TTI LD300 Service Manual page 14

Fault Finding Hints
First check that the problem is not a stability issue caused by the characteristics of a particular
supply source. Note that instability caused by inductance in the load circuit can trip the over-
voltage protection. In other cases instability can trip the excess power detection circuit.
Constant Voltage mode is particularly likely to be affected by the characteristics of the supply
source. Its use in conjunction with electronic power supplies in "constant current" mode can be
very troublesome, as their output impedance varies considerably with frequency. The use of a
battery and a medium value non-inductive resistor is recommended.
Check if the fault is an incorrect value of DMD or PROD, or whether the demand is correct but the
power stages are not correctly implementing it. Fault finding should be conducted first in constant
current mode, from a simple voltage source such as a battery; the other modes cannot work if this
does not.
Note the possibility that only one power FET is conducting current; the unit will appear to operate
normally until the current fault trips at about 50 to 55 Amps.
Power Supply PCB
The power supply PCB generates four voltages (-12, -5, +5 and +12) for the analogue circuits,
and a separate 5V for the display. The two grounds are joined on the main PCB, not at the power
supply.
It also contains the two speed fan control circuit. Normally, base current is provided to Q3 from
R157 on the main PCB, so there is a low resistance between its collector and emitter which
shorts out R150 and R154 and makes the output voltages of IC39 and IC40 about ±8V. When the
power dissipated in the load (as measured by the power limit multiplier) exceeds about 240
Watts, the output of comparator IC58-B on the main PCB pulls R157 down to -5V, turning off Q3;
the output voltages rise to ±12V and the fans run at full speed.
Each supply has a protection fuse before the rectifier. The mains transformer has a thermal fuse
built-in; if this opens the whole transformer must be replaced, as its insulation will have been
damaged.
Display PCB
The displays are driven by a six phase multiplexing scheme – four for the digits in the 7-segment
displays and two for the other annunciator lamps. The display are common cathode; the anodes
are driven high by the CMOS latches IC2 & 3 and the cathodes are pulled low by transistors in
IC1. The 56Ω resistors define the current in each segment.
For each phase the processor first disables the outputs of the anode data latches IC2 & 3 by
setting OC high, successively loads the data into the two latches, then asserts the required digit
line and sets OC low for a measured time (400µs for digits, 800µs for annunciators) to obtain the
brightness required. The six phases repeat at 300 times per second.
Control PCB
The potentiometers on the control PCB generate voltages which are a proportion of the reference
voltage FS, to control the analogue circuits. The Level A, Level B and Dropout Voltage controls
have small negative voltages applied at their counter-clockwise end to ensure that they can be
adjusted down to zero.
The switches generate logic signals used to control transmission gates on the main PCB – there
are no dynamic signals on the control PCB. The coding of each switch is shown below. All logic is
5 volt CMOS; logic high is nearly 5V, logic low is close to 0V. Any logic signal at a mid level
indicates a fault. Signals from the control PCB are pulled high by 22k pull-ups on the main PCB
and pulled low by the switch contact.
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