"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