Genesis 2 + arcade monitor

[BMF54123]

Hello, all!

I am currently trying to hook up a Sega Genesis Mk. II to a Wells-Gardner K7000 series arcade monitor, and am having a bit of trouble. I've connected RGB, sync, and ground to the monitor as usual, and I get a stable picture, but it's almost as though all the colors are the same luminance--there's a dull gray fog over the whole picture, and there's very little difference between white and black. My arcade boards look perfectly fine. I've tried connecting various resistor values across the RGB lines, as an older post suggested that the Genesis's RGB output is too high, with little success. Connecting and disconnecting ground also makes no difference. Is there something stupid I'm missing here? I'd love to play some Gunstar Heroes in glorious RGB clarity.

[RGB32E]

With my Sony PVM-2030, I ended up using resistors and capacitors to get an excellent looking picture.  Resistors are more than likely NOT needed as arcade boards output much higher voltages than any console out there.  I have many arcade boards.... and they require at least 150-ohm resistors to drop the voltage down.  I used electrolytic (polarized) capacitors.  

The voltage rating should be at least 6V (more common ones are like 16V or higher).  You might not need a cap to filter the sync... but it made an improvement on my monitor.  Let me know what works or doesn't work...  Good Luck!

[viletim]

BMF54123,

You're close. The problem is that the output of the genesis is too low! Arcade monitors like a signal of at least 3Vpp while the best your genesis can do is 1.4Vpp.

The reason you're getting lots of brightness is that there's a few volts of DC offset on the genesis' video output. A few diodes can reduce that but you'll be left with a dim picture.

To fix this you'll need a video amplifier of some sort....
I designed one for this purpose but I'm not really happy with it. I'll get around to designing a better one...eventually...

RGB32E,

puting caps in series with an arcade monitor's input is bad
er, just some general info :)

[phreak97]

why is putting caps in series bad? all they should do is remove the dc offset you talk of.

[kendrick]

Speaking strictly from a voltage point of view, putting capcitors in series has the potential to create a reverse voltage condition that might damage a capacitor or even cause it to explode. It doesn't happen often, but it's possible for one of those capacitors to discharge faster than the other, and then what you get is reverse polarity applied to the discharged component as the other capacitor discharges. Put in simple terms, you get positive voltage at the negative end of a cap, and that isn't good. The way to get around this is to use polarity-neutral capacitors, or to use a couple of diodes to create one out of regular parts.

The information above is simplified for the purpose of conversation, and I'm aware that there are some shortcomings in my description. I'm not teaching an EE-101 class, else I could go on for pages and pages.

-KKC, still with the screwed up sleep schedule after Wednesday night. Man, was that not worth staying up for.

[RGB32E]

Listen here squakers!!!  Whether or not you think you know something about capacitors... whooptie do for you.

My post describes what I USED to get a good looking picture from my Genesis 2 system on a PVM-2030 RGB/Video monitor.  It had some washed out issues, so I thought that the information might help.  Yes... under certain conditions components can fail (including injury).

As for viletim and kendrick, well all I can say to you two is: Suck it!  Have you guys ever "blown up" capacitors when using them in series from a VG system to filter the video signal.  Or perhaps you guys didn't understand what I am talking about and interchange RGB for YUV... great!!!  First off.... I do not believe that the Genesis 2 has any capacitors on board to filter the RGB signals... (encoded video signals ALWAYS go through a series resistor and capacitor after being encoded).  So adding them in might be benificial with some analog rgb displays, such as my Sony PVM.

[phreak97]

almost any device with a video output has it running through a POLARISED cap before hitting the monitor. to blow up a cap, even with a reverse voltage, you need WAY more current and voltage than a video signal will give, and any monitor that sends that much voltage back to whatever unit it is plugged into, sounds really broken.. in any case, you seem to talk as if one cap is discharging into another. seeing as theyre on completely seperate lines, i dont see how you think this. on top of that, to get a negative voltage here, you would need to get the voltage on the negative side higher than the voltage on the positive side, and i dont see how youre saying this can happen, especially with there being a dc offset on the positive side and not on the negative.

but hey, if youre really worried, you could put a signal diode or something on the positive end of the cap to prevent reverse voltage.

[Guest_viletim]

phreak97 wrote:

why is putting caps in series bad? all they should do is remove the dc offset you talk of.

That's right, but removing the dc offset isn't always a good thing.

Most video monitors have an AC coupled input. This means there's a capacitor in series with the video signal near the input stage of the monitor. The DC offset is removed and the signal is clamped to a reference voltage (ie. a more accurate/predictable dc offset is added) for further processing.

