MegaDrive 1 CXA1645 mod

Sorry I don't follow you. Should I solder the cut pin 6 to GND or just forget about this mod and restore the RGB amp?

EDIT: GND to pin 6 doesn't help.

Fudoh on shmups forum have tried this on two model 1's and it didn't help on either of them :/

 

Saw this site, thought it might have a degree of interest on the RGB issue?

http://www.retrocorner.net/index.php?Mods_/_repairs:Megadrive_2_RGB_cable_modification

 

You lifted/cut pin 6 right? Then you don't have to do anything because the oscillator causing the interference stops.

Here's what could be going on:
Pin 6 is the input to an inverter/NOT gate, pin 5 is the inverter's output which is right next to the blue input. Normally pin 5 and 6 go to a crystal which makes a feedback loop which causes the inverter to toggle at 3.58/4.43 MHz. The inverter is digital so it outputs a square wave which has high frequency components @ 10, 18, 21 MHz etc at a higher voltage than the small video signal. To save money the MD uses a single crystal oscillator that controls all timing in the console, it divides the 53ish MHz down to 3.58/4.43 and feeds the '1145's pin 6 (input) with that. Even though there is no crystal, pin 5 (output) still toggles, but I don't think it has a load to absorb any energy so it becomes a very short range transmitter.

Another possibility is that the two signal wires (PCB traces) run in parallel (as is definitely the case with the blue and composite output traces). Two conductors have a very small capacitance across them, but it's enough to have two signals couple if they are close enough and the frequency is high enough.

Another possibility (the last I think) is that there is significant switching noise from the oscillator. When some types of logic switch from one state to another they draw significant current for just an instant. All chips are wired to the power rails in parallel and same with components inside a chip. All wires have some inductance to them and when the chips switch levels and current goes through the wire, the inductance causes the component's local power rail to bounce up and down (it's local because it's separated from the main one by resistance, capacitance and inductance based on the wire's high frequency properties, length and composition). This may affect one subcircuit, one chip, or multiple chips, but basically circuits on that local rail may fail (or couple the rail jitter as interference) if they use the rails as a voltage reference which many circuits do. For this reason it's common for all digital chips to have "bypass" or "decoupling" capacitors very close to every chip's power rail. They act as a local energy storage and low pass filter to keep the their local voltage stable and noise from other chips out. I haven't checked but I don't think the MD has a bypass cap near the RGB encoder, though it probably should.

 

Hell of a post. Wish I had the wherewithal to understand it all.

Does this mean that lifting pin 5 on the CXA could fix Bearking's continuing interference issues?

 

No because pin 5 is the out of the in which is pin 6. If nothing is going in nothing is coming out...

...I think.

I'm think I'm starting to understand the issue, and also beginning to wish I had knowledge like Calpis does!

Seems weird that it works for some of us and not for others...cable issue as shown in Bacteria's post?

 

Yes exactly, if the oscillator is not running it's held in a single state = no AC = no interference

 

If that's the case then cutting the trace leading to pin 6 as far back as possible might work? But if this is the case the jailbars should have been improved some just by cutting the pin, right? It looks exactly the same as before.

I'm back to using the RGB amp btw

 

Cutting pin 6's trace wouldn't matter, it's too far from pin 4. If pin 5 (the output) actually went somewhere which I assume it doesn't, then that could be the trace possibly in parallel with pin 4.

 

Makes sense

 

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Just did this to the other Megadrive I have. Worked fine there as well.

 

Strange both EU ones? Both Fudoh's and mine are JAP.

Anyway after a lot of trial and error I think I have found the best combination of resistors to use in the RGB amp (if your MegaDrive doesn't benefit from cutting pin 6). This gives much better colors than before without colorbleeding or color changing

R,G,B should be tapped directly from the SEGA 315-5313 chip.



This gives a rocksolid picture on my XRGB-3 setup.

Again default > amp > default... :

 

Yes, both EU. Is Fudoh having the same problems? Can you link to where he's discussing it?

Looking good, anyway. I'd say brightness is a touch high this time but honestly if you'd have shown me the pictures of the RGB amp in isolation from the originals I wouldn't have known.

 

We have been discussing it in private messages

 

Start talking, Fudoh

Your input is always valuable. It was that page on scanlines that really kicked me into high gear on the whole RGB thing.

 

Quick question:

What are the model numbers of your machines - so we can figure out whether there's a difference there that could be affecting the success of this.

 

Megadrive PAL-l
Model no. 1600-05
serial 120113353

The other one is the same, serial 120029447. Both have the EXT port.

 

Motherboard version would be useful as well. They're all different even if the cases and features are identical.

I'll post mine later, can't be bothered to walk upstairs either.

 

My JAP says:

5402: VA5
5433: VA6

Yes it says both.

 

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