Advanced Bilirubin Stat-Analyzer BR2, Service Manual, Page: 17 / 69

17
The Advanced® Model BR2 Service Manual
Circuit Description
Signal-conditioning Circuit: This circuit
receives two analog photocurrent signals via
solder terminals X8 and X11 on the front left
side of the Main board, from photodiodes
Sensor 1 and Sensor 2, the 540 and 454-nm
absorbance detectors, respectively.
These photocurrents are applied to IC5 and
IC6, which are fixed-gain, high-impedance
op-amps that have been adjusted by means of
R35 and R39 (the ZERO SET potentiometers
near the left front corner of the Main board)
for outputs at each pin 6 within 0.1 mV of
zero (measured at the ends of R36 and R40
nearest each IC) with the optical light path
through the cuvette holder completely
blocked.
When a cuvette containing a water and saline
diluent “blank” solution is placed in the
cuvette chamber, the output from pin 6 of IC6
changes to its adjusted value of -25 to -29
mV, and IC5 changes to between -55 mV and
-130 mV (the difference is due to unequal
optical characteristics).
These uneven outputs are balanced (with the
water or saline diluent blank still in the
cuvette chamber) by means of R38 and R42
(the two GAIN potentiometers next to the
large, square IC7 near the left center of the
Main board) for 18 mV ±0.1 mV (represent-
ing equal 18-uA currents) measured at the
ends of R37 (TP 540) and R41 (TP 454) near-
est the two potentiometers.
The direct reference voltage is then set (with
the water or saline diluent blank still in the
cuvette chamber) by pressing the
Direct but-
ton and adjusting R43 (the SET DIRECT
potentiometer at the left front of the Main
board) for 18 mV ±0.1 mV at the end of R41
(TP 454) nearest R42.
Module IC7 is a log ratio converter which
performs the electrical analogy of the mathe-
matical equation,
The electronic circuits in the BR2 are con-
tained on two printed circuit boards, the Main
board and the Display board.
BR2100 Main Board Description
Power Supply Circuit: The power supply is
required to p15 V, - 15 V and +5 V
from either 100-120-V or 200-240-V AC
power. To accomplish this, transformer T1
has two equal primary windings which may
be connected either in series or parallel sim-
ply by positioning the small programming
card in the fuse holder/line cord socket (F1),
as marked on the card. Transformer T1 has
two secondaries, also.
One secondary provides 20-0-20 VAC at pins
8, 9 and 10. These voltages are rectified by
D11, filtered by C1 and C2, and fed to regu-
lators VR1 and VR2, which p15 V
and -15 V, respectively.
The other secondary provides 10-0-10 VAC
at pins 5, 6 and 7. These voltages are rectified
by D1 and D2, filtered by C5 and fed to regu-
lator VR3 which pr5 V.
Lamp Circuit: The function of the lamp cir-
cuit is to tightly control the optical lamp volt-
age. Reference diode D4 supplies divider-
string R2, R3 and R4 with 6.9 V; potentiome-
ter R3 (LAMP ADJ in the right center area of
the Main board) allows the voltage across the
optical lamp to be accurately set at 3.35
±0.01 V (measured between X6 and X7, the
LAMP OUTPUT terminals on the right rear
side of the Main board). IC1 and Q1 form a
current amplifier with the voltage from the
arm of R3 as its reference.
After setting the lamp input voltage, the lamp
light output is optimized, by means of physi-
cal adjustment (see Lamp Adjustment sec-
tion), for an output of -25 to -29 mV from pin
6 of IC6 in the Signal Conditioning Circuit.