BRUEL & KJAER 3923 User manual

Revision
November 1996
BE 0165 –12
Brüel & Kjær Rotating Microphone Boom Type 3923
User Manual
Rotating Microphone Boom
Type 3923
From Serial number: 1913 432

0−0Brüel & KjærRotating Microphone Boom Type 3923
User Manual
Safety Considerations
This apparatus has been designed and tested in accordance with IEC Publication
348, Safety Requirements for Electronic Measuring Apparatus (see specifications), and
has been supplied in safe condition. This User Guide contains information and warn-
ings which must be followed by the user to ensure safe operation and to retain the
apparatus in safe condition. Special note should be made of the following:
Safety Symbols
The apparatus will be marked with this symbol when it is important that the
user refers to the associated warning statements given in the User Guide.
Chassis Terminal Safety Earth Terminal Hazardous Voltage
Explosion Hazard
The Unit is not designed to be used in potentially explosive environments. It should
not be operated in the presence of flammable liquids or gases.
Warnings
• Whenever it is likely that the correct function or operating safety of the apparatus has
been impaired, it must be made inoperative and be secured against unintended opera-
tion.
• Any adjustment maintenance and repair of the open apparatus under voltage must be
avoided as far as possible and, if unavoidable, must be carried out only by trained
service personnel.
Copyright © 1979, 1996, Brüel&Kjær A/S
All rights reserved. No part of this publication may be reproduced or distributed in
any form or by any means without prior consent in writing from Brüel & Kjær
Sound & Vibration Measurement A/S, Nærum, Denmark.

Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 0−3
Contents
1. Controls and Accessories ....................................................................................... 1 – 1
1.1 Controls .................................................................................................................... 1–2
1.2 Accessories ................................................................................................................. 1–3
2. Operation............................................................................................................................. 2 –1
2.1 Power Supply............................................................................................................... 2–2
Rechargeable Batteries .......................................................................................... 2 – 2
Battery Charge and/or External Power Supply.................................................... 2 – 2
2.2 Boom Assembly ........................................................................................................... 2 – 3
2.3 Microphone Installation.............................................................................................. 2 – 4
Half-inch Preamplifier with Half-inch Microphone.............................................. 2 – 4
Half-inch Preamplifier with One-inch Microphone ............................................ 2 – 4
One-inch Preamplifier ........................................................................................ 2 – 4
2.4 Final Assembly ............................................................................................................ 2 –6
2.5 Use of the Sync. and Remote Control Socket .......................................................... 2 – 7
2.6 Replacement of Drive Belts; Cam Removal or Installation ...................................... 2 – 7
3. Accessory Bag KF 0074............................................................................................ 3 – 1
3.1 General......................................................................................................................... 3 – 2

Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 1−1
Chapter 1
Controls and Accessories

Chapter 1 — Controls and Accessories
Controls
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
1−2
1.1 Controls
Fig.1.1 Microphone Rotating Boom Type 3923
SECONDS/ REVOLUTION: Four position rotary switch for selection of the rotation speed.
In the first position (“Off”) the battery supply is disconnected. In
the three other positions, Type 3923 is powered from its internal
batteries and the rotation speed is indicated by the selector set-
ting.
BATTERY METER: Monitors battery condition when the SECONDS/REVOLUTION
selector is set to one of the operating conditions.
EXT. POWER: Seven-pin DIN socket for connection of an external DC power sup-
ply for recharging the internal batteries and/or operation using
external power. See section 2.1. The socket accepts plug JP 0710.
SYNC. AND REMOTE
CONTROL: Eight-pin DIN socket providing synchronisation signals which in- dicate the boom position. The socket also allows remote start/stop.
The socket accepts plug JP 0808.
PREAMP. INPUT: Seven-pin socket for connection of a B & K microphone preamplifi-
er. Accepts the B &K standard plug JP 0715.
OUTPUT: Seven-pin plug for the connection of Type 3923 to B&K sound
measurement instrumentation. The plug is connected, via 3m cable
and slip rings, to the PREAMP. INPUT socket.

