[TechAssist] Re: Best way to clean a head.

  • From: "Don Hartigan" <hartstv@xxxxxxxxxxx>
  • To: <techassist@xxxxxxxxxxxxx>
  • Date: Sun, 25 Nov 2001 18:01:46 -0400

I totally agree with Dennis.Normally if you ''pause'' your p/b &any portion
of the pix is clear it's a good chance your heads are ok.A customer recently
asked me about his 8mm video cam,which I no longer service,because of my 56
year old eyes &not too steady hand,if I though his video heads were bad on
his unit.Normal p/b was very snowy,BUT,if he paused or did a forward
search,the pix was clear.I told him I was almost 90% sure his heads were not
the problem.Hope I did not steer him wrong....my 0.02c...Don
----- Original Message -----
From: "Dennis Viereck" <wa6ati@xxxxxxxxxxxxxx>
To: <techassist@xxxxxxxxxxxxx>
Sent: Sunday, November 25, 2001 10:06 AM
Subject: [TechAssist] Re: Best way to clean a head.


>
> Larry:
>
> I disagree with your statement, "I takes 2 heads to complete a full
picture".
> If on head is bad and the other good you will be missing 1 field of video.
If
> you remember oUR NTSC format consists of 1 frame of video is equivalent to
two
> fields. The fields are interleaved together not appended to to the end of
the
> first field.
> Since they are interleaved together one will end up with 1 line of video
and 1
> line of snow then 1 line of video and 1 line of snow. This pattern will
continue
> throughout the entire video field.
>
> The reason you get a picture on half of the screen, usually the top, is
because
> of head protrusion. The surface of the head must impinge into the tape
slightly
> just after the entrance guide. This impingement depth must continue for
the
> entire 180 degree swipe of the video head "A" and repeat this process for
video
> head "B".
> The main factor that effects head protrusion are wear.  Obviously head
wear is
> something that only replacement is the answer. Insufficient back tension
coupled
> with head wear is usually responsible for the assessment mentioned, "even
and
> inch of video showing on the screen the heads are good" I would agree with
in
> part.
>
> As the video head travels around at 1800 rpm it creates air turbulence,
This
> turbulence does not negatively effect the entrance side of the drum
assembly
> where the tape meet the head. In fact the trailing side of the video head
chip
> had a lower pressure and will suck the tape in closer to the head. But
after the
> head has traveled 60+ millimeters to playback or record of the first video
field
> it looses the head to tape contact. One or two primary factors can cause
this,
> insufficient back tension and head wear.
>
> Any debris on the drum, or the video, or HI-FI heads can cause the tape to
be
> pushed away from the head trying to read it. This is why the air channels
in the
> drum must be kept free of any debris too. It is just as important as the
video
> head chips.
>
>
> I have a brainteaser for the group. How many miles does single video head
travel
> in 1 single pass of a 6 hour EP VHS tape?
>
> Dennis Viereck
>
>
>
>
>
>
>
>
> Ledstiles@xxxxxxx wrote:
>
> > In a message dated 11/24/2001 4:27:32 PM Pacific Standard Time,
> > billmcfarland@xxxxxxxxxxxxxx writes:
> >
> > <<
> >  > > >  Howdy Wally,
> >  > > > If the heads are bad there won't be any video showing on the
screen,
> >  if
> >  > > you have even and inch of video showing on the screen the heads are
> >  good,
> >  >>
> > Wally . This above is sooooo wrong. Most bad head problems I run across
have
> > the 1/2 good picture display. I takes 2 heads to complete a full picture
> > after all.
> > Larry
> > N.C.S.
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