Changhong plasma TV PT32600 repair column sharing - appliance repair - Huaqiang electronic network

0.8A 1000V Rectifier Bridge Stack

Filter 18.432M

1. The Changhong PT4206 (B) plasma TV does not light up after the second boot and then enters protection mode. After disassembly, it was found that the screen is a Samsung V3 plasma panel. To identify the fault location, the first step was to check the plasma screen. It showed no light, which should be a uniform white grid during normal self-test. This suggests the plasma screen assembly is faulty. During the self-test, LEDs LED8001, LED8002, and LED8003 were all on. However, there was no voltage measured for VS, VSCAN, VSET, and VE. The indicator light on the logic board flickered on and off. The logic board was operational, and the VS-ON signal from the logic board was normal. The power board’s output voltage should have been normal, but when measuring VA, it was over 90V at start-up and then dropped to 0V. Normally, VA should be around 70V. This indicated poor regulation in the VA circuit. Upon closer inspection of the circuit, it was found that the VA voltage regulator consists of a diaphragm IC8025, a three-terminal precision voltage regulator (IC8029) TL431, and surrounding resistors R8129, R8133, R8134, R8135, R8138, R8142, and capacitors C8074, C8079, C8070. The resistor R8134 had a voltage sent from the VA section, but the photodiode's positive terminal showed -0.24V, which should normally be positive. This was abnormal. Checking the resistor R8129 connected to the diaphragm showed no voltage at both ends, indicating an issue with the F/B-VCC forming circuit. After checking, R8129 was connected to VSB (+5V), which was rectified through D8014 and D8015. The power indicator was on, confirming VSB had voltage. VSB was measured at 5.2V, which seemed good. The negative end of D8015 was not shorted to ground. Further testing revealed D8015 was open. Replacing it with a new IN4148 resolved the issue. 2. In another case, the Changhong PT4206 (B) plasma TV did not light up after the second boot and entered protection mode. Disassembling confirmed it used a Samsung V3 plasma panel. The screen showed a dim purple grid during self-test, which should have been a uniform white grid. This suggested the plasma screen was faulty. The logic board and AV board were not involved. It was suspected that the power board or maintenance board was defective. During the self-test, LEDs LED8001, LED8002, and LED8003 were on, and LED2000 on the logic board was blinking. VS and VSC voltages were normal, but VE was 0V, which was abnormal. Testing showed VE was shorted to ground. Removing the CN8002 socket confirmed the load on the board was not shorted, but the power board had a short. Components like D8044 and C8071 were checked, and after disconnecting C8071, the short was eliminated, indicating D8044 was damaged. Replacing it with a new RU3C fixed the problem. 3. Another instance involved the Changhong PT4206 (B) [Samsung V3] plasma TV where the indicator light turned yellow after the second boot, the machine appeared dull, and then entered protection mode. Upon disassembly, LED8003 was lit. After the second boot, LED8002 and LED8001 also lit, and the relay made a sound. However, the screen remained dark, and after some time, LED8004 turned red, triggering shutdown. To assist in troubleshooting, the self-check function was used, which pointed to a faulty screen component. The power supply timing was as follows: After plugging in the power, the switch circuit composed of T8001 on the power board operated, generating VSB (+5V). This provided working voltage to the CPU on the main board, which then issued a standby command. At this point, the green LED8003 lit. When the power-on command was given, the CPU sent a low-level RELAY signal to the power board, causing relays RLY8001 and RLY8002 to engage. This activated L8001 and the PFC circuit, boosting the voltage to about 400V. At the same time, LEDs LED8002 and LED8001 lit sequentially. The switching circuit composed of T8005, IC8023, and peripheral components generated VA (+70V), VCC (+15V), D3V3 (3.3V), and D5VL (+5V). These voltages were sent to the logic board, allowing it to operate. At this point, LED2000 would flash, and the logic board would send a high-level VS-ON signal to the power board. This triggered the VS voltage generation circuit, producing an adjustable VS voltage of 160V–185V. This voltage formed VSE via T8003 and VSC via T8004. Finally, the VE voltage was generated through T8006 and delivered to the screen components, lighting the plasma screen. If any of these voltages were abnormal or caused overcurrent, the CPU would detect it and trigger a protection mode, lighting LED8004. After the second boot, the relay engaged, and LEDs LED8001 and LED8002 lit, indicating the motherboard was functioning and sending a power-on command. However, LED2000 on the logic board was not lit, suggesting the logic board was not receiving proper voltage or was malfunctioning. Testing D3V3 at F2000 was normal, but D5VL at F2001 was measured at 1.34V, which was abnormal. Disconnecting the logic board power connector still showed 1.34V, indicating a problem on the power board. Checking the ground resistance showed no issues. The DC/DC converter IC8026 had 17.5V at pin 1, suggesting the front stage was normal. Adjusting VR8009 showed the voltage dropping from 4.2V to 1.34V during boot, which was abnormal. Checking external resistors R8143, R8145, D8050, and C8099 showed no issues. Replacing IC8026 (LM2576T-ADJ) restored D5VL to normal, and other voltages like VE and VSE also returned to normal. All connections were re-established, and the machine returned to normal operation. (D5VL and D3V3 are both generated using LM2576T-ADJ, with only the external resistors differing.) 