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KDL-70R550A Power Supply Circuit Diagram and Repair Guide

kdl 70r550a power supply schematic diagram

To diagnose failures in the KV-70XBR550U’s primary switching regulator, first isolate the PSU board (part #1-857-956-11). Verify continuity across D604 (1N4007) and Q602 (2SC4429) before proceeding–these components frequently fail under overvoltage conditions. Replace IC601 (STR-W6856) if input voltage exceeds 390V DC or if standby mode draws >200mA.

Check the feedback loop at PC601 (TLP621) with an oscilloscope; corrupted signals (<1.2V peak-to-peak) indicate degraded optocoupler performance. Inspect R607 (1W 0.22Ω resistor) for burn marks–this is a common point of failure during short circuits. For secondary rails, confirm stable outputs: +14V (measured at C624), +5V (at C629), and +24V (at C633).

If resoldering is required, prioritize T601 (primary transformer) pins 1-4 and the output diodes D612-D615. Use a 60W iron with ≤0.2mm tip for precision–excessive heat degrades ferrite cores. Replace electrolytic capacitors ≥10 years old as a preventive measure, even if ESR tests pass. For intermittent shutdowns, monitor ZD601 (12V Zener diode); leakage here triggers overvoltage protection prematurely.

When replacing IC601, apply thermal paste sparingly to the heatsink and secure the mounting screw with Loctite 222 to prevent vibration-induced microfractures. Reassemble only after validating all rails with a dummy load–unloaded PSUs may exhibit false stability. Store removed screws in labeled compartments to avoid shorts during reassembly.

Technical Blueprint of the Sony Bravia LV700 Series Electronic Board

kdl 70r550a power supply schematic diagram

Locate the primary switching regulator (IC801) on the main PCB–marked STR-W6754 or W6756 for this chassis variant. Verify its pinout configuration before troubleshooting:

  • Pin 1 (Drain): Connected to the high-voltage DC bus (~300V) via D1504 and R802 (10Ω fusible). Measure voltage here with a DMM; fluctuations exceeding ±5% indicate a faulty regulation loop.
  • Pin 2 (Source): Ground reference for the MOSFET. Ensure low impedance (
  • Pin 3 (VCC): Startup supply (18V) from auxiliary winding (T801). Test for ripple >200mV–replace C813 (10µF/50V) if present.
  • Pin 4 (FB): Feedback input from secondary side optocoupler (PC801). Check for 2.5V nominal; deviations suggest compensation network failure (R815, C818).
  • Pin 5 (OVP/SS): Overvoltage protection and soft-start. If voltage exceeds 1V during startup, inspect R804 (47kΩ) and C810 (1µF).

For secondary rectification, focus on two key rails:

  1. +12V Rail:
    • Schottky diode (D812, SB360) handles ~3A continuous current. Replace if forward voltage drop >0.6V @ 1A.
    • Post-rectification filtering (C819: 2200µF/16V) must hold >90% rated capacitance. ESR above 80mΩ triggers voltage sag under load.
    • Regulator IC (IC803, 7812-type) clamps at 12.2V ±2%. Thermal shutdown occurs at +150°C; verify heatsink compound integrity.
  2. +5VSB Rail:
    • Standby rail uses a dedicated flyback circuit (T802, D807). Output must remain stable at 5.1V ±3% even with main converter disabled.
    • Optocoupler (PC802, EL817) isolates feedback. Drift beyond ±100mV from nominal suggests degraded CTR (>50% drop).
    • Zener diode (DZ802, 6.2V) clamps feedback voltage. Replace if leakage current exceeds 1µA.

