Philips 32PF9968/10 Service Manual page 33

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 12.7 V
(measured on decoupling capacitors 2U0W and 2U0Y and
the enable signals are "low" (active) then the output
voltages should have their normal values. +12 Vand +5 V-
POD supplies start-up first (enabled by PODMODE signal
from the standby microprocessor). There is a
supplementary condition for +12 V to start-up: if +5 V-POD
does not start up due to a local defect, then +12 V will not
be available as well. +5 V-ON supply is enabled by the
ONMODE signal (coming also from the standby
microprocessor). +1.2 V supply starts-up when +12 V
appears, then at least 100 ms later, +1.8 V, +2.5 V and
+3.3 V will be activated via the ENABLE-3V3 signal from
the standby microprocessor. If +12 V 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 +12 V is less
than 8 V then also +1.2 V will not be available. The third
DC-DC convertor that delivers +1.4 V out of +12 V is
started up when the ENABLE-1V2 becomes active (low)
and +12 V is present. The +V
the analogue version of China platforms) will generate
+33 V for the analogue tuner as soon as the 12 V/3.3 V
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 1.8 V
from +3.3 V with its output overloaded, the +3.3 V stays
usually at its normal value even though the consumption
from +3.3 V increases significantly.
• The +1.8 V and +2.5 V supply voltages are obtained via
linear stabilizer made with discrete components that can
deliver a lot of current, therefore in case +1.8 V or +2.5 V
are shortcircuited to GND then +3.3 V 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: +1.2 V 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 +1.2 V output voltage short-circuit to GND that
can generate pulsed over-currents 7 to 10 A through coil
5U00.
4. Check the over-current detection circuit (2U00 or 3U17
interrupted).
• Symptom: +1.4 V not present (even for a short while
~10 ms) while +12 V 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 +1.4 V output voltage short-circuit to GND that
can generate pulsed over-currents 7 to 10 A through coil
5U05.
• Symptom: +1.2 V present for about 100 ms, +1.8 V,
+2.5 V and +3.3 V not rising.
All manuals and user guides at all-guidesbox.com
Service Modes, Error Codes, and Fault Finding
generator (present only for
tun
Q522.1E LA
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 (+3.3 V,
+2.5 V and +1.8 V) that can generate pulsed overcurrents
7 to 10 A through coil 5U01,
5. Check the over-current detection circuit (2U04 or 3U14
interrupted).
• Symptom: +1.2 V OK, +2.5 V and +3.3 V present for
about 100 ms. Possible cause: SUPPLY-FAULT line
stays "low" even though the +3.3 V and +1.2 V is available
- the standby 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 +1.2 V or +3.3 V are higher than their normal
values - that can be due to defective DC feedback of the
respective DC-DC convertor (ex. 3U1J, 3U75).
• Symptom: +1.2 V, +1.4 V, +1.8 V, +2.5 V or +3.3 V 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 +1.2 V, 2U8P+2U0Y+3U24 for +1.4 V and
2U03+2U05+3U04).
• Symptom: +1.2 V, +1.4 V, +2.5 V and +3.3 V ok, no +V
(analogue sets only). Possible cause: the "+V
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
+V voltage can cause also +Vtun voltage to be too low or
tun
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.
5.8.4 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.
5. EN 33
tun
generator"
tun