Sega Master System / SMS to jamma RGB a little dark

grips03

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Sega Master System / SMS to jamma RGB a little dark

I have a Sega Master system converted to jamma. RGB is a little dark. I tap RGB from on IC labeled 315-5124
pin 26 = red
pin 27 = green
pin 28 = blue
pin 29 = sync
pin 32 = ground

I get about .2 to .5 volts (DC on multimeter)

If I switch to output of CXA1145. I see 2volts (dc on multimeter), but video is all washed out. And background that's suppose to be black is white-ish. any ideas on how to fix. I did a similar mod on my model 1 Genesis and it's nice and bright.
 
I've never messed w/ the RGB on the Sega systems, but I know a lot of those outputs are high impedance, so you may either need to amplify the signal (2N3904 w/ series resistor), or in the case of the one that's washed out, it may already be amplified enough, but you probably need a series resistor on the output.

In the case of the CXA1145, are you pulling directly from the chip, or at the AV Output port? It may already have proper circuitry if you pull it from the port.

DogP
 
On SMS I'm getting RGB before the CXA1145 chip, directly off the 315-5124 chip. If I try to get video off the CXA1145 chip its all washed out.

I did a test today to compare the same video game, but on different systems (SMS and Genesis). The Genesis to jamma works great. On the Genesis I got video before the voltage dividers on the input of the CXA1145. Using same game, measured DC volts with multimeter.

[Genesis]
Hang On
r=.78v
g=2.2v
b=2.48v

[SMS off 315-5124]
Hang On
r=.38v
g=.7v
b=.62v

[Genesis]
Shinobi
r=1.64v
g=1.45v
b=1.05v

[SMS off 315-5124]
Shinobi
r=.75v
g=.6v
b=.45v


On the SMS if I switch to taking video off the CXA1145 I get

[SMS off CXA1145]
Hang On
r=1.99v
g=2.35v
b=2.28v


[SMS off CXA1145]
r=2.35v
g=2.26v
b=2.12v

I should mention that with the Genesis plugged into the jamma harness and turned off the monitor is nice and black, even though the monitor is turned on.

With the SMS plugged into jamma harness and turned off the monitor is whitish, like the brightness is turned all the way up even though its not. One I turn SMS on, the games are all darker.
 
Have you tried the SMS from the A/V connector? I don't know for sure, but that's probably the most conditioned signal for using elsewhere. The pinout is here: http://www.gamesx.com/avpinouts/genesisav.htm .

And you can't really check the signal with a multimeter... on DC volts, that'll basically just tell you what the DC offset is on the signal, not really what's happening on the signal. Especially coming right off the chip, a lot of signals have a DC bias, which are then AC coupled across a capacitor after the chip. If possible, you should really check it out on an oscilloscope.

If you don't have an oscilloscope, I'd just do some trial and error of the A/V port and the CXA1145, with and without resistors, to see if you can get it to look good.

If you can't get that to look good, I'd build an amp with a 2N3904 and a resistor connected off the 315-5124 (which you know looks good, but too dark), and tweak the image with the resistor. Diagram stolen from a post on smspower.org.

Code:
.           | +5v
.           |
.           |
.video   | / c 
.in   ---|<
.        | \ e
.      NPN  |   27   video
.           +--RRR--- out

DogP
 
I tried three C1815GR NPN, with Emitter to jamma RGB, collector to +5, and base = SMS RGB. On volt meter it said negative 1.95v for all 3 lines and video was very washed out and could barely see anything. Perhaps because it was negative voltage?
 
Output of CXA1145 and video port are the same, i.e. they are directly wired. I have a oscilloscope, but its been 20 years since I used one. This one I have is a portable model that I got in the mail yesterday.

http://www.jyetech.com/en/default.php

DSO082 model

what size pot or resistor should I add to CXA1145 to make it work better. I have some resistors here.

thanks
 
I tried the SMS video out port, same washed out looks. I tried adding 75ohm, and looks pretty much the same. I tried a 220uf cap and same result. I'm ordering some of those transistors.

thanks
 
If you already have C1815GR transistors, I'd just use them... there's nothing magical about the 2N3904 (they're just REALLY common, and I've used them several times). They're so common that even Radio Shack has them. :p

When you used the C1815GR, did you use a resistor from the emitter to the JAMMA pins? You may actually need a pretty high value resistor, compared to S-Video mods. I believe most arcade monitors have an input impedance of ~4.7K ohm, rather than 75 ohm of TV. I'd try 100, 1K, 4.7K, 10K, and get a feel which range looks best (if any), and tweak it from there. The same goes for the output of the CXA1145... 75 ohm wouldn't do much going into 4.7K... I'd try some higher values.

DogP
 
I've ordered some larger resistor sizes. But yeah I had various resistor sizes in there. At one point I put 3 50K pot (26 turn) in series and tried various settings. But no luck. I've ordered the resistor sizes you mention, but I keep thinking something was skipped on the video output the the SMS. I've been reading here:

http://www.maxim-ic.com/app-notes/index.mvp/id/3303

And for some reason I think I need to add circuit in picture 4a. I plan on comparing good video from Genesis to bad video on SMS as I don't really have a reference on how stuff should work, i.e. 2v DC bias with .7vpp for the signal, blah blah blah. If someone has a good link to how arcade RGB is suppose to look on a scope that would be cool.

thanks
 
What monitor are you using? I don't know for sure, but I'd expect that the monitor would AC couple and clamp the signal at the input of the monitor... in which case, you'd really only need to worry about peak to peak voltage of the signal. The arcade monitor should take a level of 5Vpp.

