dbx 3BX III, Руководство по обслуживанию

Страница: 15 / 37

Model 3BX III Service Manual
– 11 –
4.2 Mid-band rms calibration
4.2.1 Set the controls as in 4.1.1.
4.2.2 Connect the sine-wave oscillator to both inputs. Set it to
100 Hz, +10 dBV (3.16 V). Connect the scope to the output of the
mid-band rms-detector buffer at TP12 (U711 pin 7) and set it to
5 mV/div (AC-coupled) and 2 ms/div.
4.2.3 Adjust VR706 for best rms symmetry, again as shown in
Fig. 7b.
4.2.4 Reset the oscillator to 1 kHz, +15 dBV (5.62 V).
4.2.5 Also connect a digital voltmeter to TP12.
4.2.6 Adjust VR707 for 0.00 Vdc ±10 mV. Verify that no more than
one LED in the mid-band gain-change display is lit.
4.2.7 Reduce the oscillator’s output level in 10 dB steps and
verify that the voltage at TP12 changes as follows:
Input Level TP12 Voltage Tolerance
+15 dBV 000 mV dc ±10 mV
+5 dBV +541 mV dc ±50 mV
-5 dBV +1080 mV dc ±100 mV
-15 dBV +1620 mV dc ±150 mV
4.3 High-band rms calibration
4.3.1 Set the controls as in 4.1.1.
4.3.2 Connect the sine-wave oscillator to both inputs. Set it to
1 kHz, +10 dBV (3.16 V). Connect the scope to the output of the
high-band rms-detector buffer at TP14 (U722 pin 7). Set the scope
to 2 mV/div (AC-coupled) and 0.2 ms/div.
4.3.3 Adjust VR708 for best rms symmetry, Fig. 7c.
4.3.4 Reset the oscillator to 10 kHz, -2 dBV (794 mV).
4.3.5 Also connect a digital voltmeter to TP14.
4.3.6 Adjust VR709 for 0.00 Vdc ±10 mV. Verify that no more than
one LED in the high-band gain-change display is lit.
4.3.7 Reduce the oscillator’s output level in 10 dB steps and
verify that the voltage at TP14 changes as follows:
Level TP14 Voltage Tolerance
-2 dBV 000 mV dc ±10 mV
-12 dBV +541 mV dc ±50 mV
-22 dBV +1080 mV dc ±100 mV
-32 dBV +1620 mV dc ±150 mV