TTI QL II Series, Service Manual

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IC37 is a 12-bit ADC; ADREF is adjusted for voltage and current measurements and is
approximately 4V. IC27B has a gain of –2; therefore if, for example, VM was –2V then the output
of IC27B would be +4V. Offsets are trimmed by ADOFST. To achieve 13-bit resolution two
measurements are taken with and without the 13
th
bit subtracted from the input selected by IC24C.
Any trip condition is latched by IC29 and IC30 and cleared by the microcontroller by pressing
escape. IC18 is used for synchronising the switching of the outputs on triple units.
Auxiliary Output
Each transformer has a 5Vrms winding and these are connected in series at the Auxiliary Power
pcb. Regulator IC5 provides 5V. IC1 is a step down switching regulator (buck). Microcontroller
IC11 sets the output voltage with a PWM signal via low pass filter IC4B. IC4D is the voltage error
amplifier and IC4A signals if the loop comes out of regulation. Using an internal ADC IC11
measures the output voltage and current. The opto-couplers provide isolated communication with
the processor on Output 1.
Interface Pcb
The interface board allows the unit to communicate externally via USB, RS232, LAN or GPIB. IC2
is an ARM7 processor which is programmed in-cicuit using the standard JTAG port accessed at
PJ5. IC3 is the Ethernet transceiver. IC1 is the RS232 transceiver it has a charge pump to
generate –5V. IC31, IC32 and IC33 form the GPIB interface. IC33 is a GPIB controller IC and
performs all the GPIB hardware handshaking and status reporting. IC31 and IC32 are GPIB
specific buffers.
Servicing Note
If IC31 the memory IC located on the power pcb is replaced it is very likely the check sums do not
match resulting in “error 999” being displayed. Pressing OK will get out of this condition and
initialise the EEPROM. This will only work on a single power supply or on output 2 on a triple power
supply. If IC31 has to be replaced on output 1 side of a triple power supply, output 1 and output 2
power boards will have to be interchanged, also links will have to be changed (see table under
Power Pcb desciption) so that the power supply can initialise.