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EN 38
5.
Q528.2E LB
5.8.3
DC/DC Converter
Introduction
•
The best way to find a failure in the DC-DC converters is to
check their starting-up sequence at "power-on via the
mains cord", presuming that the standby microprocessor is
operational.
•
If the input voltage of DC-DC converters is around 12.7V
(measured on decoupling capacitors 2U0W and 2U0Y and
the enable signals are "low" (active) then the output
voltages should have their normal values. +12Vand +5V-
POD supplies start-up first (enabled by PODMODE signal
from the standby microprocessor). There is a
supplementary condition for +12V to start-up: if +5V-POD
does not start up due to a local defect, then +12V will not
be available as well. +5V-ON supply is enabled by the
ONMODE signal (coming also from the stand-by
microprocessor). +1V2 supply starts-up when +12V
appears, then at least 100 ms later, +1V8, +2V5 and+3V3
will be activated via the ENABLE-3V3 signal from the
standby microprocessor. If +12V value is less than 10 V
then the last enumerated voltages will not show-up due to
the under-voltage detection circuit 7U01-1 + 6U04 and
surrounding components. Furthermore, if +12V is less than
8V then also +1V2 will not be available. The third DC-DC
convertor that delivers +1V4 out of +12V is started up when
the ENABLE-1V2 becomes active (low) and +12V is
present. The +Vtun generator (present only for the
analogue version of China platforms) will generate +33V
for the analogue tuner as soon as the 12V/3.3V DC-DC
converter will start to operate.
•
The consumption of controller IC 7U0A is around 19 mA
(that means almost 200 mV drop voltage across resistor
3U70) and the consumption of controller IC 7U0L is around
12 mA.
•
The current capability of DC-DC converters is quite high
(short-circuit current is 7 to 10 A), therefore if there is a
linear integrated stabiliser that, for example, delivers 1.8V
from +3V3 with its output overloaded, the +3V3 stays
usually at its normal value even though the consumption
from +3V3 increases significantly.
•
The +1V8 and +2V5 supply voltages are obtained via linear
stabilizer made with discrete components that can deliver
a lot of current, therefore in case +1V8 or +2V5 are short-
circuited to GND then +3V3 will not have the normal value
but much less.
•
The SUPPLY-FAULT signal (active low) is an internal
protection (error 9) of the DC-DC convertor and will occur
if the output voltage of any DC-DC convertor is out of limits
(10% of the normal value).
Fault Finding
•
Symptom: +1V2 not present (even for a short while
~10 ms)
1. Check 12 V availability (resistor 3U70, MOS-FETs 7U05
and 7U06), value of +12 V, and surrounding components)
2. Check the voltage on pin 9 (1.5 V),
3. Check for +1V2 output voltage short-circuit to GND that
can generate pulsed over-currents 7...10 A through coil
5U00.
4. Check the over-current detection circuit (2U00 or 3U17
interrupted).
•
Symptom: +1V4 not present (even for a short while
~10ms) while +12V is okay (also across input capacitors
2U8A and 2U8E).
1. Check resistor 3U3T and power MOS-FETs 7U0D-1/2.
2. Check the voltage on pin 4 (4 V).
3. Check enable signal ENABLE-1V2 (active "low").
4. Check for +1V4 output voltage short-circuit to GND that
can generate pulsed over-currents 7...10 A through coil
5U05.
•
Symptom: +1V2 present for about 100ms, +1V8, +2V5
and +3V3 not rising.
Service Modes, Error Codes, and Fault Finding
5.8.4
1. Check the ENABLE-3V3 signal (active "low"),
2. Check the voltage on pin 8 (1.5 V),
3. Check the under-voltage detection circuit (the voltage on
collector of transistor 7U01-1 should be less than
0.8 V),
4. Check for output voltages short-circuits to GND (+3V3,
+2V5 and +1V8) that can generate pulsed overcurrents
7...10 A through coil 5U01,
5. Check the over-current detection circuit (2U04 or 3U14
interrupted).
•
Symptom: +1V2 OK, +2V5 and +3V3 present for about
100 ms. Possible cause: SUPPLY-FAULT line stays "low"
even though the +3V3 and +1V2 is available - the stand-by
microprocessor is detecting that and switching "off" all
supply voltages.
1. Check the drop voltage across resistor 3U70 or 3U3T
(they could be too high, meaning a defective controller IC
or MOS-FETs),
2. Check if the boost voltage on pin 4 of controller IC 7U0A
is less than 14 V (should be 19 V),
3. Check if +1V2 or +3V3 are higher than their normal
values - that can be due to defective DC feedback of the
respective DC-DC convertor (ex. 3U1J, 3U75).
•
Symptom: +1V2, +1V4, +1V8, +2V5 or +3V3 shows a high
level of ripple voltage (audible noise can come from the
filtering coils 5U00, 5U01 or 5U04). Possible cause:
instability of the frequency and/or duty cycle of a DC-DC
converter or stabilizer.
1. Check the resistor 3U0H and 3U2E, capacitors 2U0C
and 2U0A, input and output decoupling capacitors.
2. Check a.c. feedback circuits (2U08+2U09+3U07+3U08
for +1V2, 2U8P+2U0Y+3U24 for +1V4 and
2U03+2U05+3U04).
•
Symptom: +1V2, +1V4, +2V5 and +3V3 ok, no +Vtun
(analogue sets only). Possible cause: the "+VTUN
GENERATOR" circuit (7U0P +7 U0Q + surroundings
components) is defective: check transistor 7U0P (it has to
have gate voltage pulses of about 10 V amplitude and drain
voltage pulses of about 35 V amplitude) and surrounding
components. A high consumption (more than 6 mA) from
+Vtun voltage can cause also +Vtun voltage to be too low
or zero.
Note: when a pair of power MOSFETs (7U02+7U08,
7U05+7U06 or 7U0D-1/2) becomes defective the controller IC
7U0A or 7U0L should be replaced as well.
Exit "Factory Mode"
When an "F" is displayed in the screen's right corner, this
means that the set is in "Factory" mode, and it normally
happens after a new SSB has been mounted.
To exit this mode, push the "VOLUME minus" button on the
TV's keyboard control for 5 seconds and restart the set.
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