I found the following problem really have me scratching my head:
- TV sometimes would come up like normal, then after a few minutes, lost picture (but with sound). The front panel Red LED would blink on-and-off at 1 Hertz frequency.
- Sometimes TV would just have sound on Power-up but never have picture. The LED just blink on-and-off like (1) above.
- I checked all the voltages (135V, 9V and standby voltages), all OK.
- I put the scope probe on the CRT Heater, and verified that there was the 22V P-P Half-Wave voltage as soon as the power-on button was hit, as indicated on the service manual. However, approximately half-a-second later, it would disappear. The LED continued to blink on-and-off. I did not have time to note the waveform on the IK output from the C-board, because the time that the heater voltage was on was so fast.
- During those times that the TV would start normally, the 22V P-P Heater voltage would stay on, the IK would have a 4V P-P waveform similiar to the RGB voltages. Then about 10 seconds later the picture would come on.
I guess my ultimate weapon would be to bring out the I2C monitor on the I2C Bus and see if the IC 351 (CXA2025) is reporting bad IK. But I want to exhaust other possibilities that would cause the above symptoms first, particularly that it shuts off the heater voltage only half-second after power on.
My question to the experts in the group are:
- When does the system processor sample the IK status? Is half-second after power-up too early - bearing in mind that the picture would only come up at above 10 seconds, under normal condition.
- Could it be that the flyback and the EHT circuit is intermittent, so that there is no IK feedback?
- I guess that any components failure in the IK feedback (that includes the transistor Q1790 on the C-board, the 3.9V Zener at its base, the coupling capacitor to the IK-in of IC351, and IC351 itself) can cause that symptom too. But is it likely?
Any other thoughts? Thanks. AI.