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Thread: Semi-working Genesis

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    Shakespeare Semi-working Genesis

    Hi there,
    I got a model 2, VA0 Genesis system which I modded (Mega Amp and CXA2075 video encoder swap [seen there: http://retrotimegames.com/page14.html ]).
    The minor problem:
    Blacks aren't that black and if there's bright elements on dark background, horizontally there will be even darker shadows, even though shadows aren't supposed to be there and background should be plain black.

    image upload
    The major problem:
    After unmeasured period of time of playing the console, if it's turned off, it'll not turn on until pin 14 and pin 15 are shorted for a second on the video encoder CXA2075.

    image upload

    Maybe anyone has any idea?
    Thanks in advance.

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    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by Peeteris View Post
    Hi there,
    I got a model 2, VA0 Genesis system which I modded (Mega Amp and CXA2075 video encoder swap [seen there: http://retrotimegames.com/page14.html ]).
    The minor problem:
    Blacks aren't that black and if there's bright elements on dark background, horizontally there will be even darker shadows, even though shadows aren't supposed to be there and background should be plain black.

    image upload
    The major problem:
    After unmeasured period of time of playing the console, if it's turned off, it'll not turn on until pin 14 and pin 15 are shorted for a second on the video encoder CXA2075.

    image upload

    Maybe anyone has any idea?
    Thanks in advance.
    That video issue is just from the CXA2075M upgrade. Its not really "broken" but I think a gain / brightness issue / config of the new chip (its not an exact drop in for the 1145 by any means).

    After unmeasured period of time of playing the console, if it's turned off, it'll not turn on until pin 14 and pin 15 are shorted for a second on the video encoder CXA2075.
    Dont do that, thats real bad. Pin 14 is VREF, which is not generated by the CXA2075, so that may be your entire problem right there. Its an important signal for multiple reasons. The reason the short works is that pin 15 is Cout which is roughly 1-2V so it tricks the system. You need a way to generate that signal with a new IC or a voltage divider at least

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    So, Vref is awaiting voltage from the chip, but it doesn't give it.
    And Cout just happened to be the nearest pin which gives voltage?
    But that still makes no sense, because every time I make short circuit between pin 14 and pin 15 the console is turned off and unplugged.

    U: what's the voltage required for VRef?
    Last edited by Peeteris; 04-22-2019 at 10:20 AM.

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    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by Peeteris View Post
    So, Vref is awaiting voltage from the chip, but it doesn't give it.
    And Cout just happened to be the nearest pin which gives voltage?
    But that still makes no sense, because every time I make short circuit between pin 14 and pin 15 the console is turned off and unplugged.

    U: what's the voltage required for VRef?
    look at the Sega Genesis VA0 2 service manual that omp dumped ( in the sticky thread). Also look at the CXA1145 data sheet. They will answer your questions.

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    Outrunner Eep386's Avatar
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    2V is required for the /SRES line to work properly. On the VA0~1.8 the encoder's regulator provides this voltage, but the CXA2075 doesn't provide an external regulator.

    These are two reasons why I advise against installing CXA2075's in these models, the ~VA1.8 were never really designed to use 'advanced' encoders like these and they more often than not cause dippy little issues like these.
    Though the VA2 and later Model 2's have the reset stuff handled by the 315-5684 instead so they tend to take better to these encoders, /SRES works properly at least, but they still don't really take too well to them.

    Now that you have the CXA2075 in there though, you can rig up a resistor divider to give you the voltage you need. You will need to use relatively low value resistors though to make sure the /SRES line toggles 'in time'. Using too high impedance resistors will make the /SRES line toggle rather slowly, which results in a noticeably longer delay for the system to boot up.
    Try a resistor divider of 220 ohm and 150 ohm, with 220 ohm going to 5V/Vcc and 150 ohm going to Ground. That should give you 2.02 volts, which ought to be close enough. If you'd rather make do with what you've got on hand, this calculator will help you out.
    Last edited by Eep386; 04-23-2019 at 02:08 PM.

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    Quote Originally Posted by segasonicfan View Post
    look at the Sega Genesis VA0 2 service manual that omp dumped ( in the sticky thread). Also look at the CXA1145 data sheet. They will answer your questions.
    Thank you for indicating the location of manuals, forgot that they could help.

    Quote Originally Posted by Eep386 View Post
    2V is required for the /SRES line to work properly. On the VA0~1.8 the encoder's regulator provides this voltage, but the CXA2075 doesn't provide an external regulator.

    These are two reasons why I advise against installing CXA2075's in these models, the ~VA1.8 were never really designed to use 'advanced' encoders like these and they more often than not cause dippy little issues like these.
    Though the VA2 and later Model 2's have the reset stuff handled by the 315-5684 instead so they tend to take better to these encoders, /SRES works properly at least, but they still don't really take too well to them.

