Maker Pro
Maker Pro

Tape heads, how do they work

U

Uriah

I have a tape head with three wires coming out of it. One goes to the
metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head would
I be able to see the signal? How would I hook it up to the scope? One
wire to the ground and one to one of the other lead? I have tried a few
things but don't get much of a response. Could someone give me some
details on how to see what the tape head sees on the scope?

Thanks
Uriah
 
D

Don Klipstein

I have a tape head with three wires coming out of it. One goes to the
metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head would
I be able to see the signal? How would I hook it up to the scope? One
wire to the ground and one to one of the other lead? I have tried a few
things but don't get much of a response. Could someone give me some
details on how to see what the tape head sees on the scope?

With 3 wires from the tape head and one known to be a "shield" to
ground, connect that one and either of the other two to your scope ground.
Expect likely as low as mere millivolts of audio from the remaining lead
with respect to the first two. Many scopes do not display well signals of
just a few millivolts, at least not in my experience!

- Don Klipstein ([email protected])
 
J

John Riley

Uriah said:
I have a tape head with three wires coming out of it. One goes to
the
metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head
would
I be able to see the signal? How would I hook it up to the scope?
One
wire to the ground and one to one of the other lead? I have tried a
few
things but don't get much of a response. Could someone give me some
details on how to see what the tape head sees on the scope?



I know little about this, or anything electronic, but is the fact that
pickup heads are biased alter your problem?
 
R

Rheilly Phoull

Uriah said:
I have a tape head with three wires coming out of it. One goes to
the metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head would
I be able to see the signal? How would I hook it up to the scope?
One wire to the ground and one to one of the other lead? I have tried
a few things but don't get much of a response. Could someone give me
some details on how to see what the tape head sees on the scope?

Thanks
Uriah

You would need a preamp to lift the signal.
Cheers ........... Rheilly
 
E

Eeyore

John said:
I know little about this, or anything electronic, but is the fact that
pickup heads are biased alter your problem?

Tape replay heads require no bias.

Graham
 
D

Don Bruder

Uriah said:
I have a tape head with three wires coming out of it. One goes to the
metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head would
I be able to see the signal? How would I hook it up to the scope? One
wire to the ground and one to one of the other lead? I have tried a few
things but don't get much of a response. Could someone give me some
details on how to see what the tape head sees on the scope?

Thanks
Uriah

Unless you've got a super-sensitive 'scope, you're probably going to
need to run it through an amp before you'll have much luck seeing
anything - The "raw" output from a head is tiny. I mean *REALLY* *TINY*.
 
J

John O'Flaherty

Uriah said:
I have a tape head with three wires coming out of it. One goes to the
metal shell around the tape head and the other two come out of the
thing. If I use a storage scope and ran some tape over the head would
I be able to see the signal? How would I hook it up to the scope? One
wire to the ground and one to one of the other lead? I have tried a few
things but don't get much of a response. Could someone give me some
details on how to see what the tape head sees on the scope?

There would have to be something recorded on the part of the tape
you're using. The signal output will be very low, and it will be
proportional to how fast you pull the tape across the head. The
frequency of what is played back would also be proportional to the
speed. If the tape isn't kept exactly perpendicular to the gap in the
head, the output will be drastically reduced. That might be hard to do,
holding things by hand.
 
J

John Riley

Eeyore said:
Tape replay heads require no bias.

So what are the different biases specified for different magnetic
materials on the tape?
 
M

Michael Black

John Riley" ([email protected]) said:
I know little about this, or anything electronic, but is the fact that
pickup heads are biased alter your problem?
The bias (an AC signal above the audio range) in a tape deck is to get good
performance on the tape;it's there to arrange the magnetic stream on the tape as you record (or
something like that). You can tape without bias, though the results
were never good. And the bias is only used for recording (it would
erase the tape at least partially, so if it's left on during playback,
it would affect the contents of the tape).

I'm not surprised by three leads. The case is grounded to shield it
from unwanted signals like AC floating aroud the room. The signal
out of the tape is weak, and the tape head itself is just an inductor so
it would naturally pick up things via inductive coupling if it wasn't
shielded. You only want the signals from the tape, not the rest of
the junk.

The other two wires are for the actual signal off the tape head.

Michael
 
M

Michael Black

John Riley" ([email protected]) said:
So what are the different biases specified for different magnetic
materials on the tape?
You mean like for Metal and CrO2? The difference is in the recording,
and I seem to recall the preemphasis (boosting of highs to make things
less noisy) is different to make use of the improved range from the
different type of tape, and anytime there is preemphasis, one needs
demphasis (cutting the highs to bring things back to normal) on the
playback end in order to put the sound back the way it should. If there
was no demphasis to correspond to the preemphasis, you'd end up with a
sound from the tape deck that hat too much high frequency content, ie too
shrill.