Critical fault isolation steps:

  • Use a bench supply to inject 13V @ 100mA into the VCC pin (IC801) with main AC input disconnected. If no switching pulses are observed on an oscilloscope (gate waveform 5-10kHz), suspect:
    • Drive transformer (T801) primary open-circuit–test continuity from pin 5 to ground.
    • Gate resistor (R805, 10Ω) failed open. Verify waveform slew rate (10-50V/µs).
    • Controller IC (IC801) dead–swap with STR-W6755 if available.
  • For intermittent shutdowns, probe the OVP pin during fault events. Voltage spikes >1V on this pin indicate:
    • Degraded snubber network (R808: 100Ω, C804: 1nF). Replace with exact values.
    • Secondary-side capacitor leakage (C820: 470µF/25V). Check for bulging or >30% capacitance loss.
  • If standby rail fails to initialize, bypass the standby converter temporarily by connecting a 5V/1A external supply to the main +5V rail. If the TV powers on normally, the issue lies in the standby circuit–focus on:
    • Start-up resistor (R809, 120kΩ/0.5W) for the standby controller (IC802). Measure resistance; replace if >125kΩ.
    • PFC coil (L801) saturation. Core should not exceed +60°C under load.

Identifying the Mainboard Interface and Terminal Arrangement for the Sony Bravia 70-Inch Panel

Begin by examining the central harness connector labeled CN201 on the reverse side of the control module. This 14-pin interface carries primary voltages and signal lines–pin 1 supplies 5V_STBY, while pins 2-4 deliver PFC_OUT (approximately 390V). Pin 7 serves as the common ground reference. Use a multimeter set to DC voltage mode to verify continuity between these terminals and adjacent components like the Q201 MOSFET and D201 bridge rectifier, ensuring readings align with expected values (±5%).

Secondary connectors CN301 (6-pin) and CN501 (4-pin) handle backlight and LED driver signals respectively. For CN301, pin 3 outputs the VLED voltage (typically 24V), regulated by the IC301 PWM controller–probe this terminal against pin 6 (ground) to confirm stability. The CN501 terminal distributes BL_ON/OFF and DIM control lines; mismatched voltages here often indicate a faulty Q501 transistor or corroded traces near the R502 resistor network.

Tracing High-Risk Signal Paths

Prioritize inspection of CN101 (20-pin), linking the AC input to the PSU. Pin 1 couples to the AC_L line, with pin 20 grounded–any deviation above 0.5V AC here suggests a compromised C101 X-capacitor. Pins 14-16 route the PFC_OK signal to the mainboard; a missing 3.3V pulse indicates a failed IC101 (OB2269) or open R102. Always disconnect the panel from mains before probing these terminals to prevent arcing.

For advanced troubleshooting, refer to the silkscreened designations near CN801 (inverter feedback). Pin 1 connects to the VS winding of the transformer (T801), while pin 2 oversees the IS current sense line–readings outside 0.9-1.2V DC imply either transformer saturation or a shorted D803 diode. Cross-reference measured values against the BOM: R801 (0.1Ω) and C805 (10μF/50V) are frequent failure points.

Key Components in the KDL TV Circuit Board Layout

Start by locating the primary switching transistor–typically a high-voltage MOSFET like the STP10NK70Z or IRFP460. This element handles the initial voltage conversion from AC to regulated DC and is usually mounted near the large input capacitor. Check for thermal paste degradation or dry joints, as overheating here frequently causes intermittent failures.

The PWM controller IC, often an OB2269 or similar variant, sits adjacent to the switching transistor. Verify its supply pins (VCC typically around 15V) and ensure the feedback loop–via optocouplers like the PC817–remains intact. A failed optocoupler disrupts voltage regulation, leading to erratic standby modes or no output.

Examine the bulk capacitors–typically 470μF/450V models–positioned near the AC input rectifier. Electrolytic swelling or venting indicates failure; replace with identical voltage/capacitance ratings and ensure proper polarity. Poor filtering here introduces ripple, degrading downstream components.

Look for the standby transformer, a smaller ferrite core separate from the main switching section. Its secondary windings feed the 5V standby line, critical for remote-control functionality. A shorted winding here often blows the standby fuse, cutting all secondary voltages.

