This is essentially a stable and sensitive Valve Voltmeter
of very wide range, designed to meet the special
requirements of the Radio and T.V. Engineer. It has the
advantages of an extremely wide frequency coverage and very
high input impedance, together with good accuracy and
reliability. Rapid and simple operation is a feature of the
instrument. The 171A is highly versatile, covering in
addition to D.C. and A.C. voltage measurement, a wide range
of resistance measurement. It can also be used as a direct
reading wide range output meter, scaled direct in decibels.
30 useful ranges are available. The high input impedance
allows much more accurate results than are possible with any
other type of test meter when measuring in high impedance
circuits, such as resistance coupled amplifiers. A.V.C.
networks, etc. A centre zero facility is provided on the IV
and 2.5V D.C. ranges to enable T.V. and F.M. discriminator
alignment to be carried out. The 171A can measure up to 25
KV D.C. with an additional adaptor. The instrument is of
quite exceptional utility, and forms a valuable acquisition
in any workshop or laboratory.
Basically the 171A consists of a balanced double triode twin
cathode follower bridge network, and a miniature
shunt-connected double diode R.F./A.F. probe, both
compensated against drift. The removable fully screened
probe allows connection to be made very close up to the
circuit under test, and has a frequency response of ± 2dB
from 6 c/s up to about 200 Mc/s. The D.C. or rectified A.C.
out-of-balance voltage appearing across the valve bridge
network is proportional to the input voltage to the
instrument, and is applied via the range multipliers to the
robust microammeter with 5" scale. The instrument is almost
completely free from drift after a brief warming-up period.
Accurate readings may be taken on the upper four voltage
ranges only 20 secs, after switching on. A special heater
balance compensation circuit ensures that zero and reading
accuracy are virtually unaffected by mains voltage
variations. All components are conservatively rated, and ihe
meter movement is fully protected against any possibility of
damage by overload.