    Now that you have the CXA2075 in there though, you can rig up a resistor divider to give you the voltage you need. You will need to use relatively low value resistors though to make sure the /SRES line toggles 'in time'. Using too high impedance resistors will make the /SRES line toggle rather slowly, which results in a noticeably longer delay for the system to boot up.
    Try a resistor divider of 220 ohm and 150 ohm, with 220 ohm going to 5V/Vcc and 150 ohm going to Ground. That should give you 2.02 volts, which ought to be close enough. If you'd rather make do with what you've got on hand, this calculator will help you out.
    Thank you for clarification, you even went extra mile to include beautiful calculator.
    --
    Than you for all the information, I wrote that down, because it'll probably help in the future, but for now I actually did this the wrong way.
    I know that shorting pin 14 and pin 15 is a terrible idea, but then I figured out that pin 14 needs power and pin 15 outputs way too much power (because image is too bright) and I soldered a bridge between both pins. I turned the console on and ...
    both minor and major problems appeared to be solved. Bleeding/shadowing isn't that noticeable anymore and console still turned on after playing it for 2h and turning it off, what wasn't possible earlier, because it just wouldn't turn on.
    If I'll ever notice any problems with such a 'solution', I'll immediately follow the information given above, but for now on - if it works and got rid of 2 problems, don't touch it.

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    Video Game Modder Outrunner segasonicfan's Avatar
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    Quote Originally Posted by Peeteris View Post
    Thank you for indicating the location of manuals, forgot that they could help.



    Thank you for clarification, you even went extra mile to include beautiful calculator.
    --
    Than you for all the information, I wrote that down, because it'll probably help in the future, but for now I actually did this the wrong way.
    I know that shorting pin 14 and pin 15 is a terrible idea, but then I figured out that pin 14 needs power and pin 15 outputs way too much power (because image is too bright) and I soldered a bridge between both pins. I turned the console on and ...
    both minor and major problems appeared to be solved. Bleeding/shadowing isn't that noticeable anymore and console still turned on after playing it for 2h and turning it off, what wasn't possible earlier, because it just wouldn't turn on.
    If I'll ever notice any problems with such a 'solution', I'll immediately follow the information given above, but for now on - if it works and got rid of 2 problems, don't touch it.
    This is just sad. Eep386 was super nice and went the extra mile to explain your exact problem all for you to just ignore and do some dodgy hack saying youll contact us later if your jerry rigging doesnt work out?

    No, I dont think you should bother us. You want to break your console thats fine, but please dont waste other peoples time requesting help you dont actually want.

    I advise against installing CXA2075's in these models, the ~VA1.8 were never really designed to use 'advanced' encoders like these and they more often than not cause dippy little issues like these.
    Thanks for your post, Id love to get the bottom of this. Any idea of what causes the issues? I think it may be an SCIN issue as the CXA2075 only looks for TTL input whereas the older chips can also do PLL and I believe look for an AC coupled OSC input...I have to recheck the datasheet. Also 2075 has a completely weird CVBS termination setup.
    Last edited by segasonicfan; 04-24-2019 at 05:43 PM.

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    Quote Originally Posted by segasonicfan View Post
    This is just sad. Eep386 was super nice and went the extra mile to explain your exact problem all for you to just ignore and do some dodgy hack saying youll contact us later if your jerry rigging doesnt work out?

    No, I dont think you should bother us. You want to break your console thats fine, but please dont waste other peoples time requesting help you dont actually want.



    Thanks for your post, Id love to get the bottom of this. Any idea of what causes the issues? I think it may be an SCIN issue as the CXA2075 only looks for TTL input whereas the older chips can also do PLL and I believe look for an AC coupled OSC input...I have to recheck the datasheet. Also 2075 has a completely weird CVBS termination setup.
    I'll not bother you with this question again, I wrote down every bit of information you two gave me there, if I'll need it in the future, thanks for that. If I'll fry the video encoder, I'll change it and will follow notes I took from this discussion. If I'll fry the console, that'll be a lesson for me.
    The thing is, I did this hack before I got detailed responses. While console was open and it didn't turn on again, I tried generating 1,5V (because pin 15, which I accidentally shorted with pin 14, generates 1-2V as stated by segasonicfan) to SRES/VRef line (because from what I understood from segasonicfan, this was the cause of the problem) and... nothing happened.
    Then I thought about all the situation, that the cure so far was to short pin 14 which awaits power and pin 15 which generates power while the console was off and then it would turn on normally without these pins shorted, it made me realize that maybe pin 15 accumulates excessive energy and that causes problems. I turned off the console, it wouldn't turn on again, unplugged it and let it sit down 10 minutes. After that I measured the voltage on pin 15 with multimeter and results were something >0.5V.
    This time I shorted pin 15 with just ground, unshorted and tuned on the console - it worked.
    At this point I undertood that pin 14 doesn't change a thing in this case (but it still needs power, as described there) and the problem is C_out which accumulates energy.
    So I thought, why not try to short these pins while console is on and the only thing that changed is the brightness of video output and this way it looks more like it should look.
    After unplugging the console, pin 15 now doesn't accumulate energy and that's exactly what I need in this case.
    I understand that this is not the way it should be, but leaving it the way it was earlier (withouth pins shorted) it could cause problems even worse, imo.

