Nanao MS8-26SG Pincushion issue [solved]

MagicianLord77 · Feb 23, 2023

Hi guys, I'm having an annoying pincushion issue with this Nanao MS8-26SG, some background: this issue might have started some time after recapping, I mean it used to look good after recapping but then I remember the monitor pincushion to flicker a bit by itself after "warming up", to eventually settle. This was some years ago.
Currently, pincushion does not change by itself or anything. Its just uglyish, with picture near the corners being "sucked" out, most notable with games that have rectangular shapes like Windjammers.

I thought this could be quickly adjusted using the SPC pot, but since it's hard to reach from the outside I've took some time to deal with this issue. Now I got the monitor out of the cab, and found out that the pot does not make much of a difference. Turned all the way to one side it makes picture a bit narrower and taller, turned all the way to the opposite it makes it a bit wider and shorter, but pincushion-wise, I can say either way it stays almost the same. I've contact-cleaned the pot and it seems to turn well.

I wonder which part of the chassis should I investigate further, should be the section around the SPC (3 small transistors), or the one around H-POSI a pretty important transistor (Q534), or something else?

Also, another question, since there are H-Phase and H-Posi pots for adjusting the Horizontal position, what would H-Sub1 and H-Sub2 do? They aren't on the schematic or manual, also couldn't find info on the internet. Haven't messed with them since they are glued, didn't want to take the glue off if not necessary, could they help with the pincushion?

Anyone has some suggestion what to try next to fix this pincushion issue?, some component to test/replace? Appreciate it

thanks!

Some other measurements:
B+ is ok at 92.1v
Adjusting width and pots, including H-POSI does not make difference pincushion-wise.
The four diodes around Q534 (D530, D531, D532, D533) measure good 0.5v forward voltage in circuit.

nem · Feb 23, 2023

You want to check the three transistors: Q451, Q452 and Q453.

MagicianLord77 · Feb 23, 2023

Hi nem, desoldered and individually tested Q451, Q452, Q453.
They are well recognized as transistors by the tester, and they seem to have almost equal characteristics, so seems that they're all "healthy"..
Also tested D451 and D452 (in circuit) and gave ok forward voltage (0.5v).
What could I test next? Thanks!

nem · Feb 24, 2023

Test the nearby resistors next. R454, R455, R456, etc.

d7onr8 · Mar 1, 2023

I had a pincushion problem on a Wei-Ya 3129D. Tested all the transistors, resistors, diodes, and pots but in my case it ended up being a bad capacitor so I wouldn’t rule out bad caps in the pincushion area.

MagicianLord77 · Mar 1, 2023

Hi guys, thanks for the suggestions!. I've been trying quite a bunch of stuff these last days, basically I removed, tested (everything tested OK) and replaced (just because I had parts available) every component at the pincushion area, and basically nothing changed

Every item highlighted has gone through that process:

Transistors (Q451, Q452, Q453) (C1740):
Replaced them with new C1815 ones, according to arcadepartsandrepair they're equivalent.

Diodes (D451, D452) (1SS133):
Replaced them with new 1N4148, which I'm confident they're equivalent.

Resistors (R452 to R459, R450 and R550) (carbon 1/8w):
Replaced them with new 1/4w mostly, carbon.

MLC Capacitors (C455, C453):
Replaced them with new, different shape (yellow thingy instead of round brown lentil shape) same values.

Potentiometer (VR451):
Replaced with a new pot which has a different shape but works anyway.

Electrolytic capacitors were all replaced 2 years ago, checked values and polarity of the related ones anyways.

Pic of the process:

After all this diverse shotgun approach replacement, issue persists quite the same, though at some times the monitor is making some "feints" in where suddenly the pincushion works as it should be, for some minutes, then goes back to normal. I couldn't determine a reason about that spontaneous behavior switch, it's quite random. I tried tapping with a wooden stick everywhere without success.
But just for the record, sometimes in the past before replacing anything it also did some feints so, I doubt the replacement of these components had anything to do.

Attaching full schematic with some color I've put into it. Appreciate any suggestion what to check next

. since the SPC area is mostly covered, kind of ran out of ideas.. thanks!

bigger image since for some reason it shrunk when uploaded: https://ibb.co/nDDmFLj

also, @d7onr8 the cap that caused issue in your monitor was electrolytic or solid?

d7onr8 · Mar 2, 2023

It was electrolytic. You can see my thread here.

MagicianLord77 · Mar 3, 2023

Thanks. The part of your thread that says you found no continuity in certain places inspired me to do a "shotgun reflow" of the Pincushion area and surroundings. I had reflowed some critical parts (pots, crt socket, big transistors,..) but hadn't tried doing it to the small components. So after some work making every solder joint shiny again by applying a bit of new solder wire to each one or replacing it with new solder, turned it on and pincushion has stayed good since then! (yesterday)

So as a summary, if it continues to work as it is now, I can conclude that this pincushion issue was just a case of some cold solder joints located who knows where over the SPC area, could have been the jumper wires maybe.. Issue solved

Here it is how it looks now, the new SPC area (I'm not going to put the old components back :whistling: