MegaDrive 1 CXA1645 mod

I suggested this some pages back as for a cause of EMI and I think it was tested to no success. Once the oscillator stops it doesn't really matter what the output is connected to.

Sure it's in RGB mode?? It's possible your display only "locks" to PAL and NTSC decoding is through software. If there were two crystals for the phase-locked loop, or both were decoded in software the quality would be pretty much the same. One color shouldn't bleed while the others don't unless there's a level or termination problem.

Got a 470uF cap? Maybe try that on composite/C-sync. It sounds like you do have a signal level problem. With a low level or bad time-constant/C-sync rise time the "back porch" area of the signal will be confused for sync which will mean longer sync = the raster lagging behind RGB = shifted to the left. Slowly over time the capacitor will charge up leading to a more stable image but it will never be right.

If you isolated pin 6 and you still have color over composite (RGB not connected) there's something else wrong. And if there's still interference with pin 6 disconnected then I guess you have Bearking's problem.

 

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Mine is shifted to the left but it doesn't cut off the picture. Seeing as I'm using it in 4:3 mode on a 16:10 screen it's not such a big deal but it is right against the left side where the picture cuts off.

 

I should have mentioned Csync from VDP output, which is what is on the AV out of MD2.

I also made a fix for the RGB shit-left problem but it causes 32X incompatibility

 

No picture or are the 32X-graphics misplaced?

 

So, um. Can somebody condense all this technical insight to a newb, and tell me what normally causes jailbars on a MD2 RGB cable?

 

It's not the cable.

On a MD1 it's 99% likely to be the chip mentioned in the topic title. Two of us have tested the theory and come out smiling. Two us haven't :/

As for the MD2, not sure, nobody has mentioned that yet.

 

Do all MD2s suffer from this as well? I don't own one to check.

 

I don't either, in the states here. I've half-jokingly considered trading my Genesis stuff for MD equivalents. Looking at topics like this it's much better regarded than what we got, the system's putrid composite didn't help...

I think when I find time I'll try plugging what I got into a CRT to see if that makes a difference. Could be my LCD's HDMI converter doesn't like MD video signals.

 

use c and y out for svideo i need to get some svideo connectiors and try it.

 

??? The Genesis and Mega Drive (all regions) contain identical components and circuits for each hardware revision and thus are of identical quality. The only thing that changes between all are the jumpers and the master clock frequency in order to be suitable with the region's respective video standard.

?? This thread is about RGB. S-video output works if that's what you wanted to know :shrug:

 

I know. I'd said I wasn't really serious about it. Just that Mega Drive is a far more respected and recognized brand, being what most of the world uses. Makes me feel a little left out being in the deepest Genesis region.

 

My MegaDrive 2 shows a very nice jailbar-free image The MegaDrive 2 doesn't use the CXA1145 video encoder so I don't think any models suffer from this.
The sound is supposed to be better on the MegaDrive 1 though, and I like the MD1 design much more.

 

OK, help me out then. Where can an American buy a reasonable price MD1 RGB cable? If nothing else my SMS will benefit from it.

 

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displaced GFX, and possible sync losses in some situations :/

 

I have just been re-reading this topic, and have had a few thoughts (Which could be totally wrong!)

The Sonic title screen pictures posted by Bearking on pages 1 and 2 which completely bypass the CXA1145 and uses an RGB amp still have some very faint jailbars in the image - this suggest that the jailbars are already present in the blue signal before it enters the CXA1145, and that the CXA1145 simply makes them worse.

We also know that swapping the green and blue signals at specific points in the circuit causes the jailbars to switch to the green signal instead of blue.

From looking at the circuit it seems to me that this is likely interference from the subcarrier signal as previously suggested. However, I dont think the CXA1145 is the problem, I think its caused by the subcarrier and blue signal traces being too close together.

Cutting CXA1145 pin 6 may help, but the subcarrier signal from the VDP is still being sent through the motherboard traces, so could still be a source of interference.

If im right, then we would need to stop the subcarrier signal at its source, so lifting Pin 50 on the VDP 315-5313 might cure the jailbar problem? I dont know if the VDP would be 'happy' with its leg lifted like this though.

Alternatively, you could try rewiring the subcarrier signal away from the blue signal, or you could try the same with the blue signal tapping it directly from the VDP - although be sure to add any needed resistors/capacitors. It might even be a good idea to try replacing the two capacitors on the subcarrier signal C28 and C29 shown in this schematic:-
http://emudocs.org/Genesis/mega2.png
Its well known that Sega used low quality capacitors, from the Game Gear right through to the CDX:-
http://nfggames.com/forum2/index.php?topic=2449.msg17579#msg17579
Certainly something worth investigating.

These are just some random thoughts I had - I take no responsibility if you try them and end up breaking your Mega Drive!

 

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^

Any chance you could post a pic of C28 and a little detail of the best way to disconnect it.

After I'd thought I'd succeeded with the removal of the jailbars on closer inspection I seem to have them although not as strong.

I'm no good at following schematics and I'll probably end up breaking something.

 

Removing C28 is practically the same as cutting Pin 6 on the CXA1145, so I doubt there would be much difference. The subcarrier signal is still passing through the traces near the blue signal, which I think might be the source of the problem.

Another thought I had from looking at the schematic is that the blue signal comes from VDP Pin 29, which is right next to AVcc on Pin 30, so that might also be a factor? but you cant just remove AVcc. It also shows there is a capacitor between AVcc and GND labelled C17, which might be worth investigating...