Manley VOXBOX - MVBXA 4/2003 - Manual (Page 20)

CALIBRATION PROCEDURE
This procedure has been done by the factory and should not be re-adjusted unless absolutely needed
and should only be done by a qualified technician. There are potentially painfull high voltages inside
the VOXBOX so care should be taken even by good technicians. They have been known to get a
shock but they also know to use one hand and insulated tools or gloves.
The function of each trim is labeled on the diagram on the previous page (page 18). You
will need an audio oscillator and a high impedance AC voltmeter plus a medium small tip flat
screwdriver and a medium (#2) Phillips screwdriver. After removing the top let the unit warm up
for 30 minutes so that your calibration will be actually valid. You should check basic signal flow and
familiarize yourself with the unit during this time.
1) The first step is to make the EQ / DE-ESSER gain block be “unity gain” with no EQ or Limiting. Put a
1 KHz +4.0 dBu (1.228 Volts RMS) tone from an oscillator into the INSERT INPUT and select
INSERT. Make sure EQ and DE-ESS is BYPASSED. Measure the EQ OUTPUT PHONE JACK.
It should read the same level as the input signal. If not adjust “trimmer 1” (EQ=UNITY GAIN) for
+4.0 dBu or 1.228 volts AC RMS.
2) Use the VU meter select switch to read “EQ OUT”. The VU meter should read “0”. If not, adjust
trimmer 2 (0 on VU METER = +4.0 dBu = 1.288 VAC RMS).
3) Use the VU meter select switch to read “GR”. The VU meter should read “0”. If not, adjust trimmer 3
(METER GR ZERO SET).
4) Now put the oscillator into the LINE INPUT and the AC voltmeter on the PREAMP OUT PHONE JACK.
Put the PHASE switch into the middle position (LINE) and the GAIN switch to “45”. The filter
should be set FLAT and COMPRESSOR in BYPASS and both ATTACK and RELEASE to MEDI-
UM. Verify that unity gain for the line input is corresponds on the “INPUT” knob to approximately
straight up or 11:00. Set the INPUT for +4.0 dBu (1.228 volts) measured at the PREAMP OUT.
Now take the COMPRESSOR out of BYPASS and turn up the COMPRESSOR THRESHOLD to
maximum. Select “PRE OUT” on the VU meter switch. The VU should also read “-8”. If not adjust
“trimmer 4” (COMP CAL) so that it does.
5) Same as above except switch VU to GR mode and adjust COMP GR CAL (trimmer 5). Now turn the
threshold so that there is 2 dB of compression. Verify that GR and audio “track” within .5 dB. 6)
Now put the oscillator back into the INSERT INPUT and the AC voltmeter on the EQ OUT PHONE
JACK. SELECT LIMIT on the De-ess rotary switch. Select “EQ OUT” on the VU meter switch.
Now take the DE-ESSER out of BYPASS and turn up the DE-ESSER THRESHOLD to MAXI-
MUM so that the output signal is reduced 3.5 to 4 dB (or that 40% modulaton dot)(-2 dBu or .614
AC volts) . The VU should also read approximately “-3.5”. If not adjust “trimmer 6” ( DS / LIMIT
CAL) so that it does. You can also check De-esser operation by switching the oscillator to 3 KHZ
and selecting 3 KHz on the de-esser select switch. Adjust the DE-ESSER THRESHOLD so that the
output signal is reduced 6 dB which will read -2 dBu or .614 AC volts. The VU meter should read
“-6” +/- .5 dB.
7) Same as above but switch VU to D-S and adjust METER DS CAL(trimmer 7). Verify DS and audio track.
In other words watch that “real” de-essing and “indicated” de-essing agree.
Step 4 sets up the compressor ratio, Step 5 makes the G-R meter agree. Step 6 sets up the LIMIT Threshold
and step 7 makes the D-S meter agree. Two units should be calibrated together for the LINK to perform accu-
rately. At the factory we use a test jig that injects a signal into the LINK jacks to ensure unit to unit sameness
needed for LINK to be very predictable. The above procedure will be close.
The calibration procedure is completed unless you want to double check, which we always recommend. In
that case return to step 1.
Each of the power supplies (Not B+) is regulated and there are no trims involved. The voltages should be:
+12 VDC for filaments, relays & lamps / leds.
+15 VDC for ICs used in sidechains and meter drivers
-15 VDC for ICs used in sidechains and meter drivers
+300 VDC for vacuum tube B+ supplies.
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