Sega Megadrive, PAL50 PAL60Hz switch, too much colour? - Page 1
Forum section: electronics / projects

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 08, 2014, 06:06:20 am
content: I'll try to give as much detail as possible...Basically, I have a Megadrive 1, with a 50/60Hz switch.This isn't so much a PAL/NTSC switch, as the console has a different crystal fitted to the video chip (Sony CXA1145), to clock it always at the PAL clock rate...Anyway, when in 60Hz mode, it has "too much" colour... and I'm not sure why that would be?PAL, using svideo.NTSC... I don't think Sonic is meant to be orange?Basically, the mods the console has are in the following links:50/60Hz switch -http://www.mmmonkey.co.uk/console/sega/md1switches.htmPAL60Hz mod -http://zipplet.co.uk/index.php/content/electronics_mdpal60Thanks!

author: SimpCirc
author_rank: Newbie
timestamp: March 08, 2014, 06:45:18 am
content: Question, does your electricity run at 50 or 60 Hz? I remember I played this game here in the United States and Sonic was orange like you are seeing. The TV I used had a saturation adjustment on it and I just turned it down and it fixed the problem. The over saturation wasn't so much of a problem as it was an attempt to smooth out the pixels and compensate for the low saturation of the red phosphors in CRT TV's. Try adjust the saturation on your set and see if that fixes it on 60 Hz mode.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 08, 2014, 06:53:11 am
content: It's 50Hz mains, but as I use the console in both 50, and 60Hz modes, I'd probably rather not adjust the tv.... but... if I have to, I probably will, seeing 60Hz fills the screen, and 50hz doesn't

author: SimpCirc
author_rank: Newbie
timestamp: March 08, 2014, 07:19:14 am
content: Ah, the dreaded clash between 50 and 60 Hz lol, those two frequency standards never did get along with one another. Like I said, it might work things always have a way of not doing what we expect.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 08, 2014, 07:49:42 am
content: I'm wondering if I can adjust another part of the circuit when the switch is in 60Hz mode, to reduce the colours... hmm...

author: SimpCirc
author_rank: Newbie
timestamp: March 08, 2014, 09:06:05 am
content: I was wondering that too, there may be a trim pot or adjustable inductor in it that handles just that. If your lucky the 60hz adjustment won't affect the 50hz side. If you open the mega drive up it would be awesome to see pictures. I'd check to see if you can do the adjustments externally first though. As for me I'm going to go count electric sheep lol

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 08, 2014, 09:57:20 am
content: It's rather modded internally....Now, I have an adjustable capacitor on the crystal (as per the above link) but from memory, it didn't adjust anything...Maybe its worth having a re-tweak... Although it will effect the 50, and 60Hz setting...

author: amyk
author_rank: Super Contributor
timestamp: March 08, 2014, 01:45:23 pm
content: NTSC is a lot more sensitive to phase of the signal than PAL.

author: Gribo
author_rank: Frequent Contributor
timestamp: March 17, 2014, 12:30:19 pm
content: There is a reason why NTSC = Never Twice the Same Color...

author: Holograph
author_rank: Regular Contributor
timestamp: March 17, 2014, 02:04:01 pm
content: Are you using composite video? Where are you pulling the signal from? Is it going through a 75 ohm resistor? And I believe composite is supposed to have the 220uF coupling capacitor as well, though I use RGB because composite can never look anywhere near decent, even with the best comb filter ever. Reason I am unsure here is because I'm in the US and have only worked on US and Japanese consoles, both NTSC. I haven't worked on PAL hardware, but I do know that some PAL consoles have different RGB cables than NTSC. Could be true for composite also. For example PAL SNES shouldn't have the coupling capacitors for RGB according to many people who have worked on those. Might be the case with the Megadrive and composite too. Not sure.BTW there are forums that are more specific for these types of things. Such as AssemblerGames and to some degree shmups.system11.org and such. Not that you'll get no help here (and I mod consoles myself) but you can probably find more elsewhere.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 21, 2014, 09:49:34 pm
content: The "too much" colour is when using both composite, and S-Video, its not an issue when using RGB.It seems like when set to 60Hz, it needs more/less capacitance/resistance on the video out... but doesn't need this when set to 50Hz... unless there is a happy medium to get both settings looking correct...I have a thread on AssemblerGames.. It didn't get any answers, as you'll find that generally people who do mods there just follow what they find online, without actually knowing the reasons for the mods

author: Holograph
author_rank: Regular Contributor
timestamp: March 27, 2014, 03:19:09 am
content: Well, what resistance/capacitance do you have on there? Do you mean it needs more than 75 ohms in 60Hz? Or do you have less than 75 ohms on there? Capacitors?Could always use a pole on your 50/60Hz switch to also switch the output resistance, capacitance. No long signal wires though. Suggest not using the pole of the switch for the signal if possible (sorry if that sounds too basic).http://junkerhq.net/xrgb/index.php/240p_test_suiteUse that, find peak signal level and tune your resistance accordingly, if you care a lot. Not sure what test equipment you have. Can also use it to help tune your screen by eye if that's good enough for you. Though you may not have an EPROM programmer for it anyway (handy though).Also, interesting on AG. It seemed like a better place than that to me. Guess I haven't spent "enough" time there...By the way, the CXA1145 is kinda low quality... Might want a 1645 or 2075 (the 2075 in particular draws a bunch of extra power that must be accounted for). A CXA1645 should actually be pretty cheap. Or... go RGBlots of RGB info @http://shmups.system11.organdhttp://retrogaming.hazard-city.de. You don't NEED to spend the money for an XRGB setup. Even old consoles benefit a TON going from S-video to RGB.http://www.sega-16.com/2006/03/seeing-is-believing-video-connections/.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: March 27, 2014, 11:23:11 pm
content: The composite has no capacitance/resistance added, its just whatever was on the console.Svideo, has a 75ohm, and 220uF on the Chroma, then a transistor amplifier and (probably) 27ohm resistor... can't remember the exact guide I followed.I do also have RGB out on this console, which I use with an rgb scart to ypbpr adapter, but its handy having composite and svideo at times tooI could replace this video IC with a 2075, as the extra current won't matter in my console, as I have a 2amp switchmode regulator in place of the original linear regs

author: Holograph
author_rank: Regular Contributor
timestamp: March 28, 2014, 05:21:29 pm
content: Quote from: AmmoJammo on March 27, 2014, 11:23:11 pmS-video should have the resistor and capacitor on both luma and chroma, and they should be right at the output if the circuit is well-designed. I'm not sure if you have them before the amplifier but it sounds like it from what you've mentioned here. They should be after the amplifier. At least for NTSC mode. Possibly not for PAL mode.A schematic of your mod would be nice. As you probably already know, there are several variants of this mod out there. Some are more resilient than others. Personally I usually just use op amps, which makes for a technically more complex circuit (since op amps are more complex than individual transistors), but a little easier to design since I don't need to worry about transistor beta or using the correct resistors (other than the 75 ohms) etc. Well, that and I just happen to have a better selection of op amps than transistors. I work mostly with audio.