Arcade monitors on the other hand are special, they are DC coupled all the way through. If you connect a constant +5v to the red video input the screen will go red and stay red until the voltage is removed. If you did the same to another video monitor (with 0.7V) the picture will flash red for a split second then go black.

So to connect a signal to an arcade monitor it must be DC coupled. Without a fixed DC referance in the signal it will just float around and the picture brightness will change with picture content.

if that's not enough for you then there's an app note by maxim that goes into all the details of ac coupling http://www.maxim-ic.com/appnotes.cfm/appnote_number/3303

ps tantalum electros are more fun to blow up anyway...

[kendrick]

Hey 32E, did I put something in my post you found offensive or personally insulting? I'm just putting up information; I ain't your momma and I won't go so far as to tell you what to do or not to do. If you want to disagree or do something unconventional, that's perfectly fine. But I don't see any reason for you to be unfriendly or confrontational, especially when we're talking about theory and application of electronics, rather than something controversial like topics of politics and religion.

-KKC

[Guest]

No hard feelings... ;P  Just trying to convey what works for my setup.  If you guys have actual experience with what does or doesn't work for hooking a Genesis 2 to an arcade monitor... great!  After all, that is the point of having a forum - to share ideas!

[RGB32E]

No hard feelings... ;P Just trying to convey what works for my setup. If you guys have actual experience with what does or doesn't work for hooking a Genesis 2 to an arcade monitor... great! After all, that is the point of having a forum - to share ideas!  

[BMF54123]

Okay, so...if I follow that diagram you posted earlier (minus the resistors), I should get proper colors?

Also, the picture was perfectly stable using sync straight from the console, so I don't think I need to add anything there...

[RGB32E]

Yes... that worked for me... I used the resistors as the levels were too high for my monitor (or else I would have to lower the Picture/Brightness controls and it still wouldn't be right).  

The 100uF cap on the sync was an experiment and I've forgotten the subtle change it made... so you probably wouldn't need it.  

Important: I used electrolitic (polarized) capacitors with the positive end connected to the source (genesis) and the negative side (caps usually indicate the negative end) connected to the cable conductor or connector.

I hope this works for your setup.  Its fun to play the Genesis via RGB!  It almost rivals the SNES.... well maybe not....

[viletim]

BMF54123 wrote:

Okay, so...if I follow that diagram you posted earlier (minus the resistors), I should get proper colors?

No, you won't.

RGB32E wrote:

Yes... that worked for me...

Your monitor isn't an arcade monitor, ie. a monior that is made specifically to run in an arcade machine

[RGB32E]

Your monitor isn't an arcade monitor, ie. a monior that is made specifically to run in an arcade machine

Really...?  The Sony PVM-2030 ISN'T an arcade monitor?  What?!  Thats a shocker!!!

if that's not enough for you then there's an app note by maxim that goes into all the details of ac coupling

So... Tim, have you actually hooked up a Genesis 2 to a Wells-Gardner?  If not, perhaps you should "Get A Grip" and quit farting in this forum.

I own many arcade PCBs (over 40), and they work just fine on the RGB input of my Sony monitor.  The only thing I have to do with arcade boards is place 150 ohm resistors in series on the RGB signals to drop the voltage down.  And I'll definitely have a nice chuckle if you find fault in that! ;P

If your counter intuitive statements hold true, fact is definitely stranger than fiction.

[NFG]

Please keep it civil gentlemen.  

Remember too that your personal experience is not necessarily the way everyone can expect their gear to work, so - as the sig says - try not to confuse your opinion (or experience) with fact.

[RGB32E]

All that I've tried to convey is my personal experiece.  Lawrence, do you have any knowlegde of a solution with no confusion?

[phreak97]

i think people need to make desicions based on their own experiences. if a cap blows up, wow, theres 20c down the drain. if the picture looks like crap, try another method.

[NFG]

I wasn't referring to anyone in particular, just in general.  

Rembember of course that I'm the one in charge.  =)

[Guest_viletim]

Your monitor isn't an arcade monitor, ie. a monior that is made specifically to run in an arcade machine

Really...? The Sony PVM-2030 ISN'T an arcade monitor? What?! Thats a shocker!!!

My point was that the OP has an arcade monitor, you don't. What works for you won't work for the OP

if that's not enough for you then there's an app note by maxim that goes into all the details of ac coupling

What's the point of quoting that? It was a (part of) a responce to a question by phreak97.

So... Tim, have you actually hooked up a Genesis 2 to a Wells-Gardner? If not, perhaps you should "Get A Grip" and quit farting in this forum.