Chapter 1 — Controls and Accessories
Accessories
Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 1−3
1.2 Accessories
YG 0227: Boom termination tube. The difference between this and the ex-
tension tubes is a flat mounting surface for the microphone sus-
pension. The tube is 50 cm long.
YG 0226: Extension tubes (5 are provided). Each tube is 50 cm long. Allows
selection of any boom length between 50 cm and 2 m from micro-
phone to rotational axis with a 1 kg load if proper balance is re-
quired. With lighter loads the useful boom length may be longer.
The conical front shape of the tubes ensures a firm boom assem-
bly.
UA 0476 and UA 0477: Counterweights (2.7 kg and 0.5 kg respectively) for balancing the
assembly.
UA 0502: Microphone suspension. The rubber yokes fit one-inch and half -
inch preamplifiers and attenuate possible vibrations transmitted
through the boom to the microphone.
UA 0800: Ball joint. Screws onto the end of the boom and allows mounting
of the microphone suspension UA 0502.
UA 0708: Set of spare drive belts. Replacement procedure is given in section
2.6.
ZG 0283 Battery Charger for recharging the internal NiCd cells, Allows
mains power operation of Type 3923 while recharging. See section
2.1.
DV 0225: Cable clamps for firmly attaching the cable to the boom.
KF 0074: Accessory bag. See Chapter 3.

Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
1−4

Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 2−1
Chapter 2
Operation

Chapter 2 — Operation
Power Supply
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
2−2
2.1 Power Supply
2.1.1 Rechargeable Batteries
Rechargeable nickel-cadmium cells (B & K stock number QB0008) are supplied with
Type 3923. Fully-charged batteries allow 35 hours operation. Battery condition is
monitored by the BATTERY METER. If the needle falls into the re area, the batter-
ies should be recharged. This can be done in several ways, with Type 3923 in
operation or not. See 2.1.2
Cells QB 0008 have a voltage and charge rating of 1.2 V and 4Ah and may be
recharged approximately 500 times before replacement is necessary. The need for
replacement is indicated by the batteries failing to reach a fully charged condition
after having been left on charge for 14 hours or more. Replacement of the batteries
is as follows (reference can be made to Fig.2.2, section 2.6).
1. Type 3923 should be placed so that the black bottom cover is facing upwards.
2. Release the bottom finger screw and remove the cover.
3. Open the battery compartments and remove the used batteries.
4. Insert new batteries with the polarities as indicated on the battery compart-
ments.
WARNING: There is the possibility of explosion if recharging is attempted on cells
whose polarity has been reversed. Similarly, never mix different makes or types of
battery, os batteries that have been used in other instruments. This can cause
internal reversal of battery polarity.
5. Replace the battery covers and the bottom cover of Type 3923.
2.1.2 Battery Charge and/or External Power Supply
The EXT.POWER socket allows connection of an external DC power supply for
recharging the batteries and/or operation the instrument. Socket connections are
shown in Fig.2.1.
Fig.2.1 Connections of the EXT. POWER socket (external view)
1
2
3
4
5
67
Ground
7,5-13 V
790517/1e

Chapter 2 — Operation
Boom Assembly
Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 2−3
The Battery Charger ZG 0283, delivered with Type 3923, operates from mains sup-
plies of 100, 115, 127, 200, 220 or 240V AC (50 to 400 Hz), selectable.
The ZG 0283 will commence charging as soon as it is connected to Type 3923
(whether Type 3923 is switched on or off). recharge time is approximately 14 hours
for fully discharged cells. The charger may be used for mains operation of Type
3923 whilst simultaneously recharging the batteries. In this case, however, the bat-
tery recharge time will be longer.
It is also possible to use a DC voltage to operate Type 3923, and to recharge its
internal NiCd batteries. To recharge the batteries and/or operate the instrument a
voltage between 8.3 and 13 V is required. The instrument will, however, operate
down to a lower voltage, 7.5 V, but without recharging the batteries. The maximum
current allowable for recharging is 400 mA. To check the battery condition, using
the built-in meter, the external power source must first be disconnected.
2.2 Boom Assembly
The mounting of Type 3923 is provided with a 3/8” Whitworth thread which allows
direct mounting onto a standard tripod or stand, etc.
Selection of the boom length and of the angle between the rotational plane and the
horizontal plane depends upon the standard or code of practice which must be
followed for the measurements.
Assembly of Type 3923 is as follows:
1. Mount the body of Type 3923 on its stand.
2. If no external supply is to be used, check battery condition by turning the
SECONDARY/REVOLUTION selector to “64”. If the needle of the BATTERY
METER deflects in to the red area, the batteries need recharging. This can be
done while Type 3923 is operating by connecting Battery Charger ZG 0283 to
the EXT. POWER socket. For more details on external power supply and battery
recharging, see section 2.1.
3. Lock the trunion mount.
4. Screw together the necessary number of tubes so that the total boom length is
more than the required distance between the microphone and the rotation axis.
The first tube must be the one having a flat mounting surface at one extremity.
5. Slide the boom into the rotating fixture and lock it when the required distance
between the extremity and the rotation axis is obtained.
6. Slide both counterweights onto the boom. The larger of the two should be used
for rough balance adjustment while the smaller allows fine adjustment. If neces-
sary, the small counterweight may be slid onto the microphone side of the boom.

