[net.audio] Pinch rollers and biasing

stevans (01/05/83)

I have been told not to use alcohol on the pinch-roller, because it
removes moisture from the roller, causing cracking of the surface.
Stereo Review and a few other publications disagree, but I can see
the point.

Someone has recommended lacquer thinner as a substitute, but this will
dissolves plastic very easily.  The housing of the pinch-roller on my
unit is plastic, so I am reluctant to switch unless somebody has a
non-plastic-dissolving alternative (besides water, which might not be
too bad).  Perhaps a judicious mixture of alcohol and water, say %50?

Someone else has explained biasing here on net.audio.  The recorder's
heads will not become magnetized, since the bias is an AC signal averaging
about zero.  Since the average magnetic field at the head in both recording
and playing is zero, how would a demagnetized head ever become magnetized?
And if it gets magnetized, normal use will tend to unmagnetize it again.

Am I wrong again?

                             Mark Stevans, U of Rochester, NY
                             (allegra or seismo or brl-bmd)!rochester!stevans

jim (01/05/83)

	Just to prevent more problems let me again clarify my recommendation
on the use of lacquer thinner on pinch rollers--
	1) It should only need doing about twice a year
	2) People with plastic tape shrouds around their rollers should
	   not use lacquer thinner.
	3) It should only be done extremely carefully with a cheap swab
	   (as mentioned previously, Q-tips have too much cotton) and just
	   a drop of thinner.
	4) Yes, LT does dissolve plastic, so be very careful and don't attempt
	   this on a less than fully disassembled car deck.

	Jim Morton
	GenRad Inc.
	Concord, Mass.

wildman (01/05/83)

	WE've already been over this once, but here goes again:

	THE PRIMARY CAUSE OF HEAD MAGNITIZATION IN TAPE RECORDERS
IN THE MODERN ERA IS THE TURN ON/TURN OFF TRANSIENTS THAT ARE CIRCULATED
THROUGH THE HEAD.  THESE CURRENTS ARISE WHILE THE POWER SUPPLY VOLTAGES
ARE CHANGING WHEN THE DECK IS IN FLUX BETWEEN ON AND OFF (OR THE OTHER
WAY AROUND)
	The tape itself is a very minor cause of magnitization.
If you have played a tape once on a magnitized deck, it doesn't
matter any  more, regardless of the inccorrect statements that
have appeared here regarding coercivity of magnetic recording tapes.
	For those of you who are knowlegable in circuit theory,
the cause of magnitization is the pulse that most head amplifiers
send through the head on turn on, and turn off.  The second most
significant cause is the current leakage through either the coupling
capacitors or the input stage of a directly coupled head amp.
	I repeat:
	If you have played a tape on a magnitized deck, and then
demagnitize the deck, it doesn't matter any more.
	If you want to know a lot more about this subject that you
ever cared to, read the last two years worth of the Journal of the
Audiom Engineering Society in your favorite technical library.
Use the master index at the end of each year to find the right
issues.
	Please, people, if you don't know what you are talking about,
don't talk.  If you have to, at least admit that you are speculating.
(Some people do that.  These people are usually right, which just points
out human nature.)
	I quit!

	Oh yes, lacquer thinner often has heavier hydrocarbons
that will act just like oil when you get done.  I wouldn't use it.
I use freon for general cleaning and isopropyl  when someone has
let things get out of hand to the point that I need a firehose.

dmmartindale (01/06/83)

Ok, can Freon TF be used to clean pinch rollers?  It seems to be what's
recommended for cleaning all the parts of our computer tape drive - capstan
(which seems to be rubber-covered), rubber pads on supply reel hub, heads,
everything.  The stuff is now sold by Radio Shack so should be available to
almost everyone out there.  And it doesn't attack plastic (even alcohol does
attack some plastics).  Just don't get it into any bearings - it's really
good at dissolving oil or grease.
Re magnetization of heads: There is no bias current when you're just playing
tapes, and most recorders are used for playing most of the time.  Virtually
all car decks are play-only.  I believe that the magnetism builds up during
play, not recording.

Dave Martindale

wildman (01/08/83)

Mark was right-- I did leave one thing out of my last note.
The record-on/record-off transient also can cause quite
a bit of head magnitization if the head isn't coupled through a
transformer(the usual case).  The startup and shutdown of the
erase oscillator were not carefully considered until a few years ago
in consumer machines, much to some pro's distress.

The startup transient, whether from power on or record on, is still
the primary cause now-a-days.  While it IS somewhat machine dependant,
it takes place to some extent on all machines, even (and, in fact, due to
the dircuitry required for better performance , to a greater extent) the
best of the bunch. (Direct coupled head amps have to do something with
the bias current...)

All decks do do it though, at least to some extent.  I am not
aware of any that have a really bad time of it, but I deal
mostly with higher end decks (Lowest is something like Teac V7 or so,
a nice machine) and may not enounter it often.
I do demagnitize all of my decks regularly, with the power off,
and with a line-supplied demagnitizer with a *LONG* snout.
I also use iso-propyl on the heads, and freon TF on the rubber parts.
(Yes, by ALL means keep it out of bearings!)


P.S.  Mark--I didn't mean you-- Look around this newsgroup.