NEO-GEO MVS MV-1B -> Scart switch->OSSC - washed out picture

[moldov]

Colleagues,

I'm facing the issue connecting NEO-GEO MVS MV-1B RGB modified board with OSSC via automatic scart swtich.
So I have the following setup

-> - means connected to

NEO-GEO -> Automatic SCART switch -> Mechanical SCART switch -> OSSC

RGB cable for NEO-GEO is hand made but good quality and equipped with 460 Ohm resistor on sync line and 220 Ohm resistors and 220uF capacitors on each RGB line in serial connection. 75 Ohms are not enough because picture is washed out and I was using Color Test Tables from Neo-geo HW test to adjust resistors.

Automatic Active SCART switch is like this


Mechanical SCART switch is like this


When I connect NEO-GEO -> Automatic SCART switch -> Mechanical SCART switch -> OSSC the picture is very bright and voltage level (I was using multimeter in DC postition shows 2,5V on each RGB line)

When I remove Automatic SCART switch and connect
NEO-GEO -> Mechanical SCART switch -> OSSC - the picture is awesome and color tables from HW tests I see are correct.

If I increase Resistors to 2Kohm - the colors look correct but picture is smooth and not good.

The rest of consoles I have N64, SNES, NESRGB, Saturn etc all look good with AutoSCART inline or without - no changes.
But MVS is not. Mostly I have 75 Ohm resistors in all RGB cables.

If anyone knows, could You tell me what I'm doing wrong?

Apparently active switch is amplifying the signal but why the rest of consoles show the good picture but MVS not?

Regards

[paulb_nl]

I think your Automatic SCART switch is not terminating with 75 Ohm to ground and is outputting the signal with 75 Ohm impedance causing it to be terminated with 75 Ohm from the switch and 75 Ohm in the OSSC instead of 220 Ohm+resistor in console and 75 Ohm in OSSC.

You need to output a 1.4v p.p signal with 75 Ohm impedance from the Neo-geo. Probably the easiest way is to install a video amp like THS 7314/7316 with 75 Ohm resistors on the output and use a resistor voltage divider to attenuate the input to the video amp to around 0.7v.

Most likely the switch is doing the same thing to the sync so the sync voltage after 75 Ohm termination is much too high for the OSSC and you need to attenuate that too.

[beharius]

We designed an impedance matching RGB amp for the MVS if you are interested in. It uses a THS7374 video buffer.
https://www.beharbros.com/product-page/ ... fier-board

[moldov]

paulb_nl - thanks a lot for the reply. I think I would go this way. So, as far as I understood one point of divider goes to console signal. The second one goes to GND and middle point attenuated to 0.7 will go to amplifier input. Right?

Seems that I need to power up the scope:) to be able to make proper attenuation

beharius - also, thanks a lot for Your proposal. I have plenty TH7316 so I prefer to go this way, but will have Your proposal in mind.

[paulb_nl]

Currently the resistor divider is 220 Ohm in the cable and 75 Ohm to ground in OSSC. Now you need to move that to the amp.

So you connect 220 Ohm in series between signal and amp and 75 Ohm parallel to ground.

Then if you put 75 Ohm on the output of the amp then you shouldn't need any resistors in the SCART cable.

[moldov]

As per scope I have 0.7 V signal on the output of divider. But if I connect the divider to 7316 - the chip becomes really hot.

I connected 7316 like on the picture I posted below


Is this correct way to connect?

When I disonnect the RGB lines from input of the chip - the chip is not becoming hot. But if I connect again - it's hot.

[moldov]

COlleagues thanks a lot for Your time and suggestions. I managed to find the issue. It was a spot of solder on the other side of TSOP board which I was using to install the THS7316. That's why THS7316 was heating - it was the short circuit on the output.

Thanks again for paulb_nl. I managed to get the best picture from my NEO GEO MVS now. The schematics I posted above is correct (BTW I was using 1K and 220 Ohm instead of 220 Ohm and 75 Ohm in the input circuit and 75 Ohm on output)

I spent 3 hours with the scope and now I know a lot about my game setup and real things happening in the cables, switches and upscalers:) I know what is 75 ohm impedance, I know a lot about old consoles RGB signal levels etc. So' don't hesitate to use the scope if You'd like to get the best picture from Your rare and fancy stuff.

Also I managed to fix PC Engine and MSX OSSC loosing sync issue. Once again thanks to paulb_nl. In one of the treads I found that sometimes we need to adjust "Analog sync Vth" parameter in OSSC. And that was acutually my case. The sync level from PC Engine and MSX was about 120 mV which is quite low for OSSC to have a stable sync but decreasig the "Analog synv Vth" to 101 mV i managed to obtain the proper and stable sync.

[paulb_nl]

Great to hear it is working properly.

The 7316 datasheet says a 22-100uF capacitor should be added in parallel to the 0.1uF capacitor to minimize interference.

How did you handle the sync signal? For best compatibility with SCART devices it should be around 300mV pp with 75 Ohm termination.

[moldov]

Oh yeah, I will apparently add 22-100 uF capacitor to the mods I have already (NEO-GEO MVS, PC Engine, SFC Jr, N64)

Regarding the sync - usually I put 470 Ohm inline with sync line (pin 20 on SCART plug). But I decided to remove the resitor from PC Engine and MSX cables because the signal is quite weak (around 100mV). But again - check the voltage with the scope before doing such things. As Bob from RetroRGB says it can damage Your rare and expencive eqpt.

So now with the following setup:

Consoles -> AutoSCART -> ManualSCART -> OSSC

I have about 400mV Vpp on the sync line as per my scope. Which is seems to be a bit high but within specifications and less than 1 Volt anyways

http://martin.hinner.info/vga/scart.html

Sync / Composite Video In (Scart Pin 20): The sync pulses are active low signals on the composite video line: 1.0 volts = White. 0.3 volts = Black, 0 volts = Sync ("blacker than black") So in RGB mode the line is normally 0.3v, pulsed to 0v for the sync periods.

For sure I can increase the resistance on sync line, but I keep in mind good proverb - Sometimes the best is worse than just good:)

Sync is stable, I don't see lots of noise - so pretty good, I guess.

Hope that tread will help the guys facing similar issues.