Chapter 2 — Operation
Microphone Installation
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
2−4
7. Unlock the trunion and adjust the counterweight positions until the boom is
balanced. Lock the counterweights. Note that the balance does not need fine
adjustment at this stage since the balance needs adjustment after microphone
installation.
8. Lock the trunion.
2.3 Microphone Installation
The microphone assembly should first be attached to the Microphone Suspension
UA 0502. Both one-inch and half-inch microphone assemblies may be used. The
suspension does not accept the quarter-inch Microphone Preamplifier Type 2618.
The Suspension prevents unwanted vibration being transmitted through the boom
to the microphone. It consists of a support and two sets of preamplifier yokes, for
both one-inch and half-inch preamplifiers. Installation of the microphone assembly
depends upon the microphone size and preamplifier type.
2.3.1 Half-inch Preamplifier with Half-inch Microphone
When a half-inch preamplifier (e.g. B & K Type 2669C) is used with a half-inch
microphone (or smaller cartridge with corresponding adaptor), two half-inch Yokes
DS 0412 are used (see Fig.2.2). Fit the Yokes to the mounting support and slide the
preamplifier through the Yokes, starting from the support ring. Push the preampli-
fier as far as possible to keep the influence of reflections from the suspension to a
minimum. For the best vibration insulation, the Yokes should be adjusted to be
parallel.
2.3.2 Half-inch Preamplifier with One-inch Microphone
When using a one-inch microphone mounted on a half-inch preamplifier fitted with
Adaptor DB 0375, both a half-inch and a one-inch Yoke must be used (see Fig.2.2).
The half-inch Yoke is fitted close to the support ring. Slide the half-inch preamplifi-
er (without microphone and adaptor) through the Yoke and fit Adaptor DB 0375 to
the preamplifier. The one-inch Yoke is then slid over the Adaptor and fitted to the
support. Adjust the Yokes to be parallel and mount the microphone on the pream-
plifier.
2.3.3 One-inch Preamplifier
With a one-inch preamplifier two one-inch Yokes must be used. To allow mounting
the microphone as far away from the suspension as possible, the Adaptor Ring
DP 0217 should be fitted over the cable entry of the preamplifier (see Fig.2.2).
Again, the Yokes should be adjusted to be parallel when the preamplifier and the
Adaptor Ring have been fitted.

Chapter 2 — Operation
Microphone Installation
Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 2−5
Fig.2.2 Half-inch Microphone and half-inch Preamplifier fitted to Suspension UA 0502
Fig.2.3 One-inch Microphone and half-inch Preamplifier fitted to Suspension UA 0502

Chapter 2 — Operation
Final Assembly
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
2−6
2.4 Final Assembly
Final assembly of Type 3923 is as follows:
1. Mount the boom as described in section 2.2.
2. Mount the microphone and preamplifier in Suspension UA 0502 (see section 2.3).
3. Screw the Suspension onto Adaptor UA 0800 and mount the assembly at the
extremity of the boom, orientating the Suspension as necessary.
4. Connect the preamplifier to the PREAMPLIFIER socket of Type 3923.
5. Clamp the cable along the boom using cable clamp DV 0225.
6. Unlock the trunion and adjust the boom balance. The small counterweight must
be used for fine adjustment. Lock the counterweights, and lock the trunion at
the required angle.
Note: For correct balance adjustment, the trunion must be unlocked so that the
adjustment takes place in the horizontal plane, and the boom must be set per-
pendicular to the suspension axis. Care should be taken that the cable from
Type 3923 does not introduce loading during the balance adjustment.
7. Connect the 7-pin plug of Type 3923 to the measuring instrumentation.
8. Connect the external power supply, if any (see section 2.1).
Fig.2.4 One-inch Microphone and one-inch Preamplifier fitted to Suspension UA 0502

