TTI LD300 Service Manual page 23

Metering
The metering is a completely separate function implemented with an 8-channel A to D converter
IC9, a processor IC5 which drives the multiplexed LED displays on the display PCB. The
processor detects the operating mode of the load by reading the state of the control signals
through IC1 and IC2 and scales the display readings appropriately.
There is a soldered link in the 5V power to the processor which is open circuited while the
processor is programmed at the factory. It must be shorted for normal operation. Note that AGND
is about 60mV negative relative to 0V.
To avoid interference from the multiplexed currents the power for the displays is provided by a
separate 5V supply, Vdd and Vss, which is joined to 0V only by R239.
If a fault arises, first check the supply voltages, then that the processor's reset pin is not being
held low, then check the shared oscillator signal and finally that all the analogue signals are
correct and between 0V and FS (if any signal is outside this range, all channels will read
incorrectly). Display faults such as missing segments are probably caused by the drivers on the
display PCB.
Power Stages
The purpose of the local feedback to the gate drivers is to ensure that the heat dissipation is
shared equally between the two power stages. It does not determine the accuracy of the sink
current of the load. Because of the overall current feedback through the comparison amplifier,
even if one of the power stages should fail, the unit may appear to function normally up to at least
half its rated current, until that stage reaches its over-current trip. The cause of this sort of fault is
often in the wiring between the power stage and the main PCB.
If a FET fails, it normally becomes a very low resistance short circuit between all three terminals.
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