So the switches on tape decks surve two purposes. Set the bias
and premphasis properly on record, and set demphasis properly on
playback. The switches have to be on playback only decks so you
can play back the different types of tapes, but there it only sets
the demphasis.

Again, bias on record is just to improve things, it is not a necessity
for recording on tape (though you'd be disappointed with the results
if you didn't have it.
 
E

Eeyore

Michael said:
You mean like for Metal and CrO2? The difference is in the recording,
and I seem to recall the preemphasis (boosting of highs to make things
less noisy) is different to make use of the improved range from the
different type of tape, and anytime there is preemphasis, one needs
demphasis (cutting the highs to bring things back to normal) on the
playback end in order to put the sound back the way it should. If there
was no demphasis to correspond to the preemphasis, you'd end up with a
sound from the tape deck that hat too much high frequency content, ie too
shrill.

So the switches on tape decks surve two purposes. Set the bias
and premphasis properly on record, and set demphasis properly on
playback. The switches have to be on playback only decks so you
can play back the different types of tapes, but there it only sets
the demphasis.

Again, bias on record is just to improve things, it is not a necessity
for recording on tape (though you'd be disappointed with the results
if you didn't have it.

'Disappointed with the results' would be putting it mildly !

Graham
 
M

Michael Black

Eeyore said:
Michael Black wrote:


'Disappointed with the results' would be putting it mildly !

Graham
Come to think of it, I was thinking of that cheap early sixties battery
operated reel to reel tape recorder we had. It didn't have an erase head,
just a magnet. And without giving it thought as I posted, I was thinking
that it meant there was no bias oscillator, which isn't necesarily the
case.

So in the end, I haven't a clue what it would be like without bias,
so I'll upgrade that "disappointed with the results" to more like
"you wouldn't make a tape recorder without a bias oscillator".

Michael
 
J

John O'Flaherty

Michael said:
Come to think of it, I was thinking of that cheap early sixties battery
operated reel to reel tape recorder we had. It didn't have an erase head,
just a magnet. And without giving it thought as I posted, I was thinking
that it meant there was no bias oscillator, which isn't necesarily the
case.

I had one of those, and it not only didn't have a bias oscillator, it
didn't have a capstan or pinch roller- it depended on the pickup reel
tension to move the tape. It had a real lot of flutter and wow.
So in the end, I haven't a clue what it would be like without bias,
so I'll upgrade that "disappointed with the results" to more like
"you wouldn't make a tape recorder without a bias oscillator".

Without bias, the distortion would be greater. There is a hysterisis in
the transfer function of magnetic force vs. magnetization for the
material on the tape. You have to get above a certain signal voltage to
magnetize the tape at all. If you don't have bias, you get crossover
distortion, which is worse for low level signals. The bias is a
supersonic signal applied to the head when recording. The audio signal
is added to that, so even low level audio signals result in a change
in magnetization, and a more linear magnetization pattern results.
 
E

Eeyore

Michael said:
Come to think of it, I was thinking of that cheap early sixties battery
operated reel to reel tape recorder we had. It didn't have an erase head,
just a magnet. And without giving it thought as I posted, I was thinking
that it meant there was no bias oscillator, which isn't necesarily the
case.

So in the end, I haven't a clue what it would be like without bias,
so I'll upgrade that "disappointed with the results" to more like
"you wouldn't make a tape recorder without a bias oscillator".

DC bias ! It's not very good though.

Graham
 
E

Eeyore

John said:
I had one of those, and it not only didn't have a bias oscillator, it
didn't have a capstan or pinch roller- it depended on the pickup reel
tension to move the tape. It had a real lot of flutter and wow.


Without bias, the distortion would be greater. There is a hysterisis in
the transfer function of magnetic force vs. magnetization for the
material on the tape. You have to get above a certain signal voltage to
magnetize the tape at all. If you don't have bias, you get crossover
distortion, which is worse for low level signals. The bias is a
supersonic signal applied to the head when recording. The audio signal
is added to that, so even low level audio signals result in a change
in magnetization, and a more linear magnetization pattern results.

These machines used DC bias.

Graham
 
M

Michael A. Terrell

Eeyore said:
These machines used DC bias.


That would make the frequency response even worse.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
M

Michael A. Terrell

Eyesore said:
It was miserable anyway.

Graham


It would saturate the head and raise the noise floor, not to mention,
the tape head would become permanently magnetized, and erase the tape
after a few plays.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
J

Jamie

Don said:
Unless you've got a super-sensitive 'scope, you're probably going to
need to run it through an amp before you'll have much luck seeing
anything - The "raw" output from a head is tiny. I mean *REALLY* *TINY*.
i have a nice set of active Fet probes over here that works real good
for that..
 
Top