akira HTS-797AS, Service Manual

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Model No.: HTS-797AS
Version: 1.0
TYPICAL PROBLEMS AND PROBLEM SHOOTINGS
No Power Supply
Phenomenon:
The power supply indicator does not go on and on display on VFD, the unit can not detect input
signals and not output in each channel.
Analysis:
As the unit is controlled and coordinated by CPU, either the power supply indicator, VFD, the signal
detection process or signal output are all controlled by CPU. In other words, this kind of problems not
only refers to power supply section, but also include malfunction on CPU or on sub-circuit of CPU.
Problem Shooting:
Turn on the power supply switch and first check the +5V operating voltage is normal or not, as the
+5V voltage is the core power supply for the whole circuit. If +5V voltage fails to maintain, several
parts including CPU, display drive IC can not operate normally and the corresponding phenomenon is
NO POWER SUPPLY. Check U203 (7085) pin (3) to see if the +5V voltage is stable (refer to Figure
8-4), if +5V voltage is stable then basically the problem is not caused by the power supply and CPU
together with its sub-circuit should be checked. If the +5V voltage can not be detected, pin (1) of
U203 should be checked. There should be an about 10V voltage is the pin is normally working,
otherwise it is probably the safety resistor R206 (2R2/0.5W) is blowout or burndown of the
transformer. If pin (1) of U203 is good and pin (3) no voltage, IC 7085 itself may be damaged or
short-circuit occurred in back output. The method to ascertain is that use a multimeter measure the
resistance of pin (3), the normal value can not under 500ohms.
If there is not problem with the power supply section, then check the CPU. Three aspects should be
stressed to judge whether the CPU is good or not. First, whether the working frequency of CPU U901
(89C52A) pin (18), (19) is 12M Hz (refer to Figure 8-2); Second, whether the +5V voltage is stable
from pin (40); Third, whether the initial reset voltage of pin (9) is added. If there are no problems
from three issues mentioned above, most probably the program or CPU is stoke off. Try to replace
with a new CPU.