I had a Tekken 3 board on my bench, so I took some screenshots of the color bar test on my scope. I just probed the red line, though all three will have the same signal characteristics.

You can see a single horizontal line of video in one screenshot (horizontal cursors are 63us apart, or ~15KHz). Note the falling slope of the line... that corresponds to the fading red color seen on the color bar. The other screenshot is a complete frame (horizontal cursors are 16.7us apart, or ~60Hz). Note the higher voltages at the beginning, then a gap, then higher at the end... that corresponds to where the red was actually being output (no red on the green and blue color bars in the middle).

You can see the total Vpp is just under 5V, though most signal stays under 4V. That was DC coupled, so those are the true voltages (0V to 5V).

Hopefully this will give you ideas and a reference if you look at your signals on the scope.

Edit: I'm not sure why the forum software shrunk the scope images like that, but I think you can still see sufficient details. If you need any clarification, let me know.

DogP
 

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Nice scope photos, is it a Tek scope?

Monitor is Hitachi GMK-29FS2. For some reason I thought all arcade monitors are DC coupled and that TVs are AC coupled, but I'm very green at this stuff.

I'll check out what it looks like on mine and post photos.

thanks
 
Sorry for image quality but its all my cheap scope can do.

I have 3 images one for Sega Master system (SMS) off 315-5124 IC, this produces dark images. 2nd for Sega Genesis that looks very good and 3rd SNK NeoGeo MVS board which looks great as well.

What I notice is the SMS voltage does not go as high as the other two systems. Each square is 2v. So 5v would be 2.5 squares in the grid. Genesis wave form seems to be about 2x that of the SMS and the Neogeo about 1.5x.

What would correct this? I think I have make to voltage go bigger.
 

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>Nice scope photos, is it a Tek scope?
Heh, nope... it's actually a cheap OWON: http://www.owon.com.cn/eng/pds.asp , but it works great for simple stuff like this, and it's battery powered and easily portable, and USB capable. I've got higher speed Tek CRO, but it's not as convenient, especially for screenshots.

>For some reason I thought all arcade monitors are DC coupled and that TVs are AC
>coupled, but I'm very green at this stuff.
That's very possible... I've never really looked into the specifics. I just know whatever details I've picked up along the way while doing other projects. The more I think about it, that would actually make more sense... otherwise making a solid color screen may not work properly, as there's no guarantee of AC signal on RGB (unlike TV, which has a specific signal structure).

>What I notice is the SMS voltage does not go as high as the other two systems. Each
>square is 2v. So 5v would be 2.5 squares in the grid. Genesis wave form seems to be
>about 2x that of the SMS and the Neogeo about 1.5x.
>
>What would correct this? I think I have make to voltage go bigger.
Yes, that's almost certainly the problem. The amplifier circuit I posted probably won't help, because that's an emitter follower, which has near unity voltage gain, but allows driving lower impedence circuits. There are other amplifier designs you could use (probably in conjunction with the emitter follower), but I'm not really an amplifier guy, so I couldn't be of much use, without a LOT of trial and error.

There's probably an easier way to get what you want though. One may be to use a proper amplifier IC. You just need to make sure it has sufficient bandwidth, and of course you'll need to tweak the gain to the proper levels.

The other possibility is using the output that's too bright/washed out (attenuating is much easier than amplifying). What does that look like on the scope? If it's still only 2.5Vpp, but with a DC bias, then that signal is of no use... but if it has a higher peak to peak voltage, it could be useful.

DogP
 
That TI chip would probably do the job, but yeah... that requires a fair amount of external circuitry. There's probably some wideband op-amps that would do the job that may require less support circuitry. I don't have any direct experience using any for this purpose though. Packaging will probably be a problem as well... DIP packages aren't very popular for these kind of devices.

DogP
 
Oh... well, I answered my own question. I hooked my scope up to the output on the back of my SMS, and it appears that the voltage is <2Vpp, and rides on ~2V DC offset. So, that's probably not very helpful. Some sort of video amplifier is likely going to be necessary.

DogP
 
I ordered the TLS1233N from Mouser. Will built the circuit when it gets here. Your SMS measurement matches mine. The SMS is quite a bit harder than mod 1 Genesis -due to audio amp, and now video amp. Controllers are easier as SMS controllers have no electronics. Since my cabinet is 4 button I was going wire up button 1 and 2, and then 1 and 2 with rapid fire. Hopefully I can do this with single timer IC. But first things first need to built this video amp.

thanks
 
I built this circuit and it kind of worked. But had video interference and also audio amp started to make noise with this circuit installed. Might try to build again, but there is so much extra circuitry for this IC, might just leave as is :(
 
Yeah, w/ 6dB gain, that looks like the perfect amp for you. Good to hear you got it working... and I'll keep that amp in mind if I have similar needs.

DogP
 
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