TCL color TV 2139A power + B peak capacitor desoldering causes character jitter and tailing - appliance repair

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Fault phenomenon: The TCL color TV model 2139A experiences character jitter and tailing, but it sometimes works normally. It seems like a tricky issue that requires careful analysis.

Fault analysis: Since the problem occurs intermittently, it's likely related to a loose or de-soldered component. Components involved in character display include the character oscillator, line pulse, field pulse, and the CPU. These are all potential points of failure.

Repair process: The CPU used in this model is 87CM38, which has 28 pins. Pins 28 and 29 are responsible for character shocks, while pins 26 and 27 are associated with the line and field pulses. I re-soldered the CPU and checked Q004 and Q005, which are related to the line pulse. After re-soldering, the TV was powered on again, but the fault remained unchanged.

Measuring voltages at pins 26, 27, and 28 showed 4V, 4.5V, and 4.5V respectively—these values seem normal. However, the pulse signals couldn't be directly judged from voltage alone. From the image, the pulses seemed to be arriving, so the issue might not be with the pulses themselves. Could it be a problem with Q004 or Q005? Although their breakdown would cause no characters, replacing them didn’t resolve the issue. This simple fault had no clear clues, leading me to consider the CPU as a possible culprit.

At this point, another user brought in a 21-inch Changhong TV. I decided to work on it first. After repairing the Changhong, I plugged it in and tested it. While examining the TCL’s circuit board, I noticed a slight burn mark near the field rectifier capacitor. It turned out to be C830, which is part of the B+ rectification circuit. The field power supply is derived from the high-voltage package. Re-soldering C830 fixed the issue.

To confirm if this was a common problem, I tested the same capacitor on the newly repaired Changhong TV (which uses a switching power supply for the field). When I introduced some interference to the B+ voltage, the characters became clear, though there was still a very subtle shift. There was no tailing, suggesting that the field power supply voltage was stable enough to prevent major issues.

The peak capacitor serves to protect the rectifier during power-on and power-off, but its presence or absence doesn’t affect the character display. However, a loose connection or arcing could lead to intermittent faults.