Invivo Millennia 3155A, Service Manual

Page: 16 / 76

1-4
J2 pin 6), which manipulates U1B in the same way as for a dead battery. The other part of the
AB69A board carries the transformer and the switching transistor for the flyback for +,-12 volts, and
-5 volts.
1.2.3 Power On/Off System. (Please refer to schematic 185C187, sheet 1.) Note connector P1 in
the top left corner of the schematic. This connector receives rectified and filtered power from the
AS153 AC Power Adapter through the external DC Input of the monitor. The supply receives
RAWPWR all the time, even when the front-panel switch is turned off. The current drain (from
various ICs) is minimized to extend battery life.
The power is turned on and off by indirect means to allow an orderly shutdown by the Millennia
®
3155 processor when a turn-off is requested by the user. Shutdown commands may come from the
following:
!
Loss of keep-alive pulses.
!
Turning OFF of the front-panel switch.
!
Battery discharge.
The above listed items operate as follows:
The keep-alive arrangement allows the supply to run while a stream of pulses is received from the
processor. If this stream ceases, either from a processor problem, or after an orderly shutdown, the
power supply will time-out monostable, and control voltages inside the power supply will command
the power-control ICs to turn off.
The keep-alive circuit performs the following:
a. Respond to a string of pulses to keep the power supply running.
b. Shuts off the power supply and beep if the string stops, except for:
1) Run unconditionally during the initial start-up time, except for when
the front panel power switched is toggled to OFF (then the monitor
shuts down immediately).
2) Does not shut down for any condition if the TEST switch SW1 is ON.
c. In the event of a keep-alive continuing after the front-panel switch is turned OFF,
unconditionally stops the power supply after a short pause.
The keep-alive circuit consists of gate U2D, transistor Q29, monostables U1A and U1B, and their
associated circuitry. The function of the circuit is to keep the power supply running as long as a
series of pulses are received from the processor (these pulses are referred to as the "Keep-alive
pulses").
Start up: When the front-panel switch is turned ON, a pulse is received into VSTY through resistor
R32 and diodes D5 and D12. This pulse discharges C2 into the "A" inputs of both monos, starting
them both. (Because STBY_15V is present, the monos and gate are ready to operate). Diode D47
clamps the pulse to no higher than the mono supply voltage; resistor R4 establishes the quiescent
voltage on capacitor C2 before initial operation.
For the inital operation, while the processor is starting up, there will be no keep-alive pulses. The
mono U1B has a long active interval (about 40 seconds), enough to keep the power supply running
until keep-alive pulses are received. Its /Q output pulls resistor R2 and diode D1 LOW, and keeps
the timing circuit on pin 2 of U1A low, thus keeping U1A in its active state. The outputs of U1A
continue to control both supply switching ICs (U14 and U11) in the power-on condition. This
situation is maintained until U1B times out, at which time, U1A must be receiving keep-alive pulses