LG CP79A Chassis Schematic Analysis and Component Breakdown

lg cp79a chassis circuit diagram

For immediate troubleshooting, focus on the power supply module near the rear panel–capacitors C805 (470µF/35V) and C806 (1000µF/25V) are the most frequent failure points. Replace them with low-ESR equivalents rated for 105°C if bulging is detected. Check the standby transformer T801 pins 1-4 for continuity; a reading above 2Ω indicates an open winding. Verify the gate drive IC (IC801, STR-W6754) by measuring pin 5 voltage–anything below 13V confirms a faulty internal regulator.

Signal processing issues often trace to video decoder QV01 (LG-branded TDA15043HN). Probe pins 6-8 for 1.8V; absent voltage points to a dry solder joint or failed I²C bus pull-ups (R122/R123, 10kΩ). For backlight failures, inspect the inverter board connectors J901/J902–corroded contacts are common. Replace the entire inverter strip if CCFL waveforms are distorted (use an oscilloscope with a 10x probe for accurate readings).

When replacing the EEPROM (IC1102, 24C32), ensure firmware version matches the TV model sticker–mismatched data causes erratic behavior. Desolder carefully at 350°C with a hot air station to avoid pads lifting. For intermittent shutdowns, test the thermal cutoff (RT1, 10°C thermistor) on the backlight driver–values below 5kΩ indicate thermal stress. Always discharge the main capacitor (C808, 450V/270µF) before probing to avoid lethal voltages.

Use a multimeter in diode mode to check the bridge rectifier (D801–D804, 1N4007 equivalents). Forward voltage drops should be near 0.6V–higher readings signal a blown diode. For input voltage issues, verify the fuse F801 (5A/250V) and varistor RV801 (14D471K)–both must show continuity. If the TV powers on but lacks display, measure the T-CON board LVDS signals at J101. Pin voltages should match: 12V (pins 1-2), 5V (pins 3-4), and 1.8V (pins 5-6). Deviations often stem from a faulty DC-DC converter (IC602, AOZ1016).

LG Reference Board 79A: Step-by-Step Repair & Troubleshooting

Locate the power supply section near the rear right corner – components labeled IC801 (STR-W6756) and D807 (1N4007) control primary regulation and rectification. Probe TP801 with a multimeter set to DC 200V: readings below 145V indicate a faulty bridge rectifier (BD801) or swollen capacitors (C811, C812 – 220μF/450V). Replace with exact equivalents to prevent voltage spikes damaging the main processor. For horizontal deflection failures, trace Q402 (2SC5088) on the scan sub-board – a shorted emitter-base junction (below 0.5V) confirms replacement need. Always discharge CRT anode via a 10kΩ resistor before handling flyback transformer leads.

Signal paths through the video amplifier IC301 (TDA6111) often degrade; measure pin 6 for 3.2V DC reference – deviations suggest cold solder joints at R318/R319 (470Ω). For intermittent shutdowns, focus on the standby circuit: check R830 (4.7Ω/2W) for overheating and optocoupler PC801 (TLP621) for forward voltage below 1V. Use an ESR meter to verify secondary capacitors (C822, C823 – 1000μF/25V) – replace if ESR exceeds 2Ω. Always reconnect degaussing coil securely after repairs to avoid color purity issues.

Key Components and Signal Path in LG’s CP79A Reference Board

Begin troubleshooting by isolating the power supply stages. Primary regulation centers around IC801 (STR-W6856), handling PFC and standby voltages. Verify DC outputs at C820 (390V), C836 (18V auxiliary), and C840 (5V). Abnormalities here often trace back to faulty bridge rectifiers (D801-D804) or compromised smoothing caps. Use a differential probe to measure ripple–anything above 100mV peak-to-peak suggests capacitive degradation requiring immediate replacement.

