12
FIELD ALIGNMENT
To occasionally check the accuracy of your DORROUGH Loudness Monitor,
perform the Field Alignment listed in this section. Although the meter is
designed for exceptionally stable performance, you may want to satisfy your
curiosity or verify performance after a change has been made.
All adjustments for the alignment of the LED display are located on the
Input Signal Processing Board. To gain access to these controls (40-A and
40-B Models), the instrument is placed bottom-side-up and the six screws are
removed. Once the screws are removed, place the unit right-side-up and the
cabinet can be lifted from the remainder of the chassis, exposing the adjust-
ments. Refer to Fig. 15 for circuit board part locations as you perform the
following instructions. Use Fig. 16, Fig. 17, and Fig. 18, for Panel Type
Models 10-A(B), 12-A(B), 20-A(B), and 21-A(B).
"A" Type Monitor Alignment Procedure
1. Connect the output from a test oscillator to the meter’s left input termi-
nals (see Fig. 1 ).
2. Feed a 1000 Hz sine-wave, at a reference level of “0” dB, into the meter.
3. Adjust the left input gain control VR-13 so that DS-1 is off and DS-2, -3, -4
are on.
4. Adjust the input signal to -25 dB.
5. Adjust the low PERSISTENCE level control VR-44 so only the first LED
(-25 dB on meter scale) at the bottom of the PERSISTENCE scale is
illuminated.
6. Adjust the low PEAK level control VR-46 so only the first LED (-25 dB on
meter scale) at the bottom of the PERSISTENCE scale brightens.
NOTE: This indicates that both the PEAK and the PERSISTENCE drivers are
providing current to this LED.
7. Change the input signal to +5 dB (a 30 dB increase).
8. Adjust the high PERSISTENCE level control VR-48 so that the bargraph
illuminates LEDs up to and including the +5 dB LED.
9. Adjust the high PEAK level control VR-49 so that the +5 dB LED bright-
ens.
NOTE: This indicates that both the PEAK and the PERSISTENCE drivers are
both providing current to this LED.
10. Increase input signal to +14 dB and observe that the +14 dB LED illumi-
nates.
NOTE: The +14 dB LED should indicate drive from both PEAK and PERSIS-
TENCE drivers.
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