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Here we have a Plasma TV set from Pioneer, model # PDP-434PU.
The problem is when trying to turn it on it comes on for a few seconds but then turns
off and the red LED on front of the screen flashes 4 times.
This indicates a fault on the Y assembly as we can see in the service manual.
The manual also shows which are are possible causes for this fault and what to check first.
This includes the SCAN-5V voltage and the MOSFET transistors Q2401/02 and the driver ICs IC2304/09.
Here on the bloc diagram we see that the DCDC converter which creates the SCAN-5V also creates
VF, the power supply for the driver ICs.
If the 5V don't have the correct voltage level a protection circuit is activated and the TV set.powers down
The IC2304 only gets its power supply when the MOSFETs are conducting,
the current flow to charge the bootstrap capacitor (near IC2304) is shown in the diagram.
First thing we are going to measure is the 5V at the output of the converter.
Initially it's OK, then some noise appears and then the TV powers down.
We look at it with a faster time scale. So first it's 5V then it drops to 3V.
The feedback circuit of the converter is a very common one, they use a TL431 shunt regulator
with a resistor divider. Once the voltage at the control input of the shunt regulator
is higher than 2.5V the optocoupler conducts and the voltage is limited via control of the primary switch (not shown here)
So this is rectifier for the 5V, the shunt regulator, the output capacitor for the 5V and the optocoupler for the feedback
We supply the board with 16V, shown in the lower right corner.
We will measure VF which is generated by the same converter, it's about 15V.
IC2304 is only supplied from VF when the MOSFETs are conducting.
Here we short the MOSFETs, but the voltage that appears is much lower than 15V.
We check VF on the Low-side, it also drops when shorting the MOSFETs.
Finally the 5V, initially ok, also drop to 3V when shorting the drain-source of the MOSFETs.
Apparently the IC2304 draws too much current and pulls down all voltages coming out from the converter.
It's a TND301 in a SOIC-8 package, not easily available.
We replace it with a IXDI604SIA that has similar characteristics.
We finally check all the voltages again while shorting the MOSFETs. Everything works.
Material cost, only $3. Thanks for watching. Subscribe for more repair videos!