Stage Key IC/Parts Expected Signal/Output Diagnostic Voltage (Measured @ Pin)
Primary Regulation IC801 (STR-W6856) 390V DC, 18V aux, 5V standby 390V (pin 3), 18V (pin 2), 5V (pin 1)
Inverter Driver IC701 (OZ9938GN), Q701-Q704 High-voltage AC (~700Vp-p) 12V (pin 1), PWM ~3.3V (pin 6)
Video Processing IC3001 (MT8227H) LVDS 6-bit signal 1.8V (core), 3.3V (I/O)

Signal flow splits post-tuner (TDTK-G242TH): RF input routes to IF demodulator IC101 (TDA18273HN) outputting differential baseband. This feeds the MPEG decoder (IC3001) via I²C bus (SCL/SDA on pins 18/19). Audio extraction runs parallel–analog pairs L/R exit IC3001 pins 117/118, while HDMI audio streams through IC1201 (ANX7150) and forwards SPDIF to IC3001. Check I²C integrity with a logic analyzer: missing clock pulses or stuck SDA lines point to pull-up resistor failures (R1301/R1302, 10kΩ). LCD backlight feedback loop monitors current via R713 (shunt resistor); over-voltage scenarios typically fry Q701-Q704 before triggering IC701’s OCP–inspect for burns on R711/12 (flameproof resistors) and traces for lifted pads.

Step-by-Step Trace of Power Supply Signal Routes

Locate the AC input terminal on the main board–marked CN801–and verify continuity to the EMI filter network (components L801, C803, C804). A multimeter in resistance mode should read near 0 ohms between the plug prongs and the filter output; deviations above 2 ohms indicate a faulty line choke or burned trace. Next, probe the rectifier bridge (D801-D804) for approximately 325V DC across its output pads when powered; absence suggests an open diode or failed PFC coil (L802).

Follow the high-voltage rail into the PFC controller (IC801 pin 7) and check for 16V at its VCC pin (pin 8) supplied via R805 (22kΩ, 1W). If voltage is missing, inspect Q804 (2SC2412K) and ZD803 (6.2V zener) for shorted junctions–replace immediately if leakage exceeds 0.1mA under reverse bias. Trace the boost diode (D806) output to the bulk capacitor (C812, 220µF/450V); expect 390V DC here. Ripple above 200mV p-p signals dried electrolyte or faulty gate driver (Q801, 2SK2645).

Secondary Regulation Checkpoints

Isolate the SMPS transformer (T801) primary winding resistance–0.8Ω typical for LG T928 models. Cross-reference the secondary taps against the service schematic: +12V (yellow), +5V (red), and +3.3V (orange) must align with D811-D813cathodes. Measure each rail at no-load: +12V should stabilize at 12.2V, +5V at 5.05V, and +3.3V at 3.33V; deviations exceeding ±2% require recalibrating the TL431 shunt regulator (IC853) via RV851 trimmer. Failing rails typically stem from cracked solder joints on the MOSFET heatsink (Q851-Q852) or degraded optocoupler (PC801, PC123).

Confirm the standby power path: AC passes through RN801 (10Ω fusible) into a standby transformer (T802), yielding 8V AC rectified by D815. This feeds the linear regulator (IC851, 78L05) whose output powers the MCU (IC901). Check for 5V at TP901–if absent, scope the waveform at T802 primary; expect a clean 50Hz sine with . Corrupted signals often trace back to a desoldered R810 (4.7kΩ) or leaky C824 (47µF/16V). For intermittent failures, reflow all through-hole joints on the standby board using 63/37 SnPb solder at 320°C for 3 seconds max to prevent pad lift.

Identifying Common Failure Points Using the Schematic

Trace the power delivery lines from the main transformer to the secondary regulators. Weak areas typically include:

  • Capacitors C805 (470µF/25V) and C806 (1000µF/16V) near the standby PSU–bulging or leaking electrolyte disrupts voltage stability.
  • Resistor R827 (2.2Ω/2W) in the horizontal deflection feed–overheating discolors the board, indicating imminent open-circuit failure.
  • Diode D815 (1N4007) handling flyback pulses–check for shorts under load; a dead diode causes horizontal collapse.

Inspect the vertical deflection stage by locating IC401 (TDA8172). Signal integrity hinges on:

  • R401 (1.5kΩ) feeding the ramp generator–drift beyond 1.6kΩ distorts geometry.
  • C403 (100µF/35V) smoothing the ramp waveform–ESR exceeding 2Ω introduces vertical jitter.
  • Pin 5 (output) current–excessive draw (>1.2A) points to failed IC or yoke leakage.

