Main > Monitor/Video Forum
Segai M21C XX hourglass screen
trick72:
Thanks for your explanation. So i am going to order all parts I need. The amp chip is extremely small however. I never soldered that small so I hope it is going to work out fine...
trick72:
So I managed to create the cable. The end result looks about the same like Bubblechump's although my resistors 25k are a lot larger so I could not fit them directly through the holes of the board.
But anyway, the image is much better now. Indeed with the 25K resistors as input and 500ohm as output the ringing is almost gone.
However, I notice some kind of "interference" now in the crt image. Some diagonal "transparant" lines going back and forth at various speeds. The image itself is stable but there are waving lines in them.
The VGA cable I cut and used was ofcourse fully shielded. But because I hacked it with the THS7314 and resistors this shielding is now much less "clean" and there is no shielding around my own wires and resistors.
I think this causes the interference on the crt monitor. Is there anything that could be done on this? Any tips on how I can improve shielding?
Zebidee:
Well done!
Most likely cause of your waviness is signal interference, and quite possibly some part of your THS7314 amp mod.
So check all connections again.
Another possible cause of interference is the quality of your 5v power supply. If you were to look at any 5v supply on an oscilloscope you'll see some waves, some bumpiness. If too much , it can cause interference or upset the amp's operation and affect your image quality.
So, try an alternative 5v supply.
It will not hurt to add a electrolytic cap across the 5v and GND inputs, just like the 104 cap is in bubblechump's. I suggest try values 22uF to 100uF (I just read the THS7314 datasheet, it actually suggests exactly this!). Electro caps are polarised, so positive lead to 5v, negative to ground!
Failing all that, you could add a voltage regulator to the power supply, if so would suggest a LDO (low drop out). There are many out there, possibly already in your kit. The THS7314 needs at least 3v, there are many 3.3v LDOs available (for example, MCP1700-3302E, which is rated for up to 250mA output and available in a convenient TO-92 package).
You may also want to add a current limiting resistor to the 5v supply. I can't imagine the circuit uses more than 100mA / 0.5W so a 50R 1/2W resistor?
trick72:
Good tips to get me going, Zebidee. Many thanks. I'm taking 5v from the PC power supply. (the power cables are yellow/black for 12V and red/black for 5V. So I spliced such a cable and uses the red/black for 5v. Maybe this PC power supply (I'm running mame on my cab) causes too much interference. I will try a seperate one and also the other stuff you suggested.
Thanks for your help
trick72:
So it appeared the display problem I had was due to the video amplifier of the JPAC. It was too bright causing the ringing and other stuff. I hacked a VGA cable and used the R,G,B,V+Hsync and groud cables to directly connect to the CRT. Now the picture is fine and I don't even have to use the THS7314 anymore.
Only problem I am left with is the diagonal lines scrolling from bottom left to top right or visa versa in my picture. I had this also with the THS7314 between it, and I had it also with a normal VGA cable from the JPAC to the JAMMA. The image itself is stable and fine but there are lines "scrolling" in the picture. They very in speed. They usually start to scroll slow, go faster and slowdown again until they stand still, then start to scroll the opposite side. The lines are white and look like some ground issue but I don't think that's possible, as now it's just the PC video card directly to the CRT. I made a short video and uploaded it to this post (could not upload a MP4 video file so had to ZIP it), so you can see the lines. Mostly visible in the blue background of metal slug.
I have another mame cabinet with a hantarex screen and when I hook up this screen to the PC inside the "problem cab" I don't see the diagonal interference there.
So this would leave me thinking it is a problem with this Segai screen. However, when I hook up the original Neo Geo Console running Metal Slug, I don't see any interference at all on this Segai . The diagonal lines are not there.
The video I posted here is metal slug running on groovymame (crt emudriver) on a normal PC. I have the impression the video card of the PC is generating this disturbance lines. Because every time the resolution changes it seems these lines are "reset" somehow. They don't continue to scroll regardless of what is on the screen. They seem to reset when I change games or resolutions. Why they are not visible on the hantarex screen of the other cab, but are on this Segai screen is very puzzling to me, certainly because this Segai screen can display the NeoGeo console perfectly fine... I'm not sure if some kind of EMI filter or something would solve it. I tried to connect the PC power supply to a completely different broker circuit than the CRT screen power supply, but that did not help.
I hope someone can help on this. Thanks