A
Appreciate_It
I found the following problem really have me scratching my head:
1. 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.
2. Sometimes TV would just have sound on Power-up but never have
picture. The LED just blink on-and-off like (1) above.
3. I checked all the voltages (135V, 9V and standby voltages), all OK.
4. 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.
5. 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:
1. 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.
2. Could it be that the flyback and the EHT circuit is intermittent,
so that there is no IK feedback?
3. 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.
1. 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.
2. Sometimes TV would just have sound on Power-up but never have
picture. The LED just blink on-and-off like (1) above.
3. I checked all the voltages (135V, 9V and standby voltages), all OK.
4. 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.
5. 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:
1. 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.
2. Could it be that the flyback and the EHT circuit is intermittent,
so that there is no IK feedback?
3. 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.
