Invivo Millennia 3155A, Руководство по обслуживанию

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1-7
Op amp U4B amplifies the difference of the voltage sensed across resistor R22 and the voltage from
the resistors R39, R78, and R79, and controls transistor Q11. When the current rises excessively,
transistor Q11 conducts enough to pull U11 pin 9 LOW through diode D29 to reduce the duty cycle
of the drive into transistor Q6, effectively reducing the output current from the supply. When the
current limit is not effective, diode D29 is not conducting, and the voltage on U11 pin 9 is not
affected.
Snubbers on the rectifier diodes D11 and D12 (both on AB69A) protect them from voltage spikes
arising from the leakage inductance of the transformer. Diode D10 is comfortably enough below its
voltage breakdown rating to not need a snubber; similarily, transistor Q6 is protected from damage
by its avalanche capability.
1.2.6 Clock Generator. (Please refer to 185C187, sheet 2.) The oscillator transistor Q27 operates
from VSTY to accomplish the largest voltage swing. The crystal Y1 controls the oscillator's
frequency and the output is delivered to the frequency dividers U10 and U6. The two divide-by-five
sections of U10 are cascaded to deliver 320KHz to synchronize U14. A divide-by-two section then
produces 160KHz to synchronize U11, and two further divide-by-two sections (part of U10 and half
of U6) deliver 40KHz to the A/D board. An output of 4 MHz is taken from another divide-by-two
(half of U6) for the A/D board.
1.2.7 Audio. (Please refer to 185C187, sheet 1.) Audio signals are provided by the audio amplifier
U3. Outputs go to an external speaker; inputs come from the A/D board and from a local fail-signal
oscillator U2B and U2A. Compensation and bypassing for U3 is provided by capacitors C22, C13,
C11, and resistor R15. The output capacitor C12 blocks the DC quiescent voltage from the speaker.
The fail-signal oscillator is two sections of the gate U2 wired as a free-running oscillator. It is turned
ON and OFF by the voltage into U2B pin 5. When the voltage is HIGH, the oscillator sounds
through the speaker; otherwise, it is OFF. Transistor Q31 inverts the polarity and conditions the
external FAIL_SOUND level from the A/D board (it comes in as a 5-volt logic level, and the U2
supply may be as high as 16 volts).
1.2.8 Power Sense for POWER GOOD. (Please refer to 185C187, sheet 1.) The op amp U4A
provides the nucleus of the power-sense system. It compares the voltages between a divided-down
sample of the five-volt reference from power-control IC U11 and the five- or three-volt lines
(selectable by jumpers). The circuit is a basic voltage comparator, with resistor R16 providing
hysterises, and a threshold circuit for sensing sudden drops exceeding a few hundred millivolts.
Capacitor C18 is charged to about half the sensed voltage by the shunting resistor R22 and resistors
R23 and R30. If a sudden drop comes in that swings the negative terminal of C18 below one diode
drop below the nominal voltage of 2.4 volts, the op amp output will swing negative and signal any
power problems. Capacitor C15 performs a similar operation for the 3-volt supply, if the jumper is
set accordingly. For a slow input drop, resistor R22 will cause the negative terminal of capacitor
C18 to track the dropping input voltage, causing the threshold to be about 200mV down.
1.2.9 Battery/AC Power Sense. A bicolor LED on the front panel is driven by transistors Q2 and
Q3 so that:
!
External DC Power present: LED is GREEN.
!
No External DC Power, but batteries are OK: LED is YELLOW.
!
Batteries getting LOW: LED is RED
When External DC Power is present, resistor R61 and diode D22 will keep the GREEN half of the
LED lit, while transistor Q1's saturation will keep the base voltage to transistor Q3 low enough so
that the RED half of the front-panel LED is OFF. When External DC Power fails, resistor R61
ceases to supply current to the GREEN half, and because transistor Q1 turns OFF (base drive is lost),
transistor
Q3's base will be pulled up by resistor R19, and Q3 supplies current to the RED half of the