[TechAssist] Re: Best way to clean a head.
- From: Dennis Viereck <wa6ati@xxxxxxxxxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Sun, 25 Nov 2001 06:06:55 -0800
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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