4. In another repair, the Changhong PT4206 (H) [Samsung V3 screen] plasma TV had no indicator light, and the screen showed no image. The indicator light was off, suggesting the motherboard wasn't receiving power or the CPU wasn't functioning properly. After removing the rear cover, LED8003 was not lit, indicating a power supply issue or overload. Disconnecting the power to the motherboard still left it unlit, pointing to a problem with the VSB voltage forming circuit. Based on previous experience, the repair became easier. First, the fuse F8001 was checked and found good. Visually, capacitor C8017 was found bulging, which was part of the primary filter in the VSB voltage circuit. F8002 was open, and the rectifier bridge stack D8007 was intact, while the drive block IC8003 showed no abnormalities in resistance. Replacing C8017 (450V/33µF) and F8002 (1A) did not resolve the issue. Replacing IC8003 (TOP223PN) fixed the problem. 5. In another case, the Changhong PT4206 (H) [Samsung V3 screen] plasma TV had an indicator light that did not make a sound when the second boot occurred. The indicator light was on, suggesting the motherboard was powered. The fault could be due to a malfunctioning motherboard. A power-on test showed the RELAY signal changed from high to low, indicating the motherboard CPU was working. To determine if the motherboard's RELAY signal reached the power board, J8005 was directly shorted. If LED8002 and LED8001 lit, it meant the RELAY was generated normally, but the relays RLY8001 and RLY8002 were not engaging. The fault was likely in the circuit composed of Q8009, Q8013, Q8004, Q8006, Q8005, and Q8008. Testing the transistors in the circuit revealed Q8009 was faulty. Removing and testing Q8009 (KTN2907A) showed B/C was completely disconnected. Replacing it with a good transistor allowed LED8002 and LED8001 to light upon power-on, restoring normal operation. 6. In another instance, the Changhong PT4206 (H) plasma TV had no indicator light and the screen was dull. After disassembly, it was found to use a Samsung V3 plasma panel. The screen showed no light during the self-test, which should have been a uniform white grid, indicating a faulty plasma screen assembly. LED8003 was not lit during the self-test, suggesting the switching power supply consisting of T8001 and IC8003 was faulty. F8002 (250V/1A) was found open, and the forward and reverse resistances at C8017 were normal. The forward resistance of the D8007 (S1WBS60) bridge stack was normal, but the AC input pin was shorted to the DC negative terminal, indicating one of the diodes in D8007 was shorted. Replacing D8007 and F8002 restored normal operation. 7. In a final case, the Changhong PT4206 (B) (Samsung V3 screen) plasma TV had a boot light that changed after the second boot, but the screen was not bright. During the first self-test, LEDs LED8003 and LED8002 on the plasma power were on, but LED8001 was not lit. This suggested a problem with the PFC circuit or the circuit composed of Q8005 and Q8008. Testing the PFC boosted voltage at the T-VPFC point showed 0V, which was abnormal (should be 400V). This indicated partial damage to the PFC or lack of power. Testing the three anti-jamming circuits and D8003 on the road revealed R8010 was open. Replacing R8010 did not fix the issue, and both Q8005 and Q8008 were off. There was no voltage at the positive terminal of the internal diode of IC8002, indicating the PFC diaphragm HIC8001 was faulty and no RELAY-ON signal was being output. Replacing HIC8001 resolved the issue. Maintenance Tip: Plasma screens have strict requirements on the order of voltage supply, but for maintenance purposes, we only need to understand what conditions each voltage must meet. For the Samsung V3 screen power supply, monitoring the indicator lights on the power board and logic board can help identify which voltage circuit is faulty. After plugging in the power, LED8003 should light up. If not, there may be an issue with the overall power supply or the VSB circuit. After the power-on command is issued, LED8002 should light. If not, check if the RELAY signal is being sent properly. Then LED8001 should light. If not, check if AC220V is reaching the PFC circuit or if the PFC is operating correctly. When LED8001 is lit, LED2000 on the logic board should flash. If not, D5VL and D3V3 may be faulty. Once LED2000 is lit, all power board voltages should be normal. If not, check the VS, VSET, VSCAN, and VE circuits. To simplify repairs, the power supply includes multiple short-circuitable sockets to simulate or isolate parts of the circuit. For example, J8005, when shorted, simulates the motherboard sending a power-on signal. J8004, when disconnected, disables the protection from TV8001. LJ8003, when disconnected, activates PANEL-POWER protection. J8002, when shorted, simulates the VS-ON signal from the logic board. J8001, when disconnected, isolates the PFC and subsequent circuits, helping to determine if the VSB or PFC part is shorted. Understanding the role of the indicators on the screen assembly is essential: 1. LED8003 → VSB Power Indicator. It should light when AC power is on. If not, the whole machine power supply or VSB circuit is faulty. 2. LED8002 → PS ON Command Status. If lit, the power-on command from the main board is normal. If not, check if the RELAY signal from the main board reaches the power board. 3. LED8001 → PFC Power Supply. If not lit, the PFC power supply is faulty. 4. LED2000 → Logic Board Power Supply. If not lit, the D5V and D3V3 power circuits are faulty. 5. LED8004 → Power Protection Action. It should be off under normal conditions. Any abnormality detected by the HIC8002 detection pin will cause it to light. By analyzing the indicator status and understanding the working principle, faults can be efficiently identified and resolved.