff of air will dislodge any paper fibers.  The paper and its fillers are
generally not going to harm the drum/ferrite-chips with this brief contact.
And you can pop a tape right in without waiting for solvents to evaporate.
Besides, solvents may soften any encapsulants on the chips, and cause residue
to get on the polished surface.

Once in a while, I run into a set of heads that seem to be bad, even after
cleaning with different methods.  This is characterized by poor signal
strength in all or part of the picture, sometimes one field only, and
sometimes tracking range is extremely narrow as a result.  On S-VHS units,
S-VHS recordings get noisy and may get blanked, while the same unit will work
in VHS with minor tracking problems.

Upon 30x pocket microscope inspection of the heads, I find a speck of what
appears to be a cloudy polymer, firmly bonded to the edge of one or more of
the chips. How did it get there?  My guess: too much solvent - may have
dissolved encapsulant and/or dust that was at the edge of the chip.

Solution: *carefully* drag a jeweler's flat-blade screwdriver along the chip
(under magnification). Sounds a bit risky, but this has never damaged a head
in my hands.  Follow this with a final paper-buffing, and usually the VCR is
tracking fine, with a much improved picture.  Saves mucho bucks.  'Course,
sometimes the heads are just plain worn out.

My advice: Invest in a pocket microscope before you start pricing heads.

  18.14) Advanced video head testing techniques

Assuming cleaning does not help and you have the time and inclination, some
additional test can be performed to confirm or rule out a bad set of video
heads (upper cylinder).

To check the signal from the video heads you need a circuit diagram so that
you can locate the relevant test points and expected voltage levels in the
head preamp. This will be housed in a metal enclosure, usually right next to
the head assembly (at the rear).  This should be done with an alignment tape,
but any known good recording should provide a reasonable approximation.

Other basic checks such as visual check with a magnifying glass, continuity
tests on the heads as well as power supply voltages in the preamp can also

If your VCR has 4 video heads (not including HiFi audio heads) and only
certain modes or speeds do not work, then the following procedure may permit
you to isolate the problem to a head or its preamp.  Basically, the idea is to
interchange the wiring of the two pairs of heads.  While the heads will no
longer be optimized properly for each mode, there is a good chance that they
will work well enough to determine gross changes.  For example, if SP play
originally had alternating fields of good and bad video and works moderately
well after this rewiring (but maybe with tracking noise), then you know that
the bad head is no longer being used for SP play.  Since the same head preamps
are being used, a bad head must be at fault.  Video drums where the heads are
wired with flying leads are somewhat easier to cross-wire than those with a PC
board.  This is not fun and may not work in all cases, but if you are hesitant
to risk the cost of a new head, it may be worth a try.

  18.15) Measuring video head wear

You mean your thumb isn't calibrated to the micrometer (um)?

"I'm trying my hand at VCR repair.  Sony specifies a hand-held device that
 connects to a non-rotating video head and that measures how worn the head
 is (beta machine).   I'm trying to imagine how it works.  Is it the head gap
 itself that gets bigger as the head wears?
(From: Raymond Carlsen (rrcc@u.washington.edu)).

There is (was?) a device to check for head wear by measuring the inductance
of the head itself. The head must be disconnected from the rotary transformer,
of course.  Older machines had wires on top of the head that could be easily
disconnected (like the Beta decks) without removing it.  Heads on some newer
machines must be removed from the VCR because the leads are underneath.

Apparently, the inductance of the head changes slightly as the ferrite
material wears away. The head gap itself doesn't change as the head wears.
If the gap were to open, the head wouldn't work at all, and I assume that the
inductance would be significantly different than a used good head.  In most
cases, the head chip(s) wears to the point that the tip penetration is not
sufficient to keep it in intimate contact with the tape. The heads tend to
clog more easily as they approach end-of-life. 

The inductance type of head tester does have a few drawbacks. It's rather
expensive, and to accurately tell how much usable life a head has left
using only the tester, you must have a "sample" of exactly the head under
test for comparison... each one is different. The measurement is relative,
not a "good-bad" reading. 