Nope, we don't have Genesis'. Here they're Megadrives and a Wells-Gardner is an arcade monitor like any other isn't it?

The only one farting in here is you...and it STIIIIINKS.

I own many arcade PCBs (over 40), and they work just fine on the RGB input of my Sony monitor. The only thing I have to do with arcade boards is place 150 ohm resistors in series on the RGB signals to drop the voltage down. And I'll definitely have a nice chuckle if you find fault in that! ;P

There's nothing wrong with that except that it's completely irrelevant.

If your counter intuitive statements hold true, fact is definitely stranger than fiction.

What are you on about?

The fact is that connecting the genesis (RGB) to a TV or pro monitor requires a three resistors and three capacitor. To conect a genesis to an arcade monitor requires three capacitors, three resistors, a video clamp circuit and a video amplifier circuit. What part of that don't you agree with and why?

now i'd better open a window before I suffocate....

[RGB32E]

Calm down Tim!  No hard feelings.  If you are convinced that what worked for me won't work for BMF54123, then good for you.  It seems as though you are trying to make a compelling case, but don't have the experience to back it up.  Good Day!

[phreak97]

ok, both of you quit the arguing, this has gone from a debate to an outright argument with no real proof aside from your experiences to pull the argument either way.
howabout we let BMF54123 try it the easy way before going and getting a video amp and crap that it is possible will not be needed.

BMF54123, your results?

[Guest_viletim]

Ok, it looks like i'm wrong in this particular case. I've downloaded a service manual for the Wells-Gardner K7000 and found that it has an internal video clamp! I've never seen this before in an arcade monitor (i've got 10 random circuit diagrams for arcade monitors and none feature one), but maybe it's more common among the newer models.

WG-K7000.pdf If anyone wants to take a look:
Refer to page 12, circuit diagram. Video comes in from the connector at the right hand side, through an emitter follower, through a 10� coupling cap and into IC1. Now see the internal diagram of page 17, notice the internal video clamp.

The monitor claims an input range from 1Vpp to 5Vpp which means it should support the genesis easily. Looking at the input circuit I can't see why the genesis woudn't 'just work' when connected directly. BMF54123, you did try to adjust the brightness and contrast didn't you?

Then only other possible problem I can see is that the DC biasing resistors on the input are somehow disturbing the gesesis. This can be sorted out with a series cap as RGB32E has mentioned.

[BMF54123]

Yes, I did try to adjust the brightness and contrast, with no success.

I haven't had a chance to go buy some caps yet (!@#$% job interfering with free time), but out of curiosity, I pulled a 100uF/16v cap from a broken Caller ID box and attached it to the blue line. When I did so, the yellow in the image (the SCORE/TIME/RINGS info in Sonic the Hedgehog, my test game) got noticeably brighter. Am I on the right track?

It's too bad I don't have any more broken stuff lying around...

[Aidan]

Yellow is Red+Green... What you might want to do as an experiment is to connect the capacitor to the green line - the changes should be more obvious. If you can fish any other capacitors out of the box, then you can probably use them, even if they're not the same value.

Dull gray fog is generally the result of a DC bias on the inputs, and the monitor not having AC coupled inputs. Nothing wrong there, as there's no requirement for AC coupling on inputs, especially if there's no sync signal on the RGB lines.

There's no issue with putting capacitors on each of the lines, and it's the standard method for AC coupling. I'm not quite sure why anyone was talking about putting capacitors in series, as this shouldn't be the case. Monitors have a 75 ohm resistor on the input to 'terminate' the line, and stop it 'ringing'. This has the side effect of ensuring that one side is being pulled towards ground.

[phreak97]

Aidan, series just means you need to feed the line through the capacitor. it doesnt have to mean theres more than one cap on any one line

[BMF54123]

Bah, nothing! I put a 220uF cap on each line (positive lead->console, negative lead->monitor), same friggin' results. $2.25 down the drain. :angry:

It's so weird...individual colors seem fine, nice and bright, but when they all combine to make white, it turns a dull shade of grey. You can even notice the brightness cutting off as scenes in Sonic fade in. The game uses blue tones to do its palette fades, and as scenes fade in, the blue gets brighter and brighter until red and green are added, at which point it starts getting darker, and eventually turns into that muted grey. To me, that would indicate that the combined signal is too strong, but adding resistors only makes the color worse and gives a fuzzy, unstable picture.

*sigh*...I wish I knew more about DC bias and all that nonsense...

[phreak97]

my commodore monitor was doing whacked things like that, and it was because the sync (of all things) was too strong, and had a dc offset. i but it through a cap then a 150ohm resistor and it works now. probably a completely unrelated problem, but i dont know what else to suggest.

have you tried running the color through a capacitor then through a resistor?

