BioSystems BTS 330 Service manual

SERVICE MANUAL

MS330IA031203

II

INDEX

1. INTRODUCTION 1-1

2. FUNCTIONAL PARTS DESCRIPTION 2-1

2.1. Optical system 2-1

2.1.1. Constitutive parts 2-1

2.1.2. System description 2-1

2.1.3. Physical description 2-2

2.1.4. Signal conditioning 2-2

2.1.5. Adjustment 2-7

2.1.6. Precautions and maintenance 2-7

2.2. Aspiration system 2-7

2.2.1. Components 2-7

2.2.2. System description 2-7

2.2.3. Physical description 2-8

2.2.4. Programmable parameters 2-8

2.2.5. Programming 2-8

2.2.6. Adjustment 2-8

2.2.7. Precautions and maintenance 2-9

2.3. Thermostatization system 2-9

2.3.1. Components 2-9

2.3.2. System description 2-9

2.3.3. Physical description 2-14

2.3.4. Programming 2-14

2.3.5. Adjustment 2-14

2.3.6. Precautions and maintenance 2-14

2.4. Communications system 2-15

2.4.1. Channel type 2-15

2.4.2. Channel characteristics 2-15

2.4.3. Information transmitted 2-15

2.4.4. Programming 2-16

2.5. Block diagram 2-18

2.5.1. Logarithmic amplifier 2-18

2.5.2. Analogic/Digital converter 2-18

2.5.3. Lamp control 2-18

2.5.4. Filters wheel detector 2-18

2.5.5. Filters wheel motor control 2-18

2.5.6. Temperature sensor amplifier 2-18

2.5.7. Peltier cell control 2-18

2.5.8. Pump motor control 2-19

2.5.9. Keyboard circuit 2-19

2.5.10. Printer control 2-19

2.5.11. Display circuit 2-19

2.5.12. RS-232 channel circuit 2-19

2.5.13. Fan control 2-19

2.5.14. Microcontroller 2-19

I

2.5.15. Power supply 2-20

2.5.16. Reset and battery-backup supervisory circuit 2-20

2.6. Electronic circuit description 2-20

2.6.1. Logarithmic amplifier 2-20

2.6.2. Analog to digital converter 2-22

2.6.3. Lamp control 2-22

2.6.4. Filter wheel detector 2-22

2.6.5. Filters wheel motor control 2-23

2.6.6. Temperature sensor amplifier 2-23

2.6.7. Peltier Cell control 2-23

2.6.8. Pump motor control 2-24

2.6.9. Keyboard circuit 2-24

2.6.10. Printer control 2-25

2.6.11. Display circuit 2-25

2.6.12. RS-232 channel circuit 2-26

2.6.13. Fan control 2-26

2.6.14. Microcontroller 2-26

2.6.15. Power supply 2-27

2.6.16. Microcontroll e r supervisory and reset circuit 2-27

3. CHECKS AND ADJUSTMENTS 3-1

3.1. Service menu 3-1

3.1.1. Option TEST 3-1

3.1.2. Option ADJUSTMENTS 3-2

3.1.3. Option UNLOCK/LOCK TESTS 3-2

3.1.4. Option UNLOCK/LOCK instrument 3-2

3.2. ADJUSTMENTS 3-2

3.2.1. Photometric adjustment 3-2

3.2.1.1. Materials needed 3-3

3.2.1.2. General remarks 3-3

3.2.1.3. Procedure 3-3

3.2.2. Thermostatic system adjustement 3-4

3.2.2.1. Materials needed 3-4

3.2.2.2. General remarks 3-4

3.2.2.3. Procedure 3-4

3.2.2.4. Explanation of the list parameters 3-5

3.2.2.4. Check 3-5

3.2.3. Filters wheel adjustment 3-5

3.2.3.1. Materials needed 3-6

3.2.3.2. General remarks 3-6

3.2.3.3. Manual mode 3-6

3.2.3.4. Automatic mode 3-7

3.2.4. Peristaltic pump adjustment 3-8

3.2.4.1. Materials needed 3-9

3.2.4.2. General remarks 3-9

3.2.4.3. Check method 3-9

3.2.4.4. Manual mode 3-9

II

3.2.4.5. Automatic mode 3-10

3.3. Transformer and power supply checking 3-11

3.4. Fan checking 3-12

3.5. Optical system checking 3-12

3.6. Check of the sensitivity with flow cuvette 3-13

4. CHECK TESTS 4-1

4.1. Activation of a test 4-1

4.2. Screen 4-1

4.3. Buzzer 4-1

4.4. Keyboard 4-1

4.5. Printer 4-1

4.6. Serial Port RS-232 4-1

4.7. Motors 4-2

4.7.1. Loss of steps of the peristaltic pump 4-2

4.7.2. Loss of steps of the filters wheel 4-2

4.8. Cuvette temperature 4-2

4.9. Carry Over 4-3

