2-12. To obtain permanent records of 412Areadings, connect a recorder to the
OC
AMPLIFIER OUTPUT
connector and operate the
'
412A as directed above. The output of the 412A is 1 volt at full scale; ti necessary,
Externally attenuate the 412A output to match it with recorder sensitivity. Maximum rated load current from
the 412A is 1 ma. A load res
i
stance of less than 1000 ohms may cause the load current to exceed
1 ma and thus cause errors in meter indication and amplifier gain.
SECTION 111
CIRCUIT OPERATION
3-1.
GENERAL
3-2. The Model 412A is basically a
O
to 0.9 millivolt dc voltmeter. Precision voltage dividers, shunts,
and reference resistors extend the range of the baste voltmeter and permit current and resistance
measurements as well.
3-3.
CIRCUIT OPERATION.
3-4. With the FUNCTION selector and RANGE switch properly set, voltage is applied to a photoconductive
modulator through a low-pass filter. See figure 3-1
.
The filter attenuates ac components present on any input signal, and the modulator converts the remaining
dc component to a square wave. A synchronous-motor-driven
,
light-beam chopper sets modulator frequency
at 5
/
6 power-line frequency
.
An ac-coupled amplifier amplifies modulator output about 500,000 times.
A demodulator synchronized with the modulator by the light-beam chopper, converts amplifier output to
de,
The output of the demodulator is filtered and applied through a cathode follower to 1) a feedback network,
2) the DC AMPLIFIER OUTPUT terminals and 3) an output indicator
.
The feedback network stabilizes
the dc gain of the modulator-amplifierdemodulator system to a value of 1111, thereby providing an output
of 1 volt for an input of 0.9 millivolt.
The output indicator is a 0-1 voltmeter. The POLARITY switch permits reversal of indicator connections,
if required, to obtain up-scale readings.
The POLARITY switch is disabled when the FUNCTION selector is set to OHMS.
SERVICING ETCHED CIRCUIT BOARDS
Excessive heat or pressure can lift th
e
copper strip from the board. Avoid damage by using a lo
w
power
s
olde
r
ing i
r
on (SO
w
atts maximum) and following these instructions. Copper that lifts off the board should
be cemented in place with a quick drying acetate base cement having good electrical insulating properties.
A break in the copper should be repaired by soldering a short length of tinned copper wire across the break.
Use only high qua lit y rosin core solder when repairing etched circuit boards. NEVER USE PASTE FLUX.
After soldering, clean off any excess flux and coat the repaired area with a high quality electrical varnish
or lacquer.