A few days ago, I repaired a Konka T21TE358 color TV. The issue was that the indicator light kept flashing, and it was really hard to start up. After checking the boot process, I found out that this was a reworked unit. The power supply had been replaced by the previous technician with a 3-wire module. When the main power was turned on, the indicator light flashed, and pressing the P+ key or using the remote control didn't help. I measured the B+ voltage at 50V, and the memory voltage was fluctuating between 4V. This indicated that the CPU power supply was too low.
I asked the user how long the TV had been in the shop, and they said that it was still difficult to boot after several months of repairs. The previous technician had returned it multiple times without success. During my inspection, I noticed that the absorption circuit on the board had been modified. The previous repairer had some understanding of the module but made a mistake by grounding the E pin of the optocoupler and directly connecting the C pin to the module's control line. However, this model uses a standby step-down module, and the control line shouldn't be directly connected to the optocoupler.
I removed the control line and adjusted the module's potentiometer counterclockwise by 2-3 turns. In standby mode, when the control line wasn't connected, the 3-wire module would rise by 20-30V. At this point, the CPU and memory power supplies were normal, but pressing the P+ key or using the remote still didn’t work. So I decided to try replacing the CPU.
After testing everything, I found an online schematic and discovered that the secondary 25V output from the switching power supply had an overvoltage protection circuit. If the voltage exceeded 28V, the protection would activate, shorting the standby control voltage through V957 and V956, causing the system to shut down.
I first disconnected the protection circuit and tested the TV, which worked perfectly. Then I reconnected the protection circuit and added a 700Ω resistor between the module's control line and the optocoupler’s C pin (restoring the original sampling circuit). After that, the B+ voltage stabilized at 78V, and the 8th pin of the memory was at 5V.
After the second startup, I adjusted the module's potentiometer so that the B+ voltage reached 110V and stopped rising. Then I turned it slightly clockwise to stabilize the voltage. This TV was fixed a week ago, and I hadn’t planned to post about it. But since I saw some recent posts about using modules, I decided to share my experience. I hope this can help others who are facing similar issues with these modules.
For this kind of power supply, I’d rather use a module than try to fix it manually. It saves time and avoids unnecessary complications.
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