Neo Geo MVS - Z80 Error

I bought a Neo Geo MVS on ebay recently, one that I knew had a Z80 error because it was going for what I thought was cheap. (Looking back it possibly wasn't) But I also wanted a project that I could learn from.

I now have my supergun working so I can start with fixing the board itself now, I was wondering if anyone could please give me some advice on where to start:

• The board is a MVS-1AX (soldered bios chip but when I get my desoldering station fixed I'm going to put a socket on there)
• When powering up the screen will look very much like the first picture here: http://www.assemblergames.com/forums/showthread.php?51765-Glitchy-graphics
• After that it will show 'Z80 ERROR' or, at least I think it's meant to be that, the middle of the text seems to be missing (squashed) so it looks more like Z0O
• Flicking all the DIP switches down then up doesn't fix this.
• The system will run a RAM test happily for hours without any issues.

My thoughts were to either replace the Z80 chip (I can't see any broken traces) or to prioritise getting the unibios and bypassing the Z80 check to see if the system works otherwise.

Replacing the Z80 may be difficult, the one on the board seems to be a compatible chip that's a lot smaller than a normal Z80, I wasn't sure if it was possible to connect up a larger IC using leads to the header on the board.

Does anyone else know of a better course of action for repairing this board please? Are the video issues I've pointed out indications of a possible extra fault somewhere?

Thank you

 

Looking at http://www.cpu-world.com/CPUs/Z80/Toshiba-TMPZ84C00AM-6.html it seems that I've located the chip, it runs at 6 MHZ and I have heard that the original Z80s in the MVS boards were 4 MHZ chips overclocked to 8 MHZ. Would it be worth replacing this chip with a TMPZ84C00AM-8 (http://www.cpu-world.com/CPUs/Z80/Toshiba-TMPZ84C00AM-8.html) which runs at 8 MHZ?

Sorry to keep bombarding this forum with questions, this is a hobby I'm just getting in to and I'm trying to learn as much as possible while treading lightly so I don't bork my board.

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EDIT

Just done some voltage measurements across the +5V pin into the Z80, only getting 4.6V. Turns out that my PSU is only supplying 4.6V in at the pins (11.96V at the 12V pins) from what I've read this could be causing the graphic issues.

Now could this be my supergun or my PSU? I have one wire attached to each pair of top and bottom 5V pins, could this be causing the drop? Could it be the 10A fuses that I have put on the 5V line?

I'm thinking that it's the PSU but I didn't know if someone else could give some advice please?

 

If you've not got enough power on the +5V line, turn your PSU up. Of course, if you're using an ATX PSU... well, this is exactly why you shouldn't

The tolerance of the chips would most likely be 10% though, and 10% of 5V is 0.5V... so 4.6V should be fine. That said, I do like to get boards running as close to 5V as possible, to eliminate any doubt.

Not all Z80s are drop-in replacements. I can't remember what Z80 was in the MVS - if it was 4MHz, it's a Z80A.

 

Yeah I did a bit more digging and it turned out that a 4MHz chip is fine, it seems like the one I ordered is a drop-in replacement though (same line of chips, just faster clock rate).

The graphical errors are improving when I up the voltage on the 5V line so I'm assuming that it is the voltage causing that for now, going to get a new PSU soon

I'm hoping it'll just be a broken track but that's not looking likely now...

Thanks for replying

 

Think I fixed the z80 error, replacing the bios with unibios has cured the graphical mess on boot and the game starts now. Haven't hooked up a speaker to test the sound yet though. Still have the wrong resolution on my screen and it seems and the sprites aren't showing properly (large vertical bars behind them all). Still, progress has been made

 

The Z80 on the MVS boards was the right speed, it was the DIP ones that werent.

I have had to replace quite a few of those MVS z80s, so you can expect it could just be a dead chip and no damaged traces.

 

Z80 error is a diagnosis message from the boot ROM which mean the system was unable to finish the handshake protocol it does before booting the cartridge code.