[RGB32E]

You said that arcade PCBs work fine with the Wells-Gardner right?  How did you construct your Genesis 2 cable?  Did you buy a 9 pin mini din and solder the conductors?  Or did you buy a molded cable and are reterminating one of the ends?  Are you connecting composite video instead of composite sync?  Try to re-verify all of the connections as issues do arise with cable construction.

I recall seeing something about connecting a 470 ohm resistor in series with the sync signal of a Genesis 2.

About the capacitors, having some 220uF caps around can come in handy with other related projects.

[Guest_viletim]

It's so weird...individual colors seem fine, nice and bright, but when they all combine to make white, it turns a dull shade of grey.

I think your monitor may be faulty. What you're seeing is the effect of the Automatic Brightness Limiting (beam current limiting) circuit. It's job is to monitor the current drawn from the CRT cathodes, this current is converted to a voltage and negatively fed back to the video amplifier reducing the video's contrast which, in turn, reduces the beam current. The reason it's done is to prevent an over-bright image apearing on the CRT. You can measure this ABL voltage on the base (middle leg) of the transistor Q8 (it's right near the V.Hold control).

I say it's faulty because it seems to be way over-sensitive. But then why does it work with arcade PCBs??? it's very confusing...

If you're not about to give up here's some things to try:

1) Make the Genesis output a static picture with all colours in it then connect just one colour to the monitor and measure the ABL voltage. Next connect all three colours and measure it again. Repeat with an arcade PCB and post the voltages here.

2) connect up an arcade PCB with 10k resistors in series with each colour. This will form a potiential divider with the monitor's own input impedance (Zin = ~5k) and reduce the signal amplitude to about the same as what your genesis is putting out. See if it behaves as the genesis does.

At any rate I think it's safe to say that it's going to require more that some stuff in series with the colour signals to solve this one.

[BMF54123]

RGB32E: I'm using the plug from an old universal RF adapter, which luckily had a full complement of pins already installed. I just yanked out the pins and soldered on the wires. I know they're in the right spots because I encountered the same problem a couple of years ago when, lacking a plug, I just stuck sewing pins directly into the jack.

About the caps, does it make a difference if they're rated 16v? That's the only type I was able to find. I know earlier you mentioned that they should be at least 6v. (Pardon my ignorance, I'm still very new to electronic components and voltages and whatnot.)

viletim: I highly doubt the monitor's faulty, as I haven't had trouble with any of my arcade boards, even the ones that reportedly output an overly bright signal. I think it needs a cap kit, as the picture "bounces" when things suddenly change in brightness or color (it's from a Street Fighter II' cabinet, circa 1992), but aside from that it looks great.

I can try your second suggestion, though I'm not sure how I would go about getting a static picture like that to test the ABL voltage.

I'm not quite ready to give up yet, though I am getting quite frustrated...why can't all RGB signals be created equal? :angry:

EDIT: OK, I've read up a little more on video signals and DC offsets, and I think I'm starting to understand. The video signal is like a waveform, and like all waveforms has a center point. However, the center point on the Genesis's signal is offset (too high?), so black is no longer zero volts and becomes grey, and white (all colors combined) rides way too high/low and gets cut off by the monitor's circuitry, also resulting in grey...am I right?

[Guest_viletim]

RGB32E,
My first thought was that the DC offset of genesis is causing you problems but now that I've seen the circuit of your monior and you've described the problem in more detail I can assure you that the problem lies elsewhere.

According to the monitor's spec, it can accept a video signal between 1 and 5 volts (measured from the black level to the white level) and from the circuit I can tell that the DC offset can be as high as 7 volts (measured from ground to the black level) without causing problems.

The problem is definately with the monitor, whether it's a fault or a design flaw i'm not certain. Also (from your description) I'm confident the problem is somehow related to the monitor's ABL circuit. It's the only part of the monitor that can cause something like this to happen. (ie. one colour fine, all colours crap)

Don't arcade boards have a kind of test pattern or something built in? a cross-hatch would do.

btw, you're spot on about the DC bias. Lots of monitors (yours included) have a "clamp" circuit that changes the DC bias without affecting the AC signal. Problems can also occur with some (older?) arcade monitors when the signal has no DC part at all and the video signal travels above and below ground (0v) (this is what happens when you put a cap in series. This can causes only top part of the signal to make it through and a very distorted picture resaults. Just some unrelated info...

[RFM]

The 75 ohm resistors AND the 220 microfarad capacitors are likely both needed for this to work properly.