All that said, I have been using my finger (and a microscope) for the last
10 years to tell good from bad, and to estimate usable life . I learned to
gauge the tip penetration by feeling the rotating head chips as the tape is
moving through the machine. A well-calibrated finger placed lightly on the
tape on the exit side of the drum is very revealing. Streaking or
comet-tailing on recordings of small bright objects usually means the heads
are close to being worn out (gap starting to open up). A microscope is good
to check for the more obvious broken or chipped head, and to see if a
stubborn clog is actually gone. Different VCR brands will show different
results to my finger test: old Hitachi decks will show bad heads before
they actually "feel" worn out... one usually goes first, producing
alternating fields of snow and picture. I've seen a few older Panasonics
that still worked even when the heads were worn down where they couldn't be
felt any more. How do you tell the difference between worn out heads and a
head clog? Clean the heads and try it again... then feel it. Tentel Corp. 
makes a test fixture that actually measures tip penetration, but with a bit
of experience as a guide, the best test instrument is still the mass of
grey mud between the ears. 

(From: Frank Fendley (frank.fendley@datacom.iglou.com)).

There are two methods (that I know of) which measure wear on video heads.

One is a protrusion gauge, which measures how far out the heads extend past 
the edge of the drum.   Once the heads fall below a minimum extrusion, they 
are considered worn.   It's similar to measuring the tread left on a 
automobile tire.

The other is a "Video Head Tester", sold for either Beta or VHS models.  
Essentially it is an inductance bridge.  You connect each head to the test 
leads, calibrate the unit, and then measure the inductance of the head.   The 
theory here is that as the head wears, its inductance goes down due to loss 
of the core material.   Past a certain minimum, the head is declared worn out.

Bear in mind that a head could fail both of the above tests and still give a 
good quality picture (although it is true that its days are probably 

It is also quite possible that a head could pass either or both of the above 
tests and be defective.

Are the the testers worth it?   In my opinion - probably not, unless you are a
real purist and like to have a lot of test equipment.

  18.16) More than you even wanted to know about video head tip penetration

Remember how I told you never to even think about adjusting the video head
chips themselves?  Well, it seems some people never listen :-).

(From Jerry Greenberg (jerryg@total.net)).

I researched out tip penetration specs on video heads for VHS and Beta (home)
machines.  I got a number of people interested in this one to the point that
a few machines came off the shelf, and the research began.  We had the tools
at our disposal to get this done.

The following measurements are in mm (millimeters).

Brand new heads off the shelf in both Beta and VHS were about 0.12 to 0.16 mm

At about  0.05 mm the video signal starts to degrade.  But the head will still
record well.

At about  0.02 mm the video is very noisy.   If you loosen the head and push
it out a bit, you improve it slightly.  But, the gap is now wider.  Therefore,
the signal is a bit stronger (due to the additional penetration) but has more
noise.  Overall, a slight improvement.

Also the horiz angle of the head effects the switch point a bit.  If out, a
slight horizontal jitter (flag waving) is noticed, and the PG is slightly out.
You can correct with the PG adjust to a point, but the head effective angle
from the opposite one must be better than 2 deg.

As the head wears, we found that the head surface leaves the tape too soon,
and starts reading the tape too late.  If the heads are down to about .05 mm
the effective error (both heads summed) is about 2 to 3 degrees.  The
puts it slightly out of spec for the switch-over point and causes some
instability.  This problem is also summed because the carrier output is at
about 60% of the normal amount.  As the head wears from this point, the
carrier drops more rapidly.  It is not linear.  It follows the inverse
square law factor. 

  18.17) Highlight tearing and bad video heads

Highlight tearing - trailing lines adjacent to bright areas of the picture -
often indicates a worn video head.  Sometimes, this only shows up severely
for tapes recorded AND played back on the same machine.  Why?

(From: Jaclyn (lambert@sos.net)).

The reason why it **appears** to be a record only problem is fairly 
straightforward if you understand what's up. 

When you make a recording with bad or marginal heads the resultant recording 
is poor. Perhaps a vcr with good heads will be capable of producing an 
"acceptable" picture, perhaps not. Depends on the alignment and how good those 
heads are.  When you try to play the "poor" recording back on the vcr in 
question the resulting performance is unacceptable because the heads on that 
vcr are marginal at best and are simply not going to have the gain required to 
pull the crappy recording off. Get it? It's a double whammy two fold effect. 

Video heads don't just go "bad". They wear down after time and early symptoms 
sometimes also include poor vertical stability (as is so common in Hitachi 
VCRs) and snow "lines" which hover about 2 inches from the top of the screen. 
Occasional loss of horizontal sync is also typical.

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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]