A few years ago, a neighbor's branded machine was replaced with a motherboard. It worked well for a long time until three days ago, when the trouble started. The first repair involved checking and replacing the output tube, but after reassembling and testing the +B voltage, no issues were found. The user took it back, but the part failed again just a few days later. I suspected the quality of the replacement tube wasn't good. [To be honest, the current parts available aren't great either.] So I decided to replace it with the original 2253 tube and carefully checked the surrounding circuits. I thought this time it would be fixed, but after more than a month, the tube broke again. This time, I had to take the motherboard home for further inspection. When I tested the tube on another machine, it showed no damage. However, the heat sink felt warm when touched, so I continued to disassemble the unit. The machine ran normally at first, so I reinstalled it. But within minutes, the tube failed again. I replaced it once more, and the heat sink became extremely hot—this indicated that the problem was not with the tube itself, but something else.
Could the deflection coil be faulty? I went back home and looked for a matching deflection sleeve for the picture tube. After replacing it, the screen image improved significantly, though not perfect due to a mismatch between the tube and the deflection coil. Upon inspecting the deflection coil, I noticed the internal adhesive was burnt, and the enameled wire had darkened slightly, but not severely. I replaced it with a similar one, adjusted the settings, and the results were satisfactory. Later, I tested by replacing the small power line tube with a 1651 model, and it worked fine, with normal temperature rise. [Of course, I only did this as a test. For long-term stability, I wouldn’t recommend using such a substitute.]
This repair process went through several iterations before the issue was finally resolved. The root cause of the repeated tube failures turned out to be poor insulation in the yoke coil, which eventually led to overheating and failure. It was a tricky issue that required careful diagnosis and multiple attempts to fix properly.
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