4.10. Photometry 4-3

4.10.1. Filter sensitivity 4-3

4.10.2. Electric noise 4-3

4.10.3. Stability of the readings 4-4

4.10.4. Precision 4-4

4.10.5. Accuracy 4-5

4.11. Unlock / Lock QC techniques 4-5

5. MAINTENANCE 5-1

5.1. Case replacement 5-1

5.2. Main board replacement 5-3

5.3. Change of the display board 5-3

5.4. Printer replacement 5-7

5.5. Keyboard replacement 5-7

5.6. Transformer replacement 5-9

5.7. Cuvette holder tray removal 5-9

5.8. Filters wheel replacement 5-10

5.9. Filters wheel motor replacement 5-10

5.10. Peristaltic pump replacement 5-11

5.11. Peltier cell replacement 5-11

5.12. Change of the photodiode 5-12

5.13. Fan replacement 5-14

5.14. Temperature sensor replacement 5-14

5.15. Lamp replacement 5-15

5.16. Filters replacement 5-17

5.17. Lenses replacement 5-17

5.18. General care 5-18

III

5.19. Cleaning of the optical components 5-20

5.20. Cleaning the filters 5-23

5.21. Cleaning the lenses 5-23

5.22. Cleaning the photodiode 5-23

5.23. Cleaning the aspiration system 5-23

5.24. Cleaning the flow-cuvette 5-24

5.25. General cleaning of the instrument 5-24

APPENDIX I. TECHNICAL SPECIFICATIONS I-1

I.1. Optical system I-1
I.2. Thermostatic control I-1
I.3. Printer I-1
I.4. Display I-1
I.5. Electronics I-2
I.6. Communications I-2
I.7. Installation I-2
I.8. Dimensions and weight I-2
I.9. Cuvette systems I-2
I.10. Flow system I-3
I.11.- Environmental conditions I-3

APPENDIX II. CALIBRATION TOLERANCES TABLES II-1

II.1. Photometric adjustment Tolerances II-1
II.2. Filters Wheel Adjustment II-1
II.3. AC Voltage of the Transformer Secondaries II-1
II.4. Filters sensitivity without flow cuvette II-2
II.5. Electric Noise II-2
II.6. Filters sensitivity with flow cuvette II-2
II.7. Fan Voltage Control II-3
II.8. Zero-Currents for the Sensitivity Test II-3
II.9. Heating Adjustment II-3
II.10. Peristaltic Pump Adjustment II-3

APPENDIX III. DEFAULT PARAMETERS III-1

APPENDIX IV. MAINTENANCE PLAN IV-1

IV.1. Cleaning

IV-1
IV.2.Change IV-1
IV.3. Review IV-1
IV.4.Check IV-1

IV

APPENDIX V. SPARE PARTS AND ACCESSORIES V-1

V.1.Accessories V-1
V.2.Authorized Spare Parts V-2

APPENDIX VI. PASSWORD VI-1

APENDIX VII. SOFTWARE VERSIONS VII-1

APENDIX VIII. COMPATIBILITY TABLE TO UPDATE SOFTWARE VIII-1

VERSIONS

ESQUEMAS

Shematic distribution E33000A
Monocard (1-4) E33001A
Monocard (2-4) E33001A
Monocard (3-4) E33001A
Monocard (4-4) E33001A
Display / Printer (1-2) E33003A
Display / Printer (2-2) E33003A
Keyboard-330 E33005A
Photodetector E33009A

MODIFICATIONS

1 Modification

- This is to make compatible in U7 the integrated circuits MAX691

(Maxim) or ADM691 (Analog Devices). 1-1

- Schemes 1-2

2 Modification

- This is to all devices to be only one monocard 2-1

- Schemes 2-2

3 Modification

- The problem is the transform of inverter circuit fails 3-1

- Photos 3-2

- Schemes 3-5

V

4 Modification

- Modification for recode the components to improve their description 4-1

5 Modification

- Modification for change the display circuit board version 5-1

-Schemes 5-2

VI

1. INTRODUCTION

This instrument instrument, due to its compact design and the reduced number
of components, is a simple, easy-to-maintain one. The computer design has
allowed to study the tolerance of all the optical block’s components, making
unnecessary any mechanical adjustment. Most of its parts are aluminum or
plastic-injected and so a maximum simplicity with reduced maintenance
requirements has been achieved.

