I see. I do have a good scope, I will look at those signals. It also looks
like you could get a good idea of those voltages by looking at the RGB
sample and hold capacitors on pins 19, 21, and 23.

When a gun is weak, does that mean that it can't drive the current for that
color effectively any more? I am still trying to understand the excessive
voltage at the "ABL IN VCCOMP" pin. (Pin 26). I get about 9 volts when the
picture is off, and about 8 volts when it is on.

The way I understand the circuit, the voltage at pin 26 will be about 11V if
no current is going through the secondary of the flyback transformer. As
current flows to the CRT anode, it is drawn from this node and the voltage
is reduced. This is why I keep asking about tube current. Here are the pin
summary for pin 26, and a piece of descriptive narrative about the ABL
function. Sorry to keep on about this, but to me understanding it is even
more satisfying than fixing it.

ABL control signal input and VSAW high voltage fluctuation compensation
signal input. High voltage compensation has linear control characteristics
for the pin voltage range of about 3V to 1V. ABL does not operate when the
pin voltage is 9 [V], and operates with increasing strength as the voltage
becomes lower than 9 [V].

The voltage applied to Pin 26 (ABLIN) is compared with the internal
reference voltage, integrated by the capacitor which is connected to Pin 27,
and performs picture control and brightness control. In order to adjust the
white balance (black balance), this IC has a drive control function which
adjusts the gain between the RGB outputs and a cut-off control function
which adjusts the DC level between the RGB outputs.



Thanks again for your help. Your expertise is greatly appreciated.



Dave Miller






wrote in message
oups.com...
Not exactly. Sony uses an AKB (auto-kine-bias) which during the
vertical blanking interval sends out a red line, green line, and a blue
line. The AKB circuit senses the current and as long as all three are
close and within specifications for the circuit, the video is unblanked
when the tube is warmed up. By blanking the video during the warm up
time it prevents the 'stripping' of the cathodes that happens in vacuum
tubes when full cathode voltage is applied before the tubes heat up all
the way. This circuit also fine adjusts the tube bias for each color
to keep the color temperature close to correct over the life (useful
life) of the tube.

The problem you have is either one of two:
1. One of the guns is significantly weaker than the other two, making
the AKB circuit not see the tube within specifications.
2. All three guns are weak causing the blanking.

If you have access to a good o'scope, you can sync the scope off the
vertical blanking pulse and monitor the AKB line with the other
channel. With the tv totally cold, turn it on and watch the three akb
return pulses come up. You will likely see one of the guns come up
much slower than the other two.

BTW the reason the video blinks is that with no video on screen, the
cathode current makes it just inside the window to unblank the video.
As soon as the video comes on, the sudden drop in cathode current
during the sense scan line drops below specification and the video
again blanks out. This is why a slight increase of G2 voltage will
compensate for this for a time as it tends to average out the current
draw between blanked video and un-blanked video, but it also washes out
the picture some.