Nanao MS8-26SG Green color "noise", what to check? [solved]

[MagicianLord77]

Hi guys, I'm having an issue with the Nanao MS8-26SG monitor from one of my aerocities. After having it in storage for a few months and turning it on again, I found some sort of annoying "noise" on its picture, like the brightness of some colors is varying all the time. To have a better understanding, I've put the service color bars and found that it's only the green color:

Please watch the 15sec video here: https://imgur.com/a/XT7mwBB

It can be seen that while blue and red remain perfectly stable as them should, but green and white (because needs green) are "glitchy".

-No amount of pot fiddling made any difference
-Tried connecting another jamma game board, same issue
-The monitor has been fully recapped some months ago

Could be something at the RGB inputs at the chassis or something at the output part, around the neck board?
Appreciate any suggestion on where should I look for something to check or measure

Thanks!

 

[brad808]

Can you try swapping the colours. For instance switch green and red input wires?

 

[MagicianLord77]

Can you try swapping the colours. For instance switch green and red input wires?

Yes, I forgot to put I tried that too, at the RGB connector that hangs inside the cabinet, I've swapped green with blue and blue with green, red left untouched
Found that green still glitches:

 

[brad808]

Ok that will eliminate your input signal from the board and input wiring as good if problem remain on green .

Now you can swap at the neck board, that will show you if your problem is on your main board or neck board and CRT.

Also you can easily do this without swap if you have a scope, but assuming you don't, this will be an easy method for narrowing down the problem area.

 

[nem]

My guess is the zener diodes on the neck board. When they start leaking, that's what happens.

 

[MagicianLord77]

Ok that will eliminate your input signal from the board and input wiring as good if problem remain on green .

Now you can swap at the neck board, that will show you if your problem is on your main board or neck board and CRT.

Also you can easily do this without swap if you have a scope, but assuming you don't, this will be an easy method for narrowing down the problem area.

I can try swapping them, that's a good idea!, but I also have a cheap but useful in some cases DSO 138:
What would you measure with it? Thanks!

My guess is the zener diodes on the neck board. When they start leaking, that's what happens.

Interesting, good to know, that's the first thing I'm going to check when I take the monitor out.
Looking at the schematic I found zeners just before the connector I guess that goes to the neck board (green):

Would those be the ones you are talking about?
That schematic does not include the neck board, this one does, it's not the exact same model (26-A) but here it is:

Hard to read, but I can't find any zener at the neck board, just the ones before it, from the IC101, anyways, when I open up the thing I'll look for them, but I'm starting to guess ZD111 (green color) could be the bad one, thanks!!

 

[brad808]

I do not know how to use your specific scope, but I will tell you the method to help you easily identity your problem area.

You have the schematics which you can read, you have an oscilloscope, and you have 3 identical circuits (all three colour paths are the same). Problems on the video signal are easy to troubleshoot because of this 3 identical circuits. You have two known good comparisons.

You will put on a test screen all white screen (then each signal going into the Mon will be identical)

You will compare R,G,B signals throughout the schematic from the input towards the output.

You can skip to large areas first to eliminate everything before. For example you will test RGB signals at neck board, if all 3 are identical, you will know all signal path before that is good. You will test RGB amplifier, you will test drive transistors input, etc. This will show you where the signal is identical and where the signal is changed. You can easily narrow it down breaking it into pieces on the schematic. It is much faster than it sounds.

Since you have 3 identical circuits, once you find your suspected bad component you can swap it with one from red or blue to confirm. Once you have confirmed you can order the new part and put it in .

 

[MagicianLord77]

I do not know how to use your specific scope, but I will tell you the method to help you easily identity your problem area.

You have the schematics which you can read, you have an oscilloscope, and you have 3 identical circuits (all three colour paths are the same). Problems on the video signal are easy to troubleshoot because of this 3 identical circuits. You have two known good comparisons.

You will put on a test screen all white screen (then each signal going into the Mon will be identical)

You will compare R,G,B signals throughout the schematic from the input towards the output.

You can skip to large areas first to eliminate everything before. For example you will test RGB signals at neck board, if all 3 are identical, you will know all signal path before that is good. You will test RGB amplifier, you will test drive transistors input, etc. This will show you where the signal is identical and where the signal is changed. You can easily narrow it down breaking it into pieces on the schematic. It is much faster than it sounds.

Since you have 3 identical circuits, once you find your suspected bad component you can swap it with one from red or blue to confirm. Once you have confirmed you can order the new part and put it in .

Thanks for the advice, I took out the monitor and soldered some wires at the RGB "out" from the chassis to the neck board, and with the oscilloscope compared signals between the crosshatch screen and the color bars (I don't have anything to produce a full white screen yet..)

The crosshatch screen gave some square waves at some points that I guess would be the white lines (forgot to take a picture), and the color bars that fade to black gave some triangles that I understand they would be the intensity fading from bright to black, voltage to ground:

But, when doing comparison between green and other colors, I couldn't find a difference, I guess the difference would be too subtle for this simple scope. I'd love to have a beefier scope with more resolution and more than one channel but I wouldn't use that much to justify the cost..

So I went to try replacing ZD111, desoldered it and removed it with tweezers from the other side, I didn't want to unscrew the chassis and all that stuff yet if not needed.

Marked with a sharpie the cathode of a 12v 1/2w zener that luckily already had in my parts bin from the last repair where I had a vertical collapse on this same monitor, and soldered it, I know I should have done it from the other side but I couldn't wait, it did not end looking bad to me..

(red cables were just for connecting the scope)

And the issue got solved!, green became stable again!, it was that damn diode.
Thanks a lot for your help!