Electronic adjustments are also avoided because of the modern, high­integrated circuits used. A powerful software allows the adjustment of most of
the parameters used. Corrections are made via software, using reference tool s
or calibrators.

All the electronic parts are located in two printed circuit boards (PCB’s), and
the software has a lot of check programs to make easy the search and
diagnosis of fail u res.

This manual is not only a guide for maintenance but a document for Technical
Assistance Service staff’s training. The running principles as well as the
electronic circuits are explained in order to get a global vi ew of the instrument.

Note: Throughout this manual, the Absorbance Unit is abbreviated as "Abs"
instead of "A", in order to avoid confusion with the electric current unit, the
Ampere, represented as "A".

1-1

1-2

2. FUNCTIONAL PARTS DESCRIPTION

This instrument is composed of the following functional parts:
a) An optical system to carry out the readings.
b) An aspiration system based on a peristaltic pump, to introduce the sample

in the flow cuvette.

c) A thermostatic system to carry out reactions into the cuvettes at a constant

temperature, when needed.

d) A communications interface, allowing the connection to a host computer by

means of a proper software.
e) A microcontroller-based system, wich controls most of funtional parts.
The next sections describe each one of the functional parts.

2.1. Optical system

2.1.1. Constitutive parts

Figure 2.1. shows the optical system. It consists in:
a) An halogen lamp (1)

b) A diaphragm (2)
c) A first plano-convex lens (3)
d) An interference filter (4)
e) A second plano-convex lens (5)
f) A cuvette (6)
g) A silicon photodiode (7)

2.1.2. System description

The first component is the light source: one halogen lamp (1). The di aphragm
(2) delimits a light solid angl e that is collected by the first lens (3). It sends a
parallel beam to an interference filter (4). Thi s is one of the nine possibl e ones
located in the wheel. One stepper motor drives this wheel and, controlled by
the program, place the proper filter in the opti cal axis, in front of the l i ght beam.
Each filter monochromes one wavelength with the characteristics described in
section I.1.

A second lens (5) focuses the monochromatic beam in the cuvette center (6). In
case of flow cuvettes, the light path hole itself behaves as a diaphragm,
decreasing the sensitivity, that it is lower than in the common cuvettes. The

2-1

light beam goes through the cuvette as far as a light sensor (7), that converts
the light beam into electric current.

2.1.3. Physical description

The optical system (figure 2.2) consists in an injected-aluminum holder (1)
where there are mounted the following components:

- A heat sink shape aluminium block (2) containing a lamp-holder (3). That
block includes a diaphragm (figure 2.3 (1)) that delimits the a light solid angle.

- A lens holder (4) that contains the first lens.

- A filters wheel (5), with capacity for 9 filters. This fil ters must be mounted in
special holders (6). Thi s wheel is moved through a belt (8) by a stepper motor
(7). A photointerrupter (9) allows the detection of a stem (10) that indicates the
filters wheel home position.

- A lens holder (11) that contains the second lens.

- A light protector (12), fitted together the second lens holder, avoids the
parasite light in terference.

- A cuvette-holder (13) holds both the cuvette and the photodiode detector. The
thermostatic system is fitted together (see section 2.3.).

2.1.4. Signal conditioning

(See figure 2.4) The photodiode (1) gives an electric current (If) directly
proportional to the received light. Thi s current goes to the input I
100 logarithmic amplifier (2). A reference current I
introduced by input I

I

. The output voltage of LOG-100 is:

2

f

(3) (nominal 100 nA) is

r

of the LOG-

1

V0 = k x log ----------­ I

r

In this case, k=1 and V0 = 1 volt/Abs.