It has absolutely zippo to do with whatever likely minor input requirement differences may exist between the WG and a Sony pro monitor; generally 'modern' US arcade monitors accept a wide range of signals since arcade boards aren't very consistent. Anyway, I'm not interested in joining the pissing contest over this.

Back to the real problem--the Sony CXA1645, and likely whatever other video encoders used in various Genesis Mk.II models, needs that resistor/capacitor pair on the R,G,B outputs. The 1645 spec sheet shows this and by opening a Genesis it can be verified they're not there. It can also be verified by looking at the board in a 32X which has the resistor/cap pair shortly after the RGB signals enter the system. (Other consoles like PS and Saturn have those components already on the outputs on the console's motherboard. The Genesis does NOT, so they need to be added.)


There may be another issue that's been alluded to in one or two posts, and that's the sync is either too strong, or perhaps the load it sees causes the Genesis to do something weird. Some resistor on the sync as described above may be a solution; I instead just used the composite video for the sync, which might require the LM1881 circuit depending on the monitor. In my experience WG7000s don't care and will take the video signal fine.

Anyone who thinks they know better and wishes to argue will be ignored. This is my experience with the specific items and no speculation about others.

[Guest_viletim]

RFM,
Maybe you should have read more than the first few posts... We're a bit past that stage now.

[BMF54123]

You know, you guys may be on to something with the sync being out of whack. It seems no matter how many resistors I put on the RGB lines, the gray doesn't diminish at all (the picture gets unstable and fuzzy, and the colors get dimmer, but that grey remains). It even seems to remain when I've disconnected all but one color. Wouldn't that indicate that the grey is coming from somewhere else?

I guess I'll have to mess around with the sync a bit. Are those LM1881s readily available, in case I need to use the composite video signal?

[Guest_viletim]

The sync coming from the genesis is just standard TTL negative going sync, no reason why the monitor shouldn't like that. I don't think the sync signals can cause a problem like you describe but it can't hurt to try.

First try a simple inverting schmitt trigger buffer (74LS14) on the sync line (remember that the sync will be positive on the output). If that doesn't work then use an LM1881 to strip the sync of the video. It's not a chip you'd normaly find at the local electronics shop but it's not hard to get either. Try a mail-order supplier or if you pretend to be an EE then national might send you a couple of freebies. Failing that I posted a equivalent sync stripper circuit about a week ago, see "LM1881 Alternative?".

Looking again at the service manual.... it claims an "optional" 75R video input. Does you monitor have the P7 header fitted? (located in the middle of the board)

[Aidan]

It even seems to remain when I've disconnected all but one color. Wouldn't that indicate that the grey is coming from somewhere else?

It would indicate that the source of the grey was somewhere else, possibly from the monitor's internal circuitry. However, as viletim points out, generally sync problems don't manifest themselves as a colour issue.

[phreak97]

you can get lm1881 chips from www.national.com for free, all you have to do is make up a company name, and use the email address supplied by your isp, not a free email address(hotmail). it works no probs, i've done it three times now.

i still think it could be the sync. nobody believed me when i had the exact same symptoms. but i can confirm 100% that in my cas it was whacked sync. i put the signal through a 100uF cap, then through a 150 ohm resistor and it works now.

[BMF54123]

YESSSSSSSS!



I finally got it! Hooking up the resistors and capacitors to the RGB lines by itself did nothing, but out of sheer curiosity, I attached a wire from the metal hood of the AV jack to a ground line on my JAMMA harness. The screen went totally black for a couple of seconds, then slowly faded back in--with proper contrast!

There is one minor problem left: the monitor seems to take longer than normal to adjust to changes in brightness. For example, leaving it at a mostly black screen for a while causes the black to get noticeably lighter, and suddenly changing to a colorful or white screen causes it to slowly darken. It's not incredibly bad, though, and games are very playable. Is this just an unavoidable side effect of using the cap/resistor chain?

[EDIT]
Oooooh...I feel incredibly, amazingly dumb now. Ground was supposed to have been connected from the beginning, wasn't it? The A/V plug I'm using doesn't have a metal sheath surrounding the pins, so it was never connected...

Sorry for all the trouble, guys.

[viletim]

glad to here you fixed it!

don't feel too dumb...it's an easy mistake to make (esp since the genesis doesn't have a 'proper' ground pin and lots of pinouts websites neglect to mention it at all)

and yes, that effect of brightness changing with the picture content is caused by the capacitors you put in series with the colour signals. You should remove them, the 75R resistors can go too (though in the resistor's case it won't make much difference).