Contrast/color issues on universal replacement chassis

[snappleman]

Out of curiosity I got one of those Chinese universal arcade monitor chassis and installed it onto one of my 19" CRTs. The installation process was easy and the picture looks great except it has this strange behavior where it changes tint depending on what's on the screen. For example if there is a lot of blue and green on the screen, the red will get get much more visible making the blue slightly purple etc, this happens with all three colors. I connected an MVS to it and I'll get the screen looking perfect in the service menu, but as soon as the game starts the colors/brightness/contrast all go off. The Neo Geo boot screen will flash white for a split second but then quickly go gray soon as the black Neo Geo logo comes in. This behavior is the same with consoles hooked up via SCART. I know that pure RGB that comes out of a JAMMA connector and the RGB out from SCART are different, and arcade monitors don't always look right with SCART, but as far as I know the MVS output should be correct.

I've tried setting the drive/cutoff/screen/brightness as I've been doing for years to calibrate monitors, but every control is in Chinese and there is no contrast control. At this point I'm wondering since this is a very (very...) cheaply put together part that if it isn't some bad components on the board? The RGBS connector was wired backwards which was just annoying at first (only because I got excited that they had actually used properly colored wire, just in the wrong order...) but I noticed the colors/sync go nutty a bit when I was just moving the cabling out of the way, so just touching it makes it go stupid. I could take it apart and do a recap but would a crappy/faulty flyback be causing this also?

Another thing I should mention is that the yoke on the TV measures about 13ohms on the vertical where as the chassis is rated for 6-12ohms, though I wouldn't think that would be the issue.

[snappleman]

Update: I went to discharge the monitor to check the board for faulty components, but the spark I got when discharging was pretty monstrous and gave me a solid scare (and blew back the suction cup even). I realized that I was having the flyback up way too high to get a raster with the game on, so I figured it's an RGB input voltage issue. Then I remembered the MVS I was using was the 2slot that I modified to work as a console back in 2001, and even though I connected the RGB via JAMMA I used the little 2A 5V power supply that I modified it to work with, so the video signal was just not capable of driving the monitor. I hooked up an unmodified MVS board to it and the video is fine.

[Osirus23]

Update: I went to discharge the monitor to check the board for faulty components, but the spark I got when discharging was pretty monstrous and gave me a solid scare (and blew back the suction cup even).

Damn. I usually just get a little click.

[grantspain]

shame you don't have a hv probe to see what level the eht is

[snappleman]

The flyback on this thing is suspect regardless. Whenever there's a white flash on the screen (or a solid white screen) it makes that little sizzly dying flyback pop that really makes me nervous, and the screen shifts brightness just a little bit before it recovers to white. I'm going to check the sticker on it to see if I can find a replacement somewhere just to have on hand.

[snappleman]

There it went, flyback just died after about 5 hours of actual operation. RIP. 

Now to try and find a suitable replacement for it. Thankfully I need a project right now so this is gonna get a recap and I'll change out those absolutely awful trim pots to something serviceable. I don't suppose there's any chance I could put a better quality flyback in this than a direct replacement?

[snappleman]

Waiiit, might have been premature. The flyback did the usual sizzle pop deal, but after coming back to this a few days later it seems like a really awful cheap 68uf cap was what popped right behind the flyback, along with the HOT right behind that.

I looked more carefully through everything, found 14 caps with bad ESR readings, which is the most bad caps I've ever seen on any kind of equipment. I messaged the seller on ebay and got partial (most of the cost) refund. Went ahead and recapped the entire board and was lucky to have a replacement HOT on hand, and now it "works"! I still want to find a replacement flyback, though, since this one is periodically making those awful sounds that make me nervous.

In the end it's been fun checking this chassis out, I figured since it's seemingly such a simple design that it would be somewhat more robust...

[lilshawn]

typically if there is a color shift that changes due to brightness (usually tell cause bright white turn tinted but grays and black look okay)the HV is way too high and heating up the shadow mask/aperture grille and distorting it...


turn down screen voltage on the flyback and turn up color drives to compensate the brightness.

[snappleman]

It was happening at very low screen voltage, at all settings. I ran it for a few more days and the flyback ended up fully dying (arcing and has a nice little hole in it now).

I have a couple much older boards that might work and found one that seems compatible (at least in yoke resistance which I know isn't the most accurate), which is a Pentranic 1019 CH-201.

The issue with this one is what it won't sync, as can be seen here (just imagine it jumping around):



This one has a damaged horizontal width coil, but it still seems to work:



The third leg is disconnected from the coil but it still turns and changes the horizontal size. This chassis is pretty beat up and probably should be trashed but if I can get it working then I'll use it on this monitor.

All the adjustments work, but there is no vertical hold adjustment. I also changed out most of the caps for good measure, but the sync issue persists.

[grantspain]

if that chassis has two transistors driving the vertical then its high impedance, although you would get folderover and compression if that was the case

[snappleman]

From what little info I found the monitors used on these Pentranic boards are supposedly around 2.3ohm horizontal and 14ohm vertical, though there are what looks to be 2 vertical transistors (B861 and D1138). They both were in bad shape and it had no vertical deflection, I swapped out the D1138 and it gave me some picture (what's pictured above), I'll order a B861 and see if that helps at all.

I read someplace that if you use a high impedance chassis on a low impedance yoke you damage some things pretty badly, so far this has been running relatively quietly and smoke-free. But I am super inexperienced and just now trying to learn how to work on these things so I'm sure I have everything wrong and will make tons of stupid (and expensive) mistakes. 

[jennifer]

Ya you will, wait till you blow up your first logic analyzer (dont ask🙄)...Testing the Kv out of that flyback would be my first thought, not that it is wrong at this point, but I would be more concerned about Xray radiation, ( high frequency on faulty equipment) someting nobody ever talks about mind you especially around here, ...After that I would take a close look at the quality of the components especially the caps, (personally I would match it up to a Wg chassis, and probibally just put new high temp Rubicon on there, but that just me, little eccentric)...And after that I would probibally order a few more of those boards just as a comparison against a bad board a curious assessment and spare parts for other projects ,cause that's kinda cool😜

[snappleman]

No need to wait, my work is in the pro-audio field and in my career I have already destroyed tens of thousands of dollars worth of mic pres, guitar amps and all manner of stupidly expensive gear they no longer make transformers for... 

[snappleman]

Update! I took a while to get back to this but out of curiosity I put the original chassis back in and fed it an RGB signal via SCART and the changing contrast and color balance happens still, but only on an RGB signal. The NTSC (composite and SVideo) works fine, and also YUV works fine, it's just when it's taking in an RGB signal that it happens. Whatever color is brighest on the screen will change the tone of the other colors and overall brightness of the screen.

This happens when I use official SCART cables for my consoles and also with RGB output direct from my JAMMA boards. I have a Raspberry Pi running with a Retrotink RGB hat, and I made my own VGA->SCART cable for it, and that displays just fine. Since it happens on any compatible chassis I put on this tube, I'm guessing it's the tube? Maybe the consoles and arcade boards all output RGB that's too much for the guns to handle?

At this point I'm thinking the tube might be damaged, I got it as NOS and it was unused so I figured the tube would be in good health, but maybe firing it up after all those years sitting in some storage room may have damaged it?