LG OLED Power Board EAY65689424: Tracing the Startup Path — From the Relay to the SSC3S900

When repairing LG OLED TVs you keep running into power boards that look “half-working” at first glance: standby is present, individual voltages can be measured, but the set never fully starts up. That was exactly the behaviour of an LG power supply board EAY65689424.

The first noticeable symptom was the relay. On working boards you usually hear a clear click when switching on. On this board, the relay did not switch cleanly, or did not switch at all. At the same time, the expected operating voltages were missing at the output connectors.

The fault-finding ultimately did not lead to the relay itself, but deep into the primary-side startup circuit of the power supply.

The original fault picture

The following values were measured at the connector to the mainboard:

• only about 7 V on a nominal 12 V rail
• 0 V on the 22 V rail
• the main relay did not switch cleanly
• the high-power main supply did not start

Important: a reduced voltage on a rail labelled “12 V” does not automatically mean a fault on modern TV power supplies. Some supply rails run differently in standby and only reach full voltage after the mainboard issues the power-on command.

So the real problem was not the measured 7 V voltage itself, but the question: why does the main supply not enter the operating state at all?

Why a relay can click with different loudness

An interesting repair indicator is actually the sound of a relay.

A 12 V relay needs a certain coil current to pull in the magnetic armature firmly. Simplified: I = U / R.

If the voltage across the coil drops, the current drops too. The magnetic force drops faster than the current alone.

That is why a relay might still react at, say, 8 or 9 V, but only pull in softly or quietly. This can have several causes:

• an aged small electrolytic capacitor in the auxiliary supply
• a sagging supply voltage
• a transistor that no longer switches through cleanly
• an optocoupler with reduced CTR
• a resistor that has drifted high in value
• poor solder joints
• or the relay itself

On this board, however, the relay ultimately turned out to be only a symptom.

Optocouplers as a possible age-related fault

Suspicion briefly fell on the fitted optocouplers of the EL101X-G series, specifically an EL1018.

An optocoupler internally consists of two galvanically separated parts: an infrared LED on the input side and a phototransistor on the output side.

When current flows through the LED, it produces light. This light hits the phototransistor and makes it conduct. The two sides remain galvanically isolated from each other.

A typical aging effect is that the internal LED produces less light over time. This lowers the so-called CTR – Current Transfer Ratio: CTR = (I_C / I_F) × 100 %.

An optocoupler can therefore look completely normal in a simple diode test and still fail to deliver enough output current. This is an important point for repair practice: passing the diode test does not automatically mean the optocoupler still reaches its original CTR.

For a clean check, the optocoupler should be desoldered and tested at a defined LED current. One possible test setup is, for example: +5 V through 1.6 kΩ to the anode of the LED inside the optocoupler, cathode to GND. This gives roughly 2 to 2.5 mA through the LED. The collector current of the phototransistor is then measured on the output side.

In this particular fault case, however, it turned out that the optocoupler was not the main cause.

The decisive controller: SSC3S900

Further investigation led to the SSC3S900, an LLC current-resonant off-line switching controller. This IC is responsible for starting the resonant main converter stage.

Three pins are particularly interesting here:

• VSEN – mains voltage / brown-in detection
• VCC – supply for the controller
• ST – input for the internal startup circuit

The basic sequence, simplified, looks like this: mains voltage / high-voltage bus → ST pin → internal startup current source → VCC → VCC capacitor → controller starts → LLC converter.

The controller needs a sufficiently high VCC voltage before it can begin operating.

The decisive measurement: VCC = 0 V

At the SSC3S900 it was found that: VCC against GND stayed at 0 V during the power-on attempt. That was a very important reading.

First, with the board unpowered, it was checked whether there was a short between VCC and GND. Briefly, the multimeter showed a very low resistance. After a few seconds, the reading rose noticeably.

This is typical for a capacitor: when the meter probes are applied, the capacitor is discharged and briefly looks low-resistance. As it charges up via the multimeter's test voltage, the displayed resistance rises. A permanent short on VCC could therefore be ruled out.

That left the decisive question: why is the VCC capacitor not being charged?

VSEN initially under suspicion

About 1.27 V was measured at the VSEN pin. Since this value is close to the start threshold, the first suspicion was that mains voltage detection was not enabling the controller.

The VSEN resistor chain was therefore examined more closely. The upper side consisted, among others, of two high-value resistors of about:

• 4.7 MΩ
• 4.7 MΩ

On the lower side towards GND were found:

• 110 kΩ
• 4.7 kΩ

This gives a classic high-value voltage divider. The total lower resistance is: 110 kΩ + 4.7 kΩ = 114.7 kΩ.

About 105.7 V was measured at the top of the divider. Plugging these values into the voltage-divider formula: U_VSEN = 105.7 V × (114.7 kΩ / (9.4 MΩ + 114.7 kΩ)) ≈ 1.27 V.

That is exactly the value that was actually measured. So it was clear: the VSEN divider works, on paper, exactly as the actually measured resistors would predict. The fault therefore probably did not lie in this area.

The trail leads to the ST pin

That put the startup path at the ST pin at the centre of attention. This pin feeds the controller's internal startup circuit.

If this path is interrupted, the following happens: high voltage present → interruption → ST pin → no startup current → VCC = 0 V → SSC3S900 does not start → LLC stage stays off.

And this is ultimately where the fault was.

The actual defect: cracked “152” resistors

Under magnification, several extremely small SMD resistors were visibly damaged. All of them were marked: 152.

The SMD code means: 15 × 10² = 1500 Ω, i.e. 1.5 kΩ. The resistors were in the 0402 package size.

To the naked eye they still looked fairly unremarkable at first. Only under magnification were cracks, or split packages, visible.

This also explained the previously partly implausible megaohm-range resistance readings: the actual low-resistance startup path was interrupted, so the multimeter could only measure through side paths of the circuit.

With that, the root cause was found.

Why several resistors in series?

In the high-voltage area, several resistors are often placed in series. This has several advantages:

• better distribution of the voltage stress
• lower stress per component
• better pulse/surge withstand
• lower risk of electromigration
• lower local power dissipation

That is why such a resistor chain should not simply be replaced with a single resistor of the same total value. The physical layout is part of the electrical design.

Original value or slightly higher?

Originally, several 1.5 kΩ resistors were fitted. With three resistors this gives: 3 × 1.5 kΩ = 4.5 kΩ.

For the SSC3S900, a value above this is recommended for the external startup resistor. This raised the idea of using, for example, 3 × 2.2 kΩ instead of the original 1.5 kΩ. This gives: 3 × 2.2 kΩ = 6.6 kΩ.

That is closer to the recommended range and at the same time reduces the current stress on each individual resistor.

However: anyone aiming for a most-original repair should use the original 1.5 kΩ value again. Changing to 2.2 kΩ is already a deliberate design change and should only be made once the exact circuit and function of these resistors has been understood beyond doubt.

Suitable replacement parts

Since the original resistors are only 0402 in size, an arbitrary standard resistor should not be used. Particularly interesting are:

• increased power handling
• good pulse/surge withstand
• high-quality thick-film construction

As a robust option for a deliberate switch to 2.2 kΩ, a high-power 0402 from the Vishay CRCW-HP series, for example, is a good fit.

More important than a specific manufacturer, though, is: correct package size, correct resistance value, sufficient power rating, pulse/surge withstand, and, ideally, good temperature stability.

What makes this fault particularly interesting

This defect nicely shows why repairing switch-mode power supplies is often not solved by a single obvious reading.

The original symptom was: relay does not switch properly. The first suspicion could therefore be: relay defective.

In fact, the fault chain ran through: relay does not switch → main supply does not start → controller gets no VCC → VCC is not being charged → investigate startup path → ST feed interrupted → 0402 resistors cracked.

The relay was therefore merely the visible, or audible, end of a cause lying much deeper.

Conclusion

On the LG EAY65689424, fault-finding led from a missing 12 V / 22 V main supply, via the relay and the optocouplers, ultimately to the primary-side startup circuit.

The key takeaways were:

• a quiet or missing relay click is often just a symptom
• optocouplers can age, but should be judged by their CTR, not just by a diode test
• a briefly low resistance on VCC can simply come from the VCC capacitor
• VSEN can be verified very well against the actually measured divider resistor values
• if VCC stays at 0 V despite no short being present, the startup path must be investigated
• extremely small SMD resistors can be mechanically cracked while still looking almost normal from the outside
• a good magnifier, or a microscope, is practically part of the standard kit for modern switch-mode power supply repair

In this case, the actual root cause turned out to be surprisingly unspectacular: several cracked 0402 SMD resistors in the startup path prevented the SSC3S900 from building up its supply. A small component fault with a big effect.

Safety notice

The measurements described here are partly taken on the primary side of a mains-powered switch-mode power supply. Several hundred volts can remain on capacitors there even after disconnecting from the mains. Measurements on the primary side should only be carried out with suitable test equipment and appropriate experience. Primary GND and secondary/mainboard GND must not be mixed up.