Chapter 2 — Operation
Use of the Sync. and Remote Control Socket
Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 2−7
9. Type 3923 is now ready for operation. The rotation speed is selected by setting
the SECONDS/REVOLUTION selector to the required position. For remote
start/stop, see section 2.5. The friction of the drive-belts allows turning the
boom to any start position without operating Type 3923.
2.5 Use of the Sync. and Remote Control Socket
Fig.2.1 shows the connections of the SYNC. AND REMOTE CONTROL socket,
which is situated on the motor assembly. Synchronisation with external equipment
is achieved through the operation of a microswitch, which is activated by cams
attached to the drive shaft of the boom. On delivery three cams are fitted at rota-
tional angles of 120˚. It is, however, possible to modify the number of cams as
described in section 2.6.
The microswitch is internally connected to pins 2 and 3, and when not activated
there is 5 V at pin 3. When activated by a cam a connection between pins 2 and 3 is
made and pin 3 goes to ground.
Remote control of the Start/Stop of the boom can be obtained by using pins 2, 3
and 6. Connection pin 5 to pin 2 causes the boom to stop immediately and connect-
ing pin 6 to pin 2 stops the boom at the next cam. With no connections on pins 5 or
6 the boom will continuously rotate.
2.6 Replacement of Drive Belts; Cam Removal or Installation
A spare set of drive belts is provided with Type 3923. When the belts are worn,
they normally spring out of their pulleys, thus indicating the need for replacement.
On delivery, three cams are fitted to the drive shaft. They activate a microswitch
after each 120˚ rotation. Two of the cams may be removed in order that the micros-
witch is activated only once per revolution.
Fig.2.5 Connections of the SYNC. AND REMOTE CONTROL
socket (external view showing internal connections)
1
2
63
4
5
78
To control
circuit
Microswitch
Ground
790518/1e

Chapter 2 — Operation
Replacement of Drive Belts; Cam Removal or Installation
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
2−8
To gain access to the drive belts or to the cams, the procedure is as follows:
1. Turn the body upside down and remove the cover (see Fig.2.2).
2. Disconnect the three connectors from the printed-circuit board and remove the
board.
3. Unscrew the four screws retaining the base plate on which the battery compart-
ments are fitted and the two screws fixing the little printed circuit board to the
housing.
4. Carefully slide the plate out perpendicular to the printed-circuit board holder, as
indicated in Fig.2.2. Do not attempt to lift the plate up, this might cause dam-
age to the contacts of the slip rings.
5. Change the drive belts or remove the cams as necessary (see Fig.2.2).
6. Slide the plate into place, perpendicular to the printed-circuit board holder and
replace the four base plate screws and the two screws for the little printed
circuit board.
Fig.2.6 Type 3923 without cover

Chapter 2 — Operation
Replacement of Drive Belts; Cam Removal or Installation
Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 2−9
7. Fit the printed-circuit board in place and plug the connectors in.
8. Replace the cover over the body.
9. Each time the microswitch is activated, a slight noise is produced. If necessary,
this noise may be suppressed by pushing the microswitch away form the rota-
tion axis. Carry out steps 1 to 4 of the above procedure. The microswitch is
fitted on the metal plate. Release the two screws holding the microswitch and
push the front of the microswitch away from the centre. Fasten the screws
again (steps 6 to 8).
Fig.2.7 Dismantled view of Type 3923 showing drive belts and cams

Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
2−10

Rotating Microphone Boom Type 3923
User Manual
BE 0165 –12 3−1
Chapter 3
Accessory Bag KF 0074

Chapter 3 — Accessory Bag KF 0074
General
Rotating Microphone Boom Type 3923
User Manual Brüel & Kjær
3−2
3.1 General
Accessory Bag KF 0074 is delivered as a standard accessory with Type 3923. Shows
where the different accessories should be placed in the bag.
Fig.3.1 Location of accessories in Bag KF 0074