2-2

2-3

2-4

2-5

2-6

V

is digitali zed by the double-ramp converter TSC500A (4) with a resolution of

0

10,000 counts per volt. The conversion time depends on the absorbance value
and increases together with it; for instance, 2 Abs time is 0.15 sec. This
converter is controlled by the microcontroller (5).

2.1.5. Adjustment

The global system (optical system, amplifier and converter) is not stri ctly l inear
due to the components tolerance, so that it is necessary to do some
adjustments to compensate the inherent deviations of the system.

The photometric adjustment is performed in ranges; between 0 and 2 Abs.
Several value ranges are determined and different correction factors are
applied depending on the range. As deviation also depends on the wavelength,
it is necessary to adjust with different filters. Adjustment process is described in
section 3.2.

2.1.6. Precautions and maintenance

Maintenance of the optical system should be carried out according to the
instructions given in section 5.20.

2.2. Aspiration system

2.2.1. Components

The aspiration system is composed of the following parts (figure 2.5):
a) Sipping tubing (1)

b) Flow cuvette (2)
c) Peristaltic pump (3)
d) Waste bottle (4)

2.2.2. System description

The sample is sipped by the sipping tubing (1). This tubing, Teflon made, has a
standard length and the system is adjusted in accordance with it. The sipped
sample enters into the flow cuvette (2), where readings take place. Sipping is
performed through a silicone tube by means of a peristaltic pump (3), made up
of a four-rollered rotor that is contolled by a stepper motor. Finally, the sample
ends into the waste bottle (4).

2-7

2.2.3. Physical description

The sipping tubing (figures 2.6 and 2.7) is placed on the cuvette-holder tray.
The silicone tube (1), that crosses the case through a steel guiding tube (2), is
fixed to this guiding tube by means of a silicone connector (3) and to the
cuvette (4) by means of an inlet adapter (5). The teflon tube (6) is connected to
the cuvette by an outlet adapter (7), placed in the peristaltic pump (8) and
finally connected to the waste outlet (9).

The waste bottle (1) (figure 2.8) is connected to the outlet (2) in the back of the
instrument by means of a silicone tube (3).

2.2.4. Programmable parameters

Three parameters control the sipper functioning and they must be programmed
to obtain the expected performance characteristics. Their programming is
explained in the user’s manual.

a) SAMPLE VOLUME. It is a number that indicates the volume of sample to be
sipped, in

P

l.

b) PUMP DELAY. It is the number of seconds the pump will wait since the
sipping finishes till the pump is activated again to position the sample into the
flow cuvette.

c) POSITIONING. It is the number of steps that the peristaltic pump’s motor
gives to set the sample into the flow cuvette, ensuring that it is suitably
positioned in order to be read.

2.2.5. Programming

The aspiration system’s programming must be performed in order to indicate
the instrument the volume, in microlitres, to be sipped.

2.2.6. Adjustment

The pump’s nominal flow is 110 PL/revolution, nevertheless, the accuracy of
this figure depends on the tolerances in length and diameter of the sipping
tube, that may be affected by the aging of the tube itself; this is why this value
should be adjusted from time to time. The adjustment process is explained in
section 3.5. As it can be also done by the user, it i s also explai ned in the user's
manual.

2-8

2.2.7. Precautions and maintenance

The general rules for the aspiration system’s maintenance are the same as the
ones given in section 5.24.

Weekly adjustment of the aspiration system is strongly recommended.

2.3. Thermostatization system

2.3.1. Components

Figure 2.9 shows an schema of the thermostatization system, that is composed
by the following parts:

a) A cuvette-holder (1)
b) A temperature sensor (2)
c) An amplifier of the temperature sensor (3)
d) The microcontrollers’ A/D converter (4).
e) A Peltier Cell’s in-circuit power driver(5).
f) A Peltier Cell (6)
g) A heat sink block (optical system holder) (7).

2.3.2. System description

The cuvette with the reaction mixture to be thermostatized is placed in its
holder (1). Thermal contact is settled between the cuvette and the holder.

The cuvette-holder is isolated from the optical system holder (7) and contacts
one of the Peltier Cell’s faces (6). The other cell ’s face is in contact with the
optical system holder.

The Peltier Cell pumps heat from one face to the other, depending on the
current sense. The power control circuit (5) is in charge of making that current
circulate in the adequate sense inside the Cell, in order to heat or cool
depending on the microcontroller’s instructions.

