Zenith SA2539P Green Picture

[Kirk S.]

Hi all,

Thought I had this one figured out as an H/K short however, I'm wrong...

a) isolated heater and picture is still green
b) swapped red/green signals to the crt board and still green
c) voltages to the crt board for red/green/blue all similar

Next step is to lift the final resistor to the green cathode and swap with
red or blue. Perhaps look at the cathode voltages with an o-scope? I
didn't find any shorts between the cathodes and heater terminals on the crt
board.

If I don't have an H/K short, what would cause a green picture? I do not
have full on w/retrace lines as I would have expected. Could something have
shifted in the CRT to only have the green phosphors hit? Spark gap bad?

Suggestions?

Thanks!

Kirk S.

 

[Art]

Drive transistors, jungle ic, video outputs, etc. What potentials do you
have at the cathodes, drive and output transistors??

 

[Kirk S.]

Art.

Thanks for the info. The drive and output transistors are on the main board
and I hoped that by swapping some signals on the crt board, it would reveal
the problem. I don't have a signal generator or any way to display a
picture other than snow which I would assume would be black and white. The
OSD is green as is the snow. I have an old CRT tester (B&K 465) however no
adaptor for the tube. I may try to build one out of an old crt board
socket..

My multimeter shows about +20vac at each of the crt cathodes. That seems a
bit low since the Sams PF shows +160vac on them. I'm gonna try my scope on
the signals and see what sort of peak readings I get.

I have a tube/yoke from another set however the crt numbers are totally
different: A63AAX02X vs A63KTM96X. Something tells me that I can't swap 'em
and probably would pull my hair out trying to get a decent picture even if I
could.

I'll go back to the Sams PF and check voltages at the transistors. One
thing I did note is that the red and blue both have higher voltages on the
schematic, does this mean the blue and red are adjustable while the green is
set to a reference? Or am I just nutz?

Kirk S.

 

[Kirk S.]

Ok...

Voltages to the crt board and drive transistors all appear normal according
to the PhotoFacts. In the +125 to +140 range. The +212 volt source is
measuring at about 193 volts. Is this the cause of my problems? I
disconnected the crt board and the screen was totally black.

Kirk S.

 

[Kirk S.]

Is it possible that I lost the green and blue guns? Everything I do gives
me a green screen.

Kirk S.

 

[dkuhajda]

You could have a cathode to G1 short on the green gun. You could have
super low emmisions on the red and blue and turning up the screen voltage
high enough causes the green to go way high.

I am betting on the picture tube being bad, from lots of Zenith experience.
They tend to go green.

Yes, the next step is to swap the drive between the green and red guns
right at the crt socket pins.

Do not forget to remove and check the ~150K ohm resistors on the crt board
as they often open/change values messing up the crt color.

As well you should make sure the RGB drive controls are all set midrange to
be sure the voltages to the tube are all close during testing. You will
have to reset gray scale when you have completed the repair at this point
no matter what happens due to the age of the tube.

David