[Megadrive/Genesis model 1&2] Dual Crystal PAL/NTSC color fix restoration.

Hi everyone!

This is a thread about all my researches for a universal PAL/NTSC Color restoration circuit for the Megadrive/Genesis model 1 and 2.

It's all begun with my switchless mod where I wanted to have color on PAL for a NTSC Megadrive.
For european it's easy to slap a 4.43Mhz and have a PAL60Hz console, but for NTSC it's more complicated.

This is a kind of experiemental for me, I had and still have a lot of advances and steps back.
But since some of you ask me to share my work, there you go!



/!\ I WON'T BE RESPONSIBLE IF YOU BROKE SOMETHING!! /!\



PLEASE READ:

• First: I don't have much experiment in electronics, only basic stuff.
• Second: Most of my work is based on already working circuit from Mmmonkey and Zipplet
• Finally: I may have wrong on some stuff, so don't yell at me; Please correct me, share what you know and let's improve this circuit!

What I wanted:

- exact colors on PAL and NTSC, no more derivated signal from main clock
- detection of the videot signal from the console,
- autoswitch the the right Crystal,
- can override the video signal and use the acoordinate Crystal
- can be disabled


How it's working:
- Board is feeded by +5V, Ground, Video Signal signal (like JP3/4 or Pin 7 of the video chip) and Color Signal (from pin 6 of the video chip)
- The board as to be somehow between the video chip and the signals from the motherboard
- It intercepts the Video Signal, compares it from what you want (PAL is Ground, NTSC is +5V) then a 74HC4052 open the right circuit with a 2 pins Crystal and what it needs to work.
- The automatic and override mod need to have the opposite of the video signal, thus there's a signal inverter in the design
- Basic mods are:

- Disabled: do nothing
- Automatic: detects the Video signal and acoordignly use the right Crystal
- Override: ignore Video signal and force what you what with the right Crystal​

Tests:
The tests were run on:

- Model 1 VA3-USA / CXA1145P
- Model 2 VA1.8-USA / MB3154PF
- Model 2 VA1.8-USA / KA2195D

to confirm:

- Model 2 VA1.8-USA / CXA1145M (I need to replace a KA2195D for a Svideo mod, so I'll confirm that soon)


The Results:

So far, the result is mixed.

• The color resoration IS working BUT you need a NTSC/PAL TV.
• It does work with Svideo
• As I suspected, I do have better colors even in NTSC, like when you add a 32x, but it depends of the console and the TV (or the receiver or converter). I'll try to capture that but it's hard.
• Adjusting the "waving" of the final bard with variable resistor is a pain.
• KA2195D seems to not working in PAL, which is at the same time normal from the datasheet, and weird since I thougt I had colors in PAL while debugging my protoboard.
• The NTSC50HZ (NTSC signal, 4.43Mhz Crystal) doesn't work. TV will either don't do anything, don't sync or just shit in their pants

Proto boards:

For Model 1 VA5-6, with remote:


For Model 2 VA-1.8, automatic:



Installation:

Naming convention:
- SRC: Original Video Signal. Ground for PAL, +5V for NTSC
- DST: Signal sent to the Video Chip
- INV: Inverse of the Video Signal
- COMP: What you are using to compare to the original signal
- MB: From the motherboard
- PIN: To the video Chip

Note 1: if MB and PIN are required it means they are not connected: as exemple lift the pin from the video chip, use the pad as MB and the lifted pin as PIN.
Note 2: if you don't want to use the remote and let the board as automatic, you can bridge SRC+DST and INV+COMP.


Installation in the Model 1:
- Remove the CXA,
- Put a DIP machined round holes sockets
- put the board
- place the CXA
- Either plug the remote or bridge the points

Installation in the Model 2:
- Remove the R61,
- add the correct resistor value on the board: if you are using a MB3154, then it's 4.7k, else it's a 10k
- For a override control, you need to left the pin 7 or cut the trace to the it. Before the cut should be "MB7" and after "PIN7"
- Either plug the remote or bridge the points


Exemple of protoboard in the Model 2:


Designs and Layout:

The Remote:



Model 1:


Exemple of layout (Trought hole and SMD):
Beware that the CXA is upside-down between VA3-4 and VA5-6




Model 2:


Exemple of layout:



Variation and adaptation of the original designs:

Model 1 - full auto/no remote:



To be tested:

32X - patch the missing signal:

 

F.A.Q:


- I don't understand what your circuit does.
- After a Region mod on a Sega Genesis/Megadrive model 1 or 2. By changing forcing the region (Pal or Ntsc), the frequency calculated from the main ocsillator is wrong resulting a black and white color. This circuit is design to restore color.


- If the Genesis/Megadrive have the colors in its original region, why don't you just use it?
- The color signal is approximated since it derivated from the main clock of the console. This circuit is using exact frequency.


- Ok, but does it make any change?
- Some TV are sensible to this signal. I did observe a better quality, but it depends of your TV and the video chip of your console. Unfortunately after so much time of working on it I'm not sure anymore .


- Does it get rid of the jailbars?
- I don't have jailbars on the 3 Genesis/Megadrive which where my test subjects, so for now I don't know.


- Is it compatible with the S-video mod?
- Yes it is.


- Does it interfere with the RGB?
- No. But just in case, you can add the remote which can disable the board.


- I'm in US. Does it mean I'll have colors in PAL on my TV?
- You need a TV compatible with the signal your are sending. In Europe, you can send a NTSC signal with a PAL colors: It's called PAL60. But unfortunatly, the opposite (PAL signal with NTSC colors) is not working.


- I made a board with your schematic; It works but I have a "waving" effect on the screen.
- Use the trim capacitor to stop the waving.


- Your circuit is awfully designed/too much complicated: you can do ***** or ******.
- Sure, maybe, share your knowledge and let's improve this board!


- Why are you using this component and that chip ? This is stupid!
- Yes, I'm sorry, I don't have much experiment in electronics, only basic stuff.


- I made a board like you now my console is not working again!
- I'm sorry for your console, but I did myself a lot of proto, and all of my genesis are still alive.




- To be completed -

 

To Do: I'll try to add comparative quality pics and a make a video.

 

Interesting post!

I have the same issue with my Mega Drive 1. In 60hz mode my picture gets off-canter and flickering due to the wrong frequency in the video signal. However I am looking at a more simple solution.

Would it be possible to desolder the PAL crystal of 53.203425 MHz, put it on a board with my NTSC crystal of 53.693175 MHz from aliexpress, and just put a switch between the 5V supply on the crystals and have both outputs go back the the Mega Drive motherboard?

Then I would be able to have a DPDT switch for my region mod that also switches between my two crystals? Seems pretty simple.

I understand that you want to automate your solution, but do you think the above would work?

 

Yes, this is the best solution. In fact, now you can eve use programmable oscillator. I'm currently working on an other way to do this mode. I wish I would have more time.

 

You're referring to the programmable oscillator that Micro developed? That is the best method in my opinion and is a drop in replacement for the main oscillator in the Genesis models.

You can use the same logic of your boards to do the switching and no other caps or crystals/oscillators needed.