When heating, heat is pumped from the environment (taken from the optical
system holder) to the cuvette-holder and when cooling the opposite is done.
The optical system holder has a heat sink block to cool the heat coming from
the cuvette-holder. A temperature sensor (2) gives a small voltage, directly
proportional to the cuvette-holder’s temperature, that is conditioned by one
amplifier (3) and read by the microcontroller’s A/D converter (4). The
microcontroller’s thermostatization program, depending on the programmed
temperature and the read value, activates the power control (5) heating or
cooling as required.

2-9

2-10

2-11

Fig 2.8

2-12

2-13

2.3.3. Physical description

The cuvette-holder (13) (figure 2.2) is fixed to the optical system holder (1) by
means of four screws (14) thermally isolated. The Peltier Cell (15) is located
between the cuvette-holder (13) and the optical system holder (1). The
temperature sensor, located in a plastic holder (16) is fixed to the cuvette­holder by a thread.

2.3.4. Programming

Programming the thermostatization system consists in indicating the i nstrument
which temperature is desired for the reaction mixture. This value may
introduced in several program’s points.

2.3.5. Adjustment

Components’ tolerance produce a deviation between the programmed
temperature and the real one, so it is necessary an adjustment procedure to
compensate such deviations. The thermostatization adjustment procedure is
described in section 3.3.

2.3.6. Precautions and maintenance

The thermostatization system has two critical poi nts: the good thermal contact
between both faces of the Peltier Cell with the optical system holder and the
cuvette-holder, and the good thermal contact between the temperature sensor
and the cuvette-holder.

To ensure proper conditions in both points, the following cautions should be
taken into account:

a) A thin, uniform layer of silicone must be placed in each face of the Peltier
Cell, covering the whole surface.

b) The silicone layer should neither go beyond the surface of the Cell, nor go
inside between its two faces; it would result in a thermal short-crossing that
would reduce the system’s performance.

c) The temperature sensor should have a silicone layer in order to make a
good thermal contact with the bottom of its lodging.

For assembling these components, follow the procedures described in secti ons

5.11. (changing the Peltier Cell) and 5.14. (changing the temperature sensor).

2-14

2.4. Communications system

This instrument is equipped with a communication channel all owing connection
with computers. The operation of this communication channel depends on the
program release and the computer’s applicati on software.

2.4.1. Channel type

The communication channel is a RS-232, and uses the following connection
lines:

RxD: Receiver Data
TxD: Transmitter Data
GND: Ground (0 volts)

The connector is located in the instrument’s back side, rounded by a box
labeled COM 1. The signals’ electric level meets the E.I.A. RS-232 standard.
Figure 2.10 describes the connector’s signals.

2.4.2. Channel characteristics

The serial channel is pre-programmed by the manufacturer with next default
parameters:

Baud rate: 9600
Timeout:: 0
Terminal number: 0

2.4.3. Information transmitted

The communications procedure uses a ‘sending & waiting’ protocol . Every time
the transmitter sends a message, it waits for an ‘ok’ from the receiver. If the ‘ok’
message is not received, after a programmable timeout period the message is
sended again. The CRC-16 error-detection code is used to check the received
messages.

The communications procedure works using a

MASTER-SLAV

E method, so it
is possible to communicate several instruments at one time. To achieve this,
every instrument has to be configured with a different identification terminal
number. This parameter is configured in the CONFIGURATION /
COMMUNICATIONS menu.

Communications are always started by the
SLAVE
when it is required by the

(the instrument) is in a ‘hearing’ mode, and only sends information

MASTER

.

2-15

MASTER

(the computer). The

The instrument can send the information below:

- Quality control val ues

- Concentration values
In order to get the communication between the computer and the i nstrument, a

proper computer-running software is needed. This program save the data
received from the instrument into the computer hard disk. These data are
stored in an

EXCEL

compatible format.

The file format used is showed in figure 2.11.

2.4.4. Programming

In order to get a suitable communication with a computer through the serial
channel, it is necessary to fit the configuration parameters with the computer
ones. If the computer parameters are not known, refer to its manual. Usually
they are likewise programmable, in most cases from the application programs
themselves.

The instrument can be programmed with the next parameters: transmission
speed (bauds), timeout and terminal number. In the user’s manual is explained
how to program these parameters.

