Help me identify this megadrive mod.

[Murphy245]

I opened my model 1 pal megadrive to clean it,and found that it had some modification done to it,i dont have any us/jap games to test it though.

The cartridge slot hasnt been widened though so im a bit puzzled,is it region free but can only play US games because of the slot?

Thanks for any help.

http://img509.imageshack.us/img509/6871/dscf0467w.jpg

Here one of the pins has been bridged with a resistor.

http://img515.imageshack.us/img515/6212/dscf0469i.jpg

[JDB]

I think that's called a change mod. It's done by SEGA at the factory. All the printed circuit boards are the same, but sometimes they have different (but equvalent) chips they can use, and when they use these different chips they need to do a little rewiring, which is why they print these easy access Jumping Points.

You can see on the first pic that the Toshiba chips (I think they're RAM) are skinny, and there's a row of holes for an alternative wider chip.. it just so happened on that day that the wide chips had run out. so they fitted the skinny ones and put a few jump leads to make them work.

[Murphy245]

I think that's called a change mod. It's done by SEGA at the factory. All the printed circuit boards are the same, but sometimes they have different (but equvalent) chips they can use, and when they use these different chips they need to do a little rewiring, which is why they print these easy access Jumping Points.

You can see on the first pic that the Toshiba chips (I think they're RAM) are skinny, and there's a row of holes for an alternative wider chip.. it just so happened on that day that the wide chips had run out. so they fitted the skinny ones and put a few jump leads to make them work.

Hm thats very interesting,i thought it was a little strange because it looked like it hadnt been taken apart before.

[Ace]

Huh... none of my Genesis Model 1s have a resistor on the 68000. But those wires are there due to a design flaw in the board. Instead of scrapping the board, Sega used jumper wires to correct their mistake. Even if there would be the wider RAM chips, the system would still need those wires.

[Murphy245]

Huh... none of my Genesis Model 1s have a resistor on the 68000. But those wires are there due to a design flaw in the board. Instead of scrapping the board, Sega used jumper wires to correct their mistake. Even if there would be the wider RAM chips, the system would still need those wires.

You mean i got some sort of reject board? or sega made a mistake with manufacturing and had to correct their mistake on every one?

[Chilly Willy]

It's pretty rare that mistakes aren't made on PCBs. You'd be surprised at all the electronics that come with wire fixes. Sometimes the fixes are made by changing the pinouts of custom ASICs. That's what I had to do with a board I had made some time years back.

[omp]

What everyone else has said that is a stock board, all the MD1 boards (from that series of board TMSS no EXT port) I have seen have that resistor from the Z80 chip (maybe a PAL thing). I think they do the jumper wires so as to avoid making multi-layer boards as they cost more to do vs simple jumper wires. I think Sega makes there boards very "industrial" vs Nintendo which has very neat packages etc, but the Sega stuff seems to outlast Nintendo (in my experience) stuff due such a sturdy build.

[kool kitty89]

Huh... none of my Genesis Model 1s have a resistor on the 68000. But those wires are there due to a design flaw in the board. Instead of scrapping the board, Sega used jumper wires to correct their mistake. Even if there would be the wider RAM chips, the system would still need those wires.

My first VA3 with the motorola CPU has a resistor bridging pins 6 and 16.

[m68k]

Sega was not the only one to do this. This was a very common practice in the electronics industry. Today, its a bit more rare but still happens.
Here is a story. In 1991, Apple released the new Classic II. It was a B&W Mac with a 68030, 4MB RAM, 80MB SCSI Drive, all for about 1000$. It was based off of the LC II motherboard. In the shrink process, apple removed 2 needed traces. The error was detected only after a few shipped (it only caused crashes) and corrected with a bridge wire. After they finished the first run of the PCB, they fixed the error, eliminating the need for the wire.
My Genesis VA5 also has the wires, so yours is not alone. Seems normal.

[Jorge Nuno]

Ohh... The jumper wires. 2 of them can be removed (the ones coming right from the 315-5433). I guess sega was lame and left them for a reason I can't understand. The other one (the longest) is needed, as there is no routing on the board for it.

And the resistor on the 68000 (not the z80, omp), seems to be a PAL only addition on VA6 boards. It's there to reduce the /AS signal undershoot. Maybe the NTSC ones have less undershoot due to a slight frequency not being in ressonance with the board trace.

[Devil N]

The PAL VA4 has a resistor on the 68000 as well. Case in point:

[Jorge Nuno]

Oh, so it's not a Va6-only thingy... weird

[kool kitty89]

And the resistor on the 68000 (not the z80, omp), seems to be a PAL only addition on VA6 boards. It's there to reduce the /AS signal undershoot. Maybe the NTSC ones have less undershoot due to a slight frequency not being in ressonance with the board trace.

One of my NTSC VA3s has a resistor on AS as well, albeit in a different configuration. (rather than in line with the pin and board trace, the resistor bridges AS with GND -pin 16)
BTW, what does AS stand for?

[Chilly Willy]

One of my NTSC VA3s has a resistor on AS as well, albeit in a different configuration. (rather than in line with the pin and board trace, the resistor bridges AS with GND -pin 16)
BTW, what does AS stand for?

Address Strobe. It's an active low signal output by the 68000 to indicate a valid address is present on the bus.

[TmEE]

none of my PAL VA4's have had a resistor on !AS...