TTI EX752M, Service Manual

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Measurement and Display
The measurement and display is controlled by a microcontroller, IC3. IC5, IC2 and associated
components, together with IC3, form a dual slope A-to-D converter. Multiplexer IC4, under the
control of IC3, selects the signals to be measured. The status line SW1 from the output ON/OFF
switch is read by IC3 to determine which signals are measured, as follows:
Status Voltage Measurement Current Measurement
Output Off Preset Volts (VPRT & ILO) Preset Current (IPHI & ILO)
Output On Output Volts (+SENSE & VOLO) Output Current (IOHI & ILO)
IC3 processes the measurement and drives the two multiplexed 3-digit, 7-segment, LED displays
via digit driver IC1 and segment latch IC6. Individual segment current limit is provided by RP1
and RP2. The constant current lamp, LED2, is part of this multiplexing scheme; the output on
lamp, LED1, is driven directly from the +5V auxiliary rail via R2 when the dc output switch SW1-A
is closed.
Mode Control
The three operating modes are selected by a 4-pole 3-way break before make switch, SW2.
Opto-isolators IC14, 15 and 16 on the Main pcb provide isolation of the control signals between
the 2 channels.
Isolated: The two channels operate independently and are isolated.
Parallel:
The slave side is turned off and the control of the master is changed such that it can
supply up to 4 Amps. SW2-C disconnects the voltage setting controls from the
voltage error amplifier IC7-A of the slave, forcing the slave output to 0V. Signal X is
high on IC8-A turning off the slave display. Signal P is low on IC108-B doubling the
master current limit. Whenever the parallel mode is exited the PAR signal goes high
pulsing the MUTE signal low via IC6 on the Main board, turning off both outputs
momentarily. At power-on C52 and Q25 on the Main board also mute the outputs
momentarily.
Series:
The two channels are connected in series and to the master terminals. SW2-B
disconnects the master negative terminal from its own supply and connects it to the
slave negative terminal. SW2-A disconnects the slave positive output from its
terminal and connects it to what was the master negative terminal supply thus
putting the supplies in series. Relay RL1 switches the sense connections. SW2-C
switches the input to the slave error amplifier to the master positive terminal forcing
the slave to track the master. Signal X is high on IC8-C forcing the slave current
limit to a little over 2 Amps so that the master current limit is always in control.
Signal Y is low on IC108-A doubling the preset voltage measurement. Signal X is
high on IC8-A turning off the slave display.
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