2-16

- Quality control values:
Test’s name

Control 1 or Control 2
Control’s name
Control’s lot
Concentration Date Alarm
Concentration 1 Date 1 Alarm 1
Concentration 2 Date 2 Alarm 2
Concentration n Date n Alarm n

Concentration values:
Test’s

name

Sample’s
number or
patient’s
code

Blank Factor Control-1,

Control-2
or sample
id.

2-17

Concentration Units Date

Fig.2.11

2.5. Block diagram

The block diagram (figure 2.12) is intended to give a general overview of the
the electronic circuit’s different parts.

2.5.1. Logarithmic amplifier

It converts the electric current coming from the photodiode in a voltage equal to
its logarithm (section 2.1.4.).

2.5.2. Analog to digital converter

It digitalizes the voltage coming from the logarithmic amplifier for its further
treatment by the microcontroller.

2.5.3. Lamp control

It supplies the regulated 12V that the lamp needs to work properly.

2.5.4. Filters wheel detector

It is a photointerrupter to detect the filters wheel home position. Allows the
microcontroller to know the location of each filter thus being able to position
them in front of the light beam.

2.5.5. Filters wheel motor control

This is a circuit that, by means of the microcontroller’s logic control, supplies
power to the stepper motor that moves the filters wheel.

2.5.6. Temperature sensor amplifier

This is a circuit to make the signal conditioning for the temperature sensor’s
voltage level in order to be measured by the microcontroll er’s 10-bits analog to
digital converter, with a resolution enough to adjust the system.

2.5.7. Peltier cell control

This is a microcontroller-controlled power circuit that supplies the Peltier cell
the adequate current to heat or cool the cell-holder.

2-18

2.5.8. Pump motor control

This is a circuit that, by means of the microcontroller’s logic control, supplies
the needed power to the stepper motor that moves the peristaltic pump.

2.5.9. Keyboard circuit

This circuit basi cally consists in the keyboard itself and some protection diodes.

2.5.10. Printer control

It consists in the power circuits that allow the orders coming from the
microcontroller to act on the printer, either moving the motors or heating the
thermal head points.

2.5.11. Display circuit

This circuit consists in the display itself, the control and supply lines, one
inverter circuit for the CFL backlight and one circuit to control the LCD contrast
adjustment.

2.5.12. RS-232 channel circuit

It is formed by the circuits needed to make the signal conditioning from TTL
voltage level to the RS-232 standard. The ACIA is integrated in the
microcontroller itself.

2.5.13. Fan control

This is a circuit that measures the temperature in the power supply’s heat sink
and changes the fan’s speed according to the cooling necessi ty. A more silent
working conditions are thus achieved.

2.5.14. Microcontroller

By means of its program, it is in charge of li nking and control ling almost all the
instrument systems. Only the fan’s circuit and the lamp’s one are not controlled
by the microcontroller.

2-19

2.5.15. Power supply

It is in charge of supplying the needed voltages to the different instrument
parts.

2.5.16. Reset and battery-backup supervisory circuit

When the instrument is turned on, this circuit is in charge of keeping the
microcontroller and other chips (like memories) in a ‘reset state’ until the suppl y
voltage reaches the proper working level, avoidi ng undesired effects when the
instrument is powered on, as well as protecting the RAM against erroneous
writings during the on / off voltage transitions.

Also, this circuit provi des a batery backup switchover every ti me the i nstrument
is switched ON/OFF

2.6. Electronic circuit description

This section describes the different electronic parts, following the functional
structure given in section 2.5.

2.6.1. Logarithmic amplifier

(See schema E33001A, sheet 1)
The logarithmic amplifier is formed by the hybrid circuit LOG-100 (U16). Its

supply voltage is

r

15 V. C90, C91, C96 and C97 are bypass capacitors that
store electrical charge that is released to the power line whenever a transient
voltage spike occurs. The photodiode is connected between ground and the
input I1 (1/U16). A reference current (section 2.1.4.) is generated i n the 2.5 V
voltage regulator TL431CD (U15) and the T-circuit formed by R45, R51 and
R52. Its nominal value is 100 nA. The LOG-100 has the output OUT (7/U16)
connected to the pin K1 (3/U16), thus being the logarithmic conversion
constant (K) equal to 1. The capacitor C92 is for circuit stability.

2-20