Luminance Issue ?

[jstout66]

I'm stumped....
Damn System 3 sets are too "new" for me, so if anyone worked on em in the day and saw this problem, can you point me in the right direction?
This is my 81 19" with Space Phone.
Last year some filter caps went out on the 9-160 board. Got that fixed and it's been working okay. Yesterday, turned it on, and it had a really dark picture, color looked maxed out as well. Screen would go back to normal intermittently. (picture looked like it was pulsing) To me, it looked like a luminance issue, like when a video tube went out in a tube set.
Would it be the board that plugs into the CRT?
This is one set I don't want to toss....
Zeno? Tom?

[kf4rca]

That's what I'd check. Also check the clip-on connectors from the crt board to the 9-160 module. And, of course, check the solder connections on the crt board.

[zeno]

Also check the flat resistor pkg on the 9-155. When ever
you see one check for PC cons.

Quick check on almost any in line CRT set is:
Measure DCV on any of color output collectors.
This is the CRT cathodes. Normal pix will be in the
120V range. If that goes way up when the pix darkens you have
a video problem. If it dont its almost always G-1 or G-2.
Remember low cathode =brite, high cathode = dark.

One other thing that goes is a 10mfd 415V 'lytic on the
right rear of the 9-160 near the FBT. Its the filter for the
200V supply for the outputs. Very common on all brands
but usually gives different symptoms.

good luck
Zeno
LFOD !

[Electronic M]

Originally Posted by kf4rca

That's what I'd check. Also check the clip-on connectors from the crt board to the 9-160 module. And, of course, check the solder connections on the crt board.

It is probably a good idea to check connections on the main board too...If it's had a hard life the 9-160 could have many poor joints.

[old_coot88]

Loss of luma with chroma still intact can be caused by a heater-cathode short in the CRT. Tapping the CRT neck usually reveals it.

In a solid state set with the heaters powered from a winding on the flyback, you can sometimes 'float' the whole circuit, thereby isolating the short from signal ground. Sometimes this is as simple as cutting a single trace on a pc board.

Some Sanyos were particularly prone to this. Never encountered in a Zenith before.





.

[jstout66]

I remembered that about a heater-cathode short, and am sure hoping that's not it. Picture has always been amazing on this set. It uses the tri-focus tube, that seemed to last forever in 13" and 19", but not so much in 23" & 25".

[zeno]

As a rule an H-K short on an in line CRT will give you
a brite pix of one color ( R,G, or B ) withe retrace lines.
Also set often shuts down due to overcurrent.
One exception I remember were Sylvania E08 chassis
that would turn white with retrace.

73 Zeno
LFOD !