Tascam MMR-8 - Manual (Page 128)

TASCAM MMR-8 Owner’s Manual • Chapter 8 • Maintenance and Service 128
Chapter 8 Maintenance & Service
The MMR-8 requires little maintenance other than ensuring adequate airflow through the interior of the
unit. Do not use alcohol or other common studio chemical cleaners on the front panel keys.
If a build-up of dust occurs, .vacuum and/or brush off the fan opening area of the power supply
System Input Level Calibration Using the Meter LEDs
The LED meters are factory aligned for +4 dBu analog operating levels. Thus a +4 dBu input level will
yield a 0 VU indication. There are no meter calibration trim pots in the MMR-8, only trim pots for setting
the analog input and output signal levels.
The LED meters can be set to indicate various headroom operating levels, however. The industry de facto
standard (and MMR-8 default setting) is for 0 VU to indicate an absolute level of -20 dBFS (20 decibels
below digital full scale). This setting determines the amount of headroom in the system before digital
clipping occurs. This setting is adjustable from -15 dBFS to -24 dBFS. Use Setup Menu 520 to select the
meter reference level.
For in-system calibration of the input signal levels, there is a calibration mode that uses the meter LEDs
to indicate input level in steps of .333 dB, 1.0 dB, or 3 dB. Setup Menu 521 sets the MMR into this
calibration mode. The default setting is Off for a normal meter display. To monitor the input level, use
Menu 521 to set the calibrate mode ON. At the source, adjust the input source levels so that the meter
LEDs up to and including the 0 LED are on, indicating an input level of +4 to +4.33 dBu. Once
calibration is complete be sure to restore Setup Menu 521 back to Off for regular meter indication. The
input and output calibrations can be checked with the MMR-8 rack mounted, but to actually adjust the
input or output trim pots the MMR-8 must be removed from the rack and the top cover removed.
MMR Input and Output Level Calibrations
Use the following procedure to calibrate the MMR-8 Input Converter board (MIC), and the MMR-8
Output Converter board (MOC). This procedure assumes that both of these cards are in the unit. It is
assumed that an electronics technician who has experience operating audio measurement equipment is
performing the calibration.
The following equipment is required to perform calibration adjustments:
1. A Low distortion, accurate balanced input/output audio analyzer/generator (Audio Precision System
2 or equivalent). A digital multi-meter with AC rms volts measurement capability, in conjunction
with a function generator with a sine wave output as a signal source may also be used. However, the
AP equipment will provide more accurate measurements, and the settings/displays will be in the
proper units
2. Plastic Potentiometer Adjustment screwdriver/tweaker.
3. Input and Output cables compatible with the MMR-8 analog Audio In/Out connectors (equivalent to
TASCAM DA-88 analog audio cables).
The MMR-8 has a built-in 1 kHz oscillator to check that the output levels are calibrated. To check the
output levels, select Setup Menu 530 and select the 1 kHz setting. Press STO to turn on the internal
oscillator. Make sure no tracks are selected in the Input mode, and then press the Play switch. Measure
the output levels and verify that they match the calibration settings (+4 dBu nominal=1.228Vrms,
measured at XLR pin 2(+) and 3(-) pins). If calibration is required continue to the MOC Calibration
Procedure.