snipeguy said:
My TEAC 144 is acting up again. Tracks 1 and 2 only partially erase previously recorded material. They record new stuff fine. Tracks 3 and 4 work perfectly. I sort of know how erase heads work but how do they create NO signal on the tape.
Once again any help from this wonderful cache of minds and experience would be greatly appreciated.
Forever at the repair bench,
Snipeguy
Tape has an oxide layer that can be magnatized. The magnet "domains" are either lined up (North to South) or randomized (non-magnatized oxide) or delibertly organized (recorded signal). The erase head uses a high-freq signal to randomize (non-magnetic state) the magnetic domains. The high-freq signal puts these domains in a alternating current field (+ to -, - to +) and makes the domains change position (180 degrees) so fast that the domains, as the field is taken away, fall in a random order. This is close to being in the original state of the oxide layer. Demagnatizers work the same exact way. They force the domains to fall in a disorganized state. Full magnetesation (ie: a magnet) is when the domains all line up from north to south. The magnetic "pull" is from the millions of domains combining their power all at once.
Hint: think of a single domain as a atom (it is not, but the priniple is the same) in a piece of metal. Each atom contributes its properties to make a substance magnetic, non-magnetic, an insulator etc.
Your heads probably need either re-lapping or aligning. Usually when the heads need re-lapping, the tape wears the center flat and the contact to the tape has a wider gap than when the head is of proper shape. Look at your head. You should see a wear mark in the center of the head. The wear should be:
1. Exactly in the center.
2. The wear should be very narrow.
If this is the case, then you may need the heads to be re-aligned. The suspect here is that you are having trouble with tracks 1 and 2. Usually when head wear is suspected, the first trouble spots are in the outside tracks. For you this is 1 and 4.
The third suspect could be the erase head amplifiers. I am not familiar with that machine's electronics, but I would bet that they use a single erase amplifier/oscillator for tracks 1 and 2 and another one for 3 and 4.
A possibility.
This is a pic of magnatized tape. Not all the domains are aligned.
The other pic is unmagnatized tape.
