Arcade Otaku - アーケード オタク

Hi guys, I am at an unexpected situation after giving a recapping with parts from arcadepartsandrepair to one of my 2 Nanao MS8-26SG.
The monitor was working "well" before, a bit out of focus, but having recently acquired it (it is part of an Aero City cabinet), I decied to recap it as many recommend to do with such old electronics.

After the job I plugged it in and found a better sharper image, but after a few minutes, where I was also adjusting the size/position with the pots, the image started "collapsing" until being a single line at the middle. It vibrates sometimes (video https://imgur.com/a/Tt8w8xf ), but it is pretty random, no pot wiggling or pcb poking could make the line vibrate more or less than I can say.

I've doing some stuff to the chassis with no success, neither worse or better, that is:
-Brushed and cleaned the board with compressed air
-Reflowed all suspicious joints I could see, there were some on the neck board (not related to this vertical collapse problem I guess), and some on the pcb around hot components.
-Contact-cleaning and reflowing all front pots, and some vertical (V-LIN and S.P.C.) pots.
-Replaced many 1/2w resistors with 1w ones, (when I took them out I measured them and besides being ugly and some cracked, they were fine, replaced them anyways)
-Same thing with many 1/8w resistors on the vertical oscillation town (all measured fine but were replaced) with 1/4w ones
-Put a IC socket at the HA11423 place and put a new HA11423 that I got locally (unnecesary but done)
-Put a IC socket at the PC501 (TLP521-2 octocoupler), tested the octocoupler and seemed fine.
-Took out q401, q402, q534 (D1138 transistors), measured fine with IC tester, soldered them back in because couldn't find replacement and didn't seem necessary
-Same with q451 q452 q453 (small C1740 transistors)

After all this work, the monitor still works exactly the same...

What can I do? Maybe measuring the input oscillation signal from where it originates and follow it around? Don't exactly know how to do it, this is my 1st crt chassis repair, would like to hear suggestions please!
I've got this cheap oscilloscope that can be useful, but I'm a little worried about connecting it to a high voltage point and frying it.



Thanks!

replace c434 10uf 50v, the original is a ceramic but replace with a electrolytic- the polarity should be marked on the chassis but you can also see it on the ceramic

grantspain wrote: ↑February 24th, 2021, 6:55 am replace c434 10uf 50v, the original is a ceramic but replace with a electrolytic- the polarity should be marked on the chassis but you can also see it on the ceramic

Thanks for your suggestion, I took out C434 and put a 10uf 50v nichicon, issue persists

Original cap measurement:


Here's a gif about how the monitor it is after that: And after that, I decided to measure the "waves" that the service manual says there should be at pin #7 of the IC103, connected the oscilloscope to pin #7 and #9 (gnd), and something unexpected happened, the vertical collapse was no more! and the oscilloscope was displaying the wave, but when I turned off the monitor, disconnected the scope, connected again,.. it went back to the usual collapse, and I can't replicate it, does not happen anymore.
I measured the horizontal wave at pin #11 and the wave is there just as the service manual says, but the vertical pin #7 displays nothing, I mean, something flat.

measuring pin #11 (horizontal wave)


Does somebody in which way the oscilloscope would affect the circuit? Maybe adding some capacitance between #7 and GND.

Trying to make it work again, I took out C435 (2200pF) and C430 (220pF), measured them fine, put them back again, no success.. This is my map:
Appreciate any suggestions!

you have full raster, i think the problem is the custom sync module

disconnect video signal and increase screen volts a little on the flyback, see if you do indeed have a full raster

grantspain wrote: ↑February 24th, 2021, 8:12 pm you have full raster, i think the problem is the custom sync module

disconnect video signal and increase screen volts a little on the flyback, see if you do indeed have a full raster

Thanks, I disconnected the video signal, turned on the monitor and turned the screen pot on the flyback, this is what I got:
The line doesn't vibrate like with the game, stays still.

Then, turned the screen pot back as it was, connected the game board, and the line has the usual vibration as the gif from the previous post, found out that the V-POSI pot works fine, it moves the line a bit to the right or the left, but V-SIZE, V-LIN, V-HOLD, don't seem to do anything.

Took some measurements at the IC102 "pcb sync" and IC103 "pcb def" outputs and inputs, and could confirm that the H & V signals travel well from the IC102, thru the octocoupler, to their IC103 inputs, but at the IC103 outputs only the H is good, the V is pretty flat. Wrote that on the schematic:

if its a solid vertical collapse then its not sync module, odd issue though a si am sure i can see something in the video like interference in the other parts of the screen
maybe a broken track somewhere, is it affected by physical persuasion?

Without video signal, with the screen pot cranked up we could say it's a solid vertical collapse, a perfect still line.
With a video signal, with the screen pot at normal, it's a vibrating collapse, sometimes it's a line, but sometimes it display a bit of the game, just like the video
I poked the pcb in many ways, first lightly with a wooden stick and then harder with the plastic end of a screwdriver, all over the solder side and some on the component side, no hit made the display react, it just vibrates in it's own way, unaffected by the persuasion..

Luckily this journey is over!, the part which was causing all this issue was ZD431 (24v 0.5w zener), the one next to r430 at pin#7 of the IC103. It didn't seem physically faulty, but after taking it out (random components measuring), measuring with a multimeter gave around 300ohm resistance between its legs on both ways! that's very bad. Replaced it, and got a perfect vertical deflection going on, it now works as it should, with no weird vibrations or anything.
Thank you grantspain for your assistance!

Btw, about adjusting the SCREEN flyback pot, I should turn on the monitor without signal, front brightness pot to minimum, and set the screen flyback pot to just after the raster lines disappear right? Thanks

good job, very few people would have been able to find that so your tenacity has paid off, breaking down diodes are difficult to diagnose

screen volts, just adjust it with signal to best you can attain with no flyback lines

Excellent!

great catch and good fix

Thank you all!, I was becoming more and more hopeless each time thought some component was cause of the issue and then turned out it wasn't, until found it! It feels great! lol. Now got a really good looking monitor for enjoying! Next stop, installing the capacitor kit on the other nanao, hope nothing weird happens this time