I opened the meter of an Hallicrafters SX-117 to mechanically adjust the zero.
Once open, the meter appeared to be of the moving iron type, that is with no
moving coil. But I was surprised to see that that meter has no spiral torsion
springs either; I can only see a metal (?) dish mounted on the axis, part of
which lies inside a rather flat coil.

With no spring, I cannot understand what forces the needle to stay at zero scale
in absence of current. Therefore I cannot figure out what I should do to adjust
the zero.

The HT-44 as well as the EICO 723 use the same type of meter. Dumping is poor,
and the needle keeps banging for a while when you apply a DC current.

73

Tony I0JX

"Antonio Vernucci" wrote in message
...
I opened the meter of an Hallicrafters SX-117 to
mechanically adjust the zero.
Once open, the meter appeared to be of the moving iron
type, that is with no
moving coil. But I was surprised to see that that meter
has no spiral torsion
springs either; I can only see a metal (?) dish mounted on
the axis, part of
which lies inside a rather flat coil.

With no spring, I cannot understand what forces the needle
to stay at zero scale
in absence of current. Therefore I cannot figure out what
I should do to adjust
the zero.

The HT-44 as well as the EICO 723 use the same type of
meter. Dumping is poor,
and the needle keeps banging for a while when you apply a
DC current.

73

Tony I0JX
I don't know for certain about the meter in the SX-117
but a lot of instruments use what are called taut-band
suspension. The meter coil or vane and pointer are suspended
on a flat ribbon running along the axis of rotation and
fastened at the ends. The band acts as both support for the
moving mechanism and as the restoring spring. It is
essentially a torsion spring.
The advantage of this system is that it eliminates the
bearings along with the friction and hysterisis they cause.
Taut-band meters have very good repeatability and can be
very accurately calibrated. Hewlett-Packard began to use
taut-band meters in their instruments along around the early
1960's and they became common around that time. Taut-band
meters do have zero-adjustments but they may be internal.
The damping of any meter is a matter of the back EMF of
the meter. No meter running open circuit will be well
damped. It may be that the testing circuit does not have low
enough resistance to damp the meter. If you rotate the meter
open circuit the pointer wil move around a lot. If you short
the terminals it will barely move at all.


--
---
Richard Knoppow
Los Angeles, CA, USA

Hi, Tony

I have a "Shurite" brand iron vane panel meter, similar to what you
describe. The meter's zero adjust screw is connected to a semicircular
metal disk mounted at a right angle to the axis of the pointer shaft.
There is no contact between the semicircular disk and the pointer
assembly. I suppose zeroing is based on adding or counteracting
residual magnetism in the fixed iron vane. I don't see any torsion
springs in the Shurite meter.

An iron vane meter was used in the Heath DX-35, and I remember the
underdamped response. You could practically monitor your sending by the
sound of the meter pin hitting the end stops.

73,
Ed Knobloch

Antonio Vernucci wrote:
I opened the meter of an Hallicrafters SX-117 to mechanically adjust the
zero.
Once open, the meter appeared to be of the moving iron type, that is
with no
moving coil. But I was surprised to see that that meter has no spiral
torsion
springs either; I can only see a metal (?) dish mounted on the axis,
part of
which lies inside a rather flat coil.

With no spring, I cannot understand what forces the needle to stay at
zero scale
in absence of current. Therefore I cannot figure out what I should do to
adjust
the zero.

The HT-44 as well as the EICO 723 use the same type of meter. Dumping is
poor,
and the needle keeps banging for a while when you apply a DC current.

73

Tony I0JX

but a lot of instruments use what are called taut-band suspension. The meter
coil or vane and pointer are suspended on a flat ribbon running along the axis
of rotation and fastened at the ends. The band acts as both support for the
moving mechanism and as the restoring spring. It is essentially a torsion
spring.
The advantage of this system is that it eliminates the bearings along with
the friction and hysterisis they cause. Taut-band meters have very good
repeatability and can be very accurately calibrated. Hewlett-Packard began to
use taut-band meters in their instruments along around the early 1960's and
they became common around that time. Taut-band meters do have zero-adjustments
but they may be internal.
The damping of any meter is a matter of the back EMF of the meter. No meter
running open circuit will be well damped. It may be that the testing circuit
does not have low enough resistance to damp the meter. If you rotate the meter
open circuit the pointer wil move around a lot. If you short the terminals it
will barely move at all.

Richard Knoppow
Los Angeles, CA, USA


Hi Dick,

in taut band meters the taut band replaces the torsion springs. But the
Hallicrafters meter has a rigid axis (not taut band), and has neither torsion
springs nor a moving coil, so it must work on a different principle.

Thank for the advice and 73

Tony I0JX



the Hallicrafters

Hi, Tony

I have a "Shurite" brand iron vane panel meter, similar to what you describe.
The meter's zero adjust screw is connected to a semicircular metal disk
mounted at a right angle to the axis of the pointer shaft. There is no contact
between the semicircular disk and the pointer assembly. I suppose zeroing is
based on adding or counteracting residual magnetism in the fixed iron vane. I
don't see any torsion springs in the Shurite meter.

An iron vane meter was used in the Heath DX-35, and I remember the underdamped
response. You could practically monitor your sending by the sound of the
meter pin hitting the end stops.

73,
Ed Knobloch

Hi Ed,

yes, the meter is just as you wrote. I did not know the name "iron vane meter",
good to know.

Carefully examining the meter, I found it very similar to what you wrote. On the
axis I can see two parallel round disks (orthogonal to the axis) one at the axis
center and one at its extreme:

- the first disk is fixed to the meter axis, and part of it lies inside the
coil. When current flows through the coil, the disk (and the axis) rotates. I
presume that the magnetic properties of that disk are not homogeneous (maybe
half disk is magnetic and the other half is non magnetic)
- the second disk is instead not fixed to the axis. Its position can be changed
by a means of a screw placed at the axis end (just like the zeroing screw of
normal meters). I presume that also this disk is non-homogeneous. Rotating the
disk changes the meter zero. I tried and it works!

Thanks and 73

Tony I0JX