Check the tuner assembly U101 (TUA6037). Critical failure signatures include:

  1. No IF signal at TP101–verify L103 (15µH) and R101 (22Ω) continuity; open L103 kills reception.
  2. Agc line (pin 4)–voltage below 1.5V suggests Tuner failure; measure across R112 (4.7kΩ).
  3. 33V tuning supply at D104 (ZD33)–missing rail locks all channels.

Probe the audio path starting at IC201 (TDA7266). Key diagnosis steps:

  • Mute pin (pin 7)–should stay above 3.8V; lower voltages trigger unwanted cut-off.
  • Output capacitors C211/C212 (1000µF/16V)–high ESR (>0.8Ω) reduces bass output.
  • R205/R206 (10Ω) series resistors–discoloration signals power stage overload.

Video Signal Chain Weak Links

Focus on the jungle IC U801 (LA76810). Measure these nodes:

  • RGB cut-off pins (27-29)–voltages below 1.2V darken the display.
  • Abl supply (pin 22)–missing 9V dims OSD and external inputs.
  • Crystal X801 (4.43MHz)–failure locks color subcarrier, desaturating hues.

Quick Isolation Test Sequence

  1. Disconnect the yoke–if linearity improves, check C405 (4.7µF/50V) for leakage.
  2. Bridge R827 with a 1W resistor–if raster returns, replace the original.
  3. Inject a 1Vpp test signal at TP102–no sync indicates U101 or IF filter failure.
  4. Monitor TP201 for 5Vpp sine wave–absent signal confirms IC201 output stage loss.

Locating and Testing Critical Voltage Measurements

Start by identifying the power supply board’s output connectors. Use a multimeter set to DC voltage mode (20V range) to probe the solder-side terminals of the main capacitor–typically marked C805–where the primary +B voltage (135V–145V) should stabilize. If readings drop below 120V, suspect a failing switching transistor (Q801) or compromised feedback loop from the optocoupler (IC802).

Check the secondary rails by tracing the diode rectifiers:

  • +24V rail (feeds audio amp IC401): Test at D804 cathode; expect ±1V tolerance.
  • +12V rail (supplies tuner and small-signal ICs): Measure at C812; deviations below 11.3V indicate faulty LDO (IC803).
  • +5V standby: Probe C815–must hold 4.8V–5.2V even in standby mode; instability here confirms corrupt standby regulator (IC801).

For horizontal deflection stages, verify the H-out transistor (Q501) collector voltage matches schematic specs (usually 110V–130V). Below 90V suggests a shorted flyback winding or defective yoke assembly. Isolate by disconnecting the yoke plug; if voltage recovers, replace the yoke.

Low-voltage areas require micro-probing:

  1. Signal processor IC (IC201) – Pin 8 (VCC) must read 8.0V±0.2V; lower indicates failing linear regulator.
  2. Vertical IC (IC301) – Check pin 2 for 24V; absent voltage points to open fuse resistor R302.
  3. EEPROM (IC901) – Pin 8 (5V) tolerates no less than 4.75V; ripple exceeding 50mV damages stored data.

Hot-ground measurements demand isolation transformer use. Clip the black lead to chassis ground, then probe these hot-side nodes:

  • Switching transistor drain (Q801) – Should pulse between 280V–320V during startup.
  • Snubber network (C806, R805) – AC ripple above 2Vpp confirms failed snubber.
  • PFC coil (L801) – Between 380V–400V DC; deviations indicate PFC IC (IC804) failure.

Signal-path anomalies often stem from incorrect bias lines. For video output stages:

  • RGB drivers (Q701–Q703) – Each emitter should sit at 6.2V; drops suggest open coupling capacitors (C704–C706).
  • Backlight inverter – High-side transistor (Q601) drain must alternate between 500V–700V; absence confirms inverter IC (IC601) lockout.

Log voltage trends during startup. Note transient spikes: a 30% overshoot on the +B rail within the first 200ms flags a weak soft-start circuit (D802, R806). For intermittent faults, freeze the board at 4 °C; temperature-sensitive shorts (e.g., tantalum capacitors) manifest as 10%–30% voltage sags when cooled.