Philips 32PF9968/10 Service Manual page 34

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EN 34 5. Q523.1U LA
5.8.4 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 stand-by μP is operational.
• If the input voltage of DC-DC converters is around 12.7V
(measured on decoupling capacitors 2U03/2U93/2U6S
and 2U6R) and the enable signals are low (active) then the
output voltages should have their normal values.+12V and
+5V-POD supplies start-up first (enabled by POD-MODE
signal from the stand-by microprocessor). There is a
supplementary condition for +12V to start-up: if +5V-POD
doesn't start up due to a local defect then +12V will not be
available, as well.+5V-ON supply is enabled by the ON-
MODE signal (coming also from the stand-by
microprocessor) and is coming up a little bit later (20ms)
due to the slower rise time needed to charge the USB
decoupling capacitor 2N31. +1V2 supply starts-up when
+12V appears, then at least 100ms later, +1V8, +2V5 and
+3V3 will be activated via ENABLE-3V3 signal from the
stand-by microprocessor.If +12V value is less than 10V
then the last enumerated voltages will not show-up due to
the under-voltage detection circuit 6U10 + 7U10 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 analog
version of China platforms) will generate +33V for the
analog tuner as soon as the 12V/3.3V DC-DC converter
will start to operate.
• The consumption of controller IC 7U00 is around 19mA
(that means almost 200mV drop voltage across resistor
3U01) and the consumption of controller IC 7U64 is around
12mA.
• The current capability of DC-DC converters is quite high
(short-circuit current is 7 to 10A), therefore if there is a
linear integrated stabilizer 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
~10ms)
1. Check 12V availability (resistor 3U01, MOS-FETs
7U03 and 7U08), value of +12 V, +12 V switch (7U14
+ 7U16 and surrounding components) and +5V-POD.
2. Check the voltage on pin 9 (1.5V),
3. Check for +1V2 output voltage short-circuit to GND that
can generate pulsed over-currents 7...10 A through
coil 5U02.
4. Check the over-current detection circuit (2U20 or 3U40
interrupted).
• Symptom: +1V4 not present (even for a short while
~10ms) while +12V is ok (also across input capacitors
2U6S and 2U6R).
1. Check resistor 3U7B and power MOS-FETs 7U61-1/2
2. Check the voltage on pin 4(4V)
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...10A through coil
5U60
Service Modes, Error Codes, and Fault Finding
5.8.5
5.8.6
5.8.7
5. Check the over-current detection reference (2U65 +
3U7C) and the boot components (2U66 + 6U60).
Symptom: +1V2 present for about 100ms, +1V8, +2V5
•
and +3V3 not rising
1. Check the ENABLE-3V3 signal (active LOW),
2. Check the voltage on pin 8 (1.5V),
3. Check the under-voltage detection circuit (the voltage
on collector of transistor 7U10-1 should be less than
0.8V),
4. Check for output voltages short-circuits to GND (+3V3,
+2V5 and +1V8) that can generate pulsed over-
currents 7...10A through coil 5U01,
5. Check the over-current detection circuit (2U18 or 3U31
interrupted).
• Symptom: +1V2 OK, +2V5 and +3V3 present for about
100ms. Possible cause: SUPPLY-FAULT line stays low
even though the +3V3 and +1V2 is available - the stand-by
μP is detecting that and switching off all supply voltages.
1. Check the drop voltage across resistor 3U01 or 3U7B
(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
7U00 is less than 14V (should be 19V),
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.3U47, 3U77, 3U7L,
3U7J or 3U70).
• Symptom: +1V2, +1V4, +1V8, +2V5 or +3V3 shows a high
level of ripple voltage (audible noise can come from the
filtering coils 5U01, 5U02 or 5U60). Possible cause:
instability of the frequency and/or duty cycle of a dc-dc
converter or stabilizer.
1. Check the resistor 3U32 and 3U7D, capacitors 2U17
and 2U19, input and output decoupling capacitors.
2. Check a.c. feedback circuits
(2U23+2U24+3U55+3U63 for +1V2,
2U6D+2U6E+2U6G+3U6A+3U7E for +1V4 and
2U07+2U08+3U17+3U24 for +3V3), compensation
capacitors 2U25, 2U34, 2U36, 2U37, 2U40, 2U43,
2U68 and 2U6B.
• Symptom: +1V2, +1V4, +2V5 and +3V3 ok, no +Vtun
(analog sets only). Possible cause: the "+VTUN
GENERATOR" circuit (7U24+7U26+surroundings
components) is defective: check transistor 7U24 (it has to
have gate voltage pulses of about 10V amplitude and drain
voltage pulses of about 35V amplitude) and surroundings
components. A high consumption (more than 6mA) from
+Vtun voltage can cause also +Vtun voltage to be too low
or zero.
Note: when a pair of power MOSFETs(7U01+7U06,
7U03+7U08 or 7U61-1/2) becomes defective the controller IC
7U00 or 7U64 should be replaced as well.
Display Option code
Attention: In case you have replaced the SSB, always check
the display option code in SAM, even if you have picture. With
a wrong display option code it is possible that you will have
picture, but in certain conditions you can have unwanted side-
effects.
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.
Sanken display supply
The 47" sets in this chassis come with a Sanken buy-in supply.
The result of a failing Sanken display supply, e.g. one of the