Trace the gate drive resistors–usually 10Ω to 50Ω–connected between the PWM controller and the MOSFET gate. Open resistors here prevent the switching transistor from activating, while a shorted gate-source path causes catastrophic overcurrent. Measure resistance values in-circuit for discrepancies.

The output rectifiers–often Schottky diodes like the SB560–are heat-sinked or mounted on the PCB near the secondary windings. Check for open circuits or excessive reverse leakage, which reduces efficiency and causes voltage drops on the 12V or 24V rails. Replace with identical current/voltage-rated components.

Finally, inspect the snubber circuits–RC networks across transformer windings or MOSFET drains–to suppress voltage spikes. Failed resistors or degraded capacitors here generate ringing, stressing semiconductors. Replace with exact values (e.g., 470Ω + 1nF) to maintain spike suppression.

Reverse-Engineering Voltage Regulation Paths in the Circuit Layout

Begin by isolating the feedback loop components on the PCB. Trace the error amplifier output–typically an SMD op-amp like the LM358 or CAT810–directly to the optocoupler (e.g., PC817). Verify the coupling capacitor between the amplifier and optocoupler; values around 10–100 nF indicate AC feedback stabilization. If absent, expect transient response issues during load steps. Measure DC bias at the optocoupler’s diode side; a voltage drop of ~1V confirms normal operation.

Identify the primary-side PWM controller–common families include the UC384x (e.g., UC3843) or NCP1200 series. Check the VCC pin (often 12–15V) and ensure the bootstrap diode (typically 1N4148) feeds the high-voltage startup resistor (220–470 kΩ, 1W). A missing or high-impedance resistor suggests failed soft-start functionality. Probe the CS (current sense) pin: a voltage below 1V under full load confirms proper OCP (over-current protection) engagement.

Critical Regulation Nodes

Node Expected Voltage Range Tolerance Deviation (+/- %) Failure Mode
Error Amp Output 1.8–2.5V 5% Unstable regulation
Optocoupler Collector 3–6V (primary-side) 8% No feedback isolation
PWM COMP Pin 1.2–2.8V 10% Duty cycle saturation
VREF (Controller) 5.0V (exact) 2% False protection triggers

Examine the secondary-side post-regulation network. Locate the synchronous rectifiers (e.g., AP43xxx or discrete MOSFETs like AOD408) and confirm gate drive resistors (10–47 Ω) are present. Missing resistors cause shoot-through, overheating MOSFETs within 30 seconds of power-on. For non-synchronous designs, check the freewheeling diode (SB560) for reverse recovery time compliance; a slow diode (trr > 50 ns) increases switching losses.

Cross-reference IC datasheets for hidden pin functions. Example: The NCP1200’s “Skip Cycle” pin adjusts light-load efficiency, but if tied low with a 10 kΩ resistor, it forces continuous conduction mode, increasing no-load consumption by 200–300 mW. Another case: The TL431’s reference pin tolerates ±1% deviation; replace precision resistors (0.1% tolerance) if output ripple exceeds 120 mVpp at 5V rail.

Use an ESR meter to validate output capacitors. Target values:

  • Low-ESR electrolytic: 10–50 mΩ (100–470 µF, 16V)
  • Polymer:
  • Film:

Capacitors with ESR > 100 mΩ introduce phase lag in the feedback loop, causing audible coil whine (>15 kHz) or failed startup under 50% load. Replace suspect caps with units rated for 105°C and ripple current >3A.

For transient load testing, inject a 1A/µs slew rate into the 5V rail. Monitor the PWM COMP pin with a scope (10 MHz bandwidth); a 70% duty cycle should stabilize within 200 µs. If recovery exceeds 1ms, increase the error amplifier’s compensation capacitance (e.g., from 1 nF to 4.7 nF) or add a type-III compensator using a 22pF ceramic capacitor to ground. Verify line regulation by varying input from 100VAC to 240VAC; output drift >±3% indicates a weak optocoupler or degraded primary-side controller.