Author Topic: k7500 chassis golden tee washed out and no color (Read 3956 times)

mclovin · « **on:** November 13, 2017, 03:57:14 pm »

Just picked up a new game that has a K7500 chassis and only a black screen. I have neck glow and can hear the monitor start up, but just black screen. I am not good with monitors so I’m looking for some troubleshooting advise on where I should start?

Thanks, love this place!

mclovin · « **Reply #1 on:** November 13, 2017, 06:42:55 pm »

After some troubleshooting I got a very dim and green picture by turning up the screen pot on the flyback. So I removed the chassis and looks like there is some small hairline cracks on the flyback. So I’ve ordered a new flyback, cap kit and output transistors for the neckboard as they looked all burnt.

Am I on the right track?

ed12 · « **Reply #2 on:** November 13, 2017, 11:12:33 pm »

yup
right track

ed

mclovin · « **Reply #3 on:** November 24, 2017, 09:55:35 pm »

Ok I checked the caps, replaced any bad ones and replaced flyback. It would appear othinf has changed. I can get a very washed out picture that I can either make very blue, green or red by adjusting the neckboard. Other than that I get no color, or blacks.

I’ve attached a few pics just to so you can get an idea of what is happening. I also noticed 3 resistors on the neckboard get extremely hot. Not sure if this is related or normal but I would doubt it.

Can anyone help?

The pics were too large to attach. Try to view them here.

http://s350.photobucket.com/user/mikehuntstinks1/library/

Thanks in advance

mclovin · « **Reply #4 on:** November 24, 2017, 09:58:46 pm »

Here is a pic of the super hot resistors.

ed12 · « **Reply #5 on:** November 24, 2017, 10:24:50 pm »

if u did not do a full cap kit u are asking >begging< for problems
also check the video ic >it is on the neck board<

ed

mclovin · « **Reply #6 on:** November 24, 2017, 11:07:18 pm »

Hi Ed, Sorry what is that and I do I check it?

ed12 · « **Reply #7 on:** November 24, 2017, 11:34:16 pm »

do u have the schematic ?
the neck board "where them 3 hot resisters are" should have a ic on it.
u can also chk your main drive voltage on the collector of the 3 finals
but I am leaning towards weak video drive >over all lumance<
hence the reason I pointed u towards the ic on the neck board "if there is 1 ?"
just run the well's # in google .

ed

mclovin · « **Reply #8 on:** November 25, 2017, 12:49:26 am »

I think all I have is the 3 output transistors for color. Could these be bad?

lilshawn · « **Reply #9 on:** November 25, 2017, 12:02:05 pm »

LM1202/1203 or whatever it is... video amp. faulty or low vcc to this chip will cause video issues. if voltage is good, replace it.

mclovin · « **Reply #10 on:** November 25, 2017, 12:11:18 pm »

Is this on the neckboard?

ed12 · « **Reply #11 on:** November 25, 2017, 03:22:57 pm »

Symptom: No picture. Has raster.

Diagnosis: Color goes straight to the neckboard, but swapping neckboards did not bring back the picture. Started looking at the Auto-Beam limiting circuit and the Brightness and Contrast circuits in the 800 section. Noticed that a partial cap kit had been done to the chassis, but none in the 800 circuit. Replaced the caps with new ones and picture was back.

full cap kit + look at the ic on the neck board

ed

mclovin · « **Reply #12 on:** November 25, 2017, 09:32:58 pm »

Even if the caps test good, should they still be replaced?

mclovin · « **Reply #13 on:** November 26, 2017, 03:29:02 pm »

Where would you guys recommend picking up an LM1203N chip from?

ed12 · « **Reply #14 on:** November 26, 2017, 05:03:53 pm »

a=never go by if it looks ok.. bad call just replace with new from a kit.
b=mouser /digikey / ebay

ed

lilshawn · « **Reply #15 on:** November 26, 2017, 06:30:21 pm »

usually what happens is the little rubber plug on the bottom that seals in the juices gets hard and shrinks...allowing for the electrolytes to slowly evaporate out over time. a capacitor can LOOK okay, but be bad. They don't have a turkey probe on them that pop out when they are done. the ones you see where the tops are bubbled (or burst an leaked out) out have failed for a completely different reason.

the only true way to tell if it needs to be replaced or not is to use an analyzer and measure its capacitance (ESR and DCR)... but for what they cost, I just replace them all... Than to try and hunt and peck a bad one out.

The chip can be had off ebay for a couple bucks. They aren't a real high ticket or highly sought after item so not really subject to counterfeiting (yet)

mclovin · « **Reply #16 on:** November 26, 2017, 06:49:00 pm »

I have a capacitance meter and if they measure within 10% if their UF I haven’t replaced them. I’ll replace R811 as it is reading OL.

mclovin · « **Reply #17 on:** November 28, 2017, 04:30:53 pm »

Success! Shiny and bright like a brand new penny R811 was the culprit. Just thought I would pass on the info.

ed12 · « **Reply #18 on:** November 28, 2017, 05:16:29 pm »

congrads

ed

lilshawn · « **Reply #19 on:** November 28, 2017, 05:58:42 pm »

Nice. If i recall the 800 series circuit feeds the brightness.

mclovin · « **Reply #20 on:** November 28, 2017, 11:49:35 pm »

I found through google that R811 is a common failure and will cause lack of color and brightness. Got lucky 🍀


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