Using Universe bios will simply bypass the handshake test which happens before the control is handed to code into the cartridge. So you didn't fix anything in the end. You won't have any sound if the Z80 is faulty. Also there's a (slim) possibility that the SM1 ROM is faulty, then that would stop it from booting without it having any fault.

Unfortunately blown Z80 is a common fault on Neo Geo hardware. Since it runs at 3.57Mhz a Z80A should work fine.


By the way, SM1 means "System M1" and SFIX mean "System FIX". SFIX is the ROM with font for the FIX layer during diagnosis mode.

Even if I am wrong and it does run at 4Mhz, a Z80A is *actually* rated for 4Mhz so it is NOT working on the wrong frequency.

Even the 6Mhz Toshiba SOP Z80s do blow fairly often so the clock IS NOT the issue here at all.

 

I thought universal bios only bypassed the checks if a certain button was pressed? Either way the old bios must have been faulty because I don't get the green blocky screen on boot now (from what I know the boot screen is in the bios chip)

I'll try it with a speaker attached tomorrow and see if I can get any sound

 

Actually, you can use a button combo to force it to skip checks but it save that setting on the nonvolatile memory so you can keep the checks off if you want.

I don't remember well but I think that "blocky green screen" seem to be how the SNK BIOS do some of it's hardware tests. Razoola had it removed from his BIOS and he use his own test code I believe.

 

The thing is, the boards with the "fault" arent using a Z80A, they are just using a Z80 which has a rated speed slower than 3.57 or 4mhz. The Z80A (or B, C or E) will work fine. The silkscreen even says Z80A, but they didnt put one in.

But your other point is quite right - I have replaced many dead SOP Z80s that were on MVS boards which are rated 6Mhz

 

I've NEVER seen a Neo Geo with a 2Mhz Z80. I don't think a 1975 Z80 (non A) chip will even work reliably enough at 4Mhz to pass a quality check on a manufacturing plant...

 

Its all over the neogeo forums and channelmaniacs repair logs (as well as others).

http://www.neo-geo.com/forums/showthread.php?224316-Z80-error&p=3313437&viewfull=1#post3313437
http://www.neo-geo.com/forums/showt...es-on-1-slot&p=2675451&viewfull=1#post2675451
http://newlifegames.net/nlg/index.php?topic=152.85;wap2
http://newlifegames.net/nlg/index.php?topic=152.60;wap2

(same issue on CPS1):
http://newlifegames.net/nlg/index.php?topic=155.10;wap2

 

That's a 4 slot and the Z80 in it is a Z80A. Z0840004PSC is a 4Mhz part. There might be a buffer clash or a power issue somewhere on these boards but they never shipped with a wrong part in them. Because it says "Z80 CPU" under the part designation people automatically assume it's the wrong part.

 

Well I connected a speaker and sure enough all I'm getting is a low-level hissing sound. Damn.

I can't see any problems with any traces so it probably is the chip itself, the replacement should be arriving soon.

Does anyone have any idea about what could be causing the graphical issues please? (Vertical lines behind sprites and overscan on the right-hand side)

 

Seems this is what happens when you take the word of someone who apparently know what they are doing as correct.

Checked on the 6 slot ive got here and it seems you are quite right.

 

If you have an EPROM programmer, you can burn a Neo Diagnostics BIOS set. You'll need a spare cart for the CHA board.

 

I can program eproms (I think, haven't tested it yet). I have puzzle bobble as my only game, would I be able to remove the eproms and use this?

Thank you

 

You're supposed to stuff the diagnostic ROM on the BIOS socket and test with the CHA board of the game connected to the slot. I don't know why it has to be only the CHA board, though ... I would first try the whole cart first.

 

Aren't you meant to also put a different programmed eeprom into the CHA board in order to test the Z80? I can do the bios part no problem.

 

You know that's what the SM1 is supposed to do when NO CART is connected, right ? It's there to give the SNK bios means to test the Z80.