Calibration; Required Equipment; +48V Adjustment; Microphone Gain And Metering - Benchmark PRE420 Instruction Manual

Calibration

CAUTION: ROUTINE CALIBRATION
SHOULD NOT BE REQUIRED. THE
FOLLOWING PROCEDURE SHOULD ONLY
BE USED BY A QUALIFIED TECHNICIAN.
This procedure will restore the PRE420 to
factory calibration if controls have been
moved from factory settings.

Required Equipment

Calibration requires an audio generator, an
audio level meter, and a VOM (multimeter).
ALL ADJUSTMENTS SHOULD BE MADE BY
A QUALIFIED TECHNICIAN IN THE
ORDER LISTED BELOW:
1. Unless otherwise noted, set all
microphone "Gain" controls fully
clockwise, all "Pan" controls on center
detent, "Monitor" level fully clockwise,
and "Main" level fully clockwise.

+48V Adjustment

2. Using a DC voltmeter, measure the
voltage across D23 and adjust "+48V
Adjust" (R107) until the meter reads
48V.

Microphone Gain and Metering

3. Release all push buttons on front
panel.
4. Turn all microphone "Gain" controls
fully clockwise.
5. Set audio tone generator to 1 kHz with
an output level of -36 dBu if you wish
to set maximum gain to 60 dB (factory
default). Set generator to -46 dBu if
you wish to set maximum gain to 70
dB, or -26 dBu if you wish to set
maximum gain to 50 dB.
6. Connect tone generator to microphone
1 input.
PRE420 Instruction Manual – rev. E
7. Connect audio level meter to channel 1
direct output.
8. Adjust channel 1 "Max Gain Lim"
(R53) until output level is +24 dBu at
channel 1 direct output.
9. Adjust channel 1 "Meter Threshold"
(R13) so that CH1 red LED just turns
on. This sets the meter to the factory
default threshold of +24 dBu.
10. Repeat steps 6 - 9 for channels 2
through 4 to adjust "Max Gain Lim"
(R112, R170, and R253) and "Meter
Threshold" (R71, R131, and R203).

Monitor Gain Balance

11. Press "Solo 1" button. All other
buttons must be released. This feeds a
mono signal to the solo bus.
12. Microphone 1 "Gain" control must be
fully clockwise.
13. Set "Monitor" level control at
maximum gain (fully clockwise).
14. Set audio tone generator to 1 kHz with
an output level of -36 dBu (or level
used in step 3).
15. Connect tone generator to microphone
1 input.
16. Connect audio level meters to control
room L & R outputs.
17. Adjust "Monitor Gain Bal" (R284) until
the control room L & R outputs are
balanced.

Microphone Pan Balance

18. Release all push buttons on front
panel.
19. Set "Pan" controls on center detent.
20. Microphone "Gain" controls should be
fully clockwise.
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