    Yes, I could open the console now again and make voltage divider for pin 14 and ground pin 15 with some resistor, but my console was screwed and unscrewed so many times that if I'll unscrew it one more time, it'll not be able to screw it back, sadly.

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    Outrunner Eep386's Avatar
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    Quote Originally Posted by segasonicfan View Post
    This is just sad. Eep386 was super nice and went the extra mile to explain your exact problem all for you to just ignore and do some dodgy hack saying youll contact us later if your jerry rigging doesnt work out?

    No, I dont think you should bother us. You want to break your console thats fine, but please dont waste other peoples time requesting help you dont actually want.
    Nah, no need to get so combative. It's his console, let him butcher it as he sees fit.

    That said, what he's doing is directly shorting COUT (15)with the line that is normally driven by the encoder's VRef (14). On the CXA2075 this pin is indicated as NC (no connection), so it's 'dead' internally to the encoder. However, the external filtering circuitry is still connected to the VRef pin.

    My guess as to why this 'works' is because there's some voltage being generated through COUT (the video signal), and it's apparently enough to 'kick over' the comparator op-amp that controls the /SRES line (which is driven by Pin 14 almost directly). On the 2075, COUT has a 75 ohm driver behind it so it will not *immediately* run out of current, but I honestly wouldn't leave it like this for a long period of time, as it's placing undue stress on COUT, it wasn't supposed to work like this.

    As to why the picture is improved with this hack, my guess is there's a current bias issue somewhere, and the load placed on COUT just so happens to bring it down to a more 'normal' level. But I stress that this is just a guess.

    Quote Originally Posted by segasonicfan
    Thanks for your post, Id love to get the bottom of this. Any idea of what causes the issues? I think it may be an SCIN issue as the CXA2075 only looks for TTL input whereas the older chips can also do PLL and I believe look for an AC coupled OSC input...I have to recheck the datasheet. Also 2075 has a completely weird CVBS termination setup.
    The big problem I have with 'upgraded' encoders like these, is they change or drop features that the original circuit depended on. As I mentioned before in the case of the VA0~1.8s, the CXA2075's don't provide a voltage reference, which for some bizarre reason Sega also uses to drive the onboard sound and reset logic. (The VA2 and later model 2's have the 315-5684 take care of the voltage reference, so this doesn't apply to those consoles.) Additionally, it's obvious the CXA2075 requires some additional circuitry and/or rework to really work properly in a Genesis, if the noise problems Peeteris is talking about are any indication. (No idea as to what else is needed, though.) Also, the CXA2075 may draw a bit too much power according to the original engineer, so I don't really know how well that will work out in the long run.

    I've also managed to trick a rather nice composite picture out of the stock CXA1145M and MB3514 encoders these consoles typically come with, so I often don't have much of a need nor desire to change up to a difficult-to-acquire encoder that is such a noxious fiddle to use. (Sadly I can't say the same for the KA2195D, there's almost no way to make that junkheap look decent.)

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    Quote Originally Posted by Eep386 View Post
    I've also managed to trick a rather nice composite picture out of the stock CXA1145M and MB3514 encoders these consoles typically come with, so I often don't have much of a need nor desire to change up to a difficult-to-acquire encoder that is such a noxious fiddle to use. (Sadly I can't say the same for the KA2195D, there's almost no way to make that junkheap look decent.)
    Believe me, if there would be CXA1145M or MB3514 chip inside this particular console, I wouldn't touch it.
    Even with all this hassle, change from KA2195D to CXA2075 was worth it.

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    OPN2 in DNA Hedgehog-in-TrainingRoad Rasher Peeteris's Avatar
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    Hello my fellow members.
    Eventually I made voltage divider from resistors as that's right thing to do. Sadly, as I anticipated, legs of the screwing holes broke off and I can't really screw back the console, but that's another problem.

    I started to use RGB scart with this console and
    Oh boy, I don't know what to do.
    20190820-001135.jpg
    Vertical lines are eye gnawing.
    Is there a cure? Maybe some capacitor or resistor values in the mod tutorial are wrong or should I change encoder to other model (CXA1145M or MB3514) if that's possible?

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