341
A
343A
neon lamps DS1 and
DS2 and resistor R98. This
circuit is
activated
if the instrument output voltage
becomes exces-
sively
high on the
10 or 100 volt ranges,
for
example, if the
main
amplifier opened and the series
regulator were
driven
fully on. Under these
conditions, DS1 and DS2 would fire
and
a
positive
trigger voltage would be developed
at the
anode of CR67. Mien the voltage
at
the
gate of CR29 is
sufficiently high,
CR29 conducts and removes
one
input
to
the AND gate. This de-energizes
the high
voltage
relay and
places the instrument in STANDBY.
To reset the relay, it
is
necessary to momentarily set the FUNCTION
switch to
STANDBY/RESET
and back to ON.
3-38.
HIGH
VOLTAGE POWER
SUPPLY
3-39.
Diodes CR53 and CR54 and capacitors
C55
through
C58 comprise a full-wave voltage doubler. The
voltage
supplied to the doubler depends
on the setting of
the range switch and the
most significant decade switch.
On the
10
and
100 volt ranges, the high voltage output is
fixed
at
about
500 volts. On the
1000
volt range, the
eleven
position decade switch selects
a voltage ranging
from
500 to 1500 volts.
3-40.
METER CIRCUIT
3-41.
The meter circuit is connected between +
and
-
SENSE for voltage
readings and is connected in
shunt
with R70 for output current readings. In current
measure-
ment mode, the meter current includes
beta string current.
This introduces a small error,
which
is negligible
on the 10
and
100 volt ranges. On the 1000
volt
range,
the error will
be equal
to +1
milliamp.
3-42.
BETA STRING
3-43.
The beta string
is a
six-decade
resistive network
(seven
decades on the
343A),
which provides
precise con-
trol
of feedback
to
the
chopper-stablized
amplifier,
there-
by controlling instrument
output. The first decade
of
the
341
A
and the
first and second decades of the
343A incor-
porate
linearity adjustments, which
are adjusted during
calibration to ensure exact resistance
ratios. The resistors
and switches
which comprise the beta string
are located on
reference
board and sample
string assemblies.
3-6
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