Fault phenomenon: A TCL color TV model 2139A shows character jitter and tailing, but sometimes it works normally. It seems like a tricky issue that needs careful analysis.
Fault analysis: Since the problem is intermittent, it’s likely caused by a loose or de-soldered component. Components related to characters include the character oscillator, line pulse, field pulse, and the CPU.
Repair process: The CPU used in this model is an 87CM38 with 28 pins. Pins 28 and 29 are responsible for character drive, while pins 26 and 27 are connected to the line and field pulses. After re-soldering the CPU and checking the solder joints on Q004 and Q005 (which are associated with the line pulse), the fault still persisted.
Measuring the DC voltages at pins 26, 27, and 28 showed 4V, 4.5V, and 4.5V respectively, which seem normal. However, the pulse signals couldn’t be directly confirmed without an oscilloscope. From the image display, the pulses appeared to be arriving, but the issue remained. Could it be a faulty Q004 or Q005? Replacing them didn’t solve the problem, leaving me puzzled. Was it the CPU?
At this point, I received another 21-inch Changhong TV for repair. I decided to work on that first. After fixing it, I tested it and then connected the TCL TV again. While examining the switch transformer on the TCL’s circuit board, I noticed a slightly burnt capacitor near the field rectifier. Could this be causing unstable field power, leading to abnormal CPU field pulses?
Upon closer inspection, the capacitor C830 was part of the B+ rectification circuit, supplying power to the field section. After resoldering C830, the fault disappeared. To confirm, I tested the same fix on a newly repaired Changhong TV, where the field power came from the switching power supply. When I introduced some interference to the B+ voltage, the characters became clear and stable. This suggested that the original issue was due to a poor connection or arcing on the peak capacitor.
Although the peak capacitor doesn’t directly affect the character signal, its failure—due to a loose solder joint or sparking—can cause instability in the field power supply, indirectly affecting the display.
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