Troubleshooting a MV-1FZ with the infamous "Z80 ERROR"

Hello, I've already posted about this in the introductory section, but for the sake of completeness I'll try to recap everything important in here, and hopefully the experts in this forum will be able to shed some light on the matter.

A few days ago I recovered two MV-1FZ boards in non-working state from a closing arcade repair/reseller shop.

Both of the boards were stuck in watchdog loop. I've been able to fix the first one by repairing a trace and straightening a bent pin which was shortcircuiting a line in the cart riser.

The second one also had an interrupted trace between A8 bios pin and the cpu (corroded platethrough under the BIOS). Patching it took it out of the watchdog loop, but now the board gives me a Z80 ERROR.

Here are the tests i did:

• Burnt a diagnostic bios, all test passed (couldn't test the Z80 without the proper cart, though)
• Burnt an UNIBIOS 3.0 image and put it in the 1FZ: It plays games just fine... without any audio
• Tested Z80 connections using this table
• Tested 6116 and SM1 connections using the tables found at Jamma Nation X
• Removed the cart riser during tests to avoid potential problems related to that part

Any idea how can I proceed any further? I ordered a diagnostic cart from Jamma Nation X, to help me debug the Z80, as I'm starting to suspect a defective component (I hope it's not one of the proprietary chips...), but I still feel I cannot completely exclude interrupted traces. Anyone has an idea of what other chip connections I should check? Is there a YM2610 connection table available? Maybe tomorrow I'll double-check the connections again, just to be sure I did not miss anything...

BTW, the board does show some darkening of traces and slight signs of corrosion. The problem was NOT caused by the battery, which was still sane (I removed it anyway, as it is a timebomb).

Advices? Ideas? Things I should check?

I desoldered and socketed the SM1 chip.
On the PCB under it, there is the following part number: 23C1000-SM1. Is it compatible with a 27C1000?
I tried reading it in my GQ-4X programmer and the result differs from the sm1.sm1 file used by MAME in that the MAME dump replicates the content after 64k, my dump has data only in the first 64k, while the rest is empty.

Can I consider the SM1 chip as sane?

You will also need to check connections to the Neo-D0, YM2610, Neo-C1 and possibly the LSPC2.

Good to know there is still hope before replacing components.
Are there connections tables available for those chips too or all I have got is on neogeodev wiki?

You need to make the diagnostics cart so you can see what is going on with the Z80. You can use any MVS CHAR board as long as it has a Neo-ZMC1 and S1 chips on it. Just replace the M1 with the one you download.

Xian Xi said:

You need to make the diagnostics cart so you can see what is going on with the Z80

Click to expand...

I think I don't have the correct EPROM available right now (the M1 is a 27C1001, right?), so I ordered a diagnostic cart from Jamma Nation X (which, if I got it right, is you? ).
In the meantime I can go on checking connections. Fixing this machine is not urgent, I'm working on it to learn something.

The SM1 with only 64KB is normal. On your board the A16 pin should be grounded so the second 64KB bank is never used. Z80 can't access that much without extra help anyway. As long as that first half is good then your SM1 is fine.

hkz said:

I think I don't have the correct EPROM available right now (the M1 is a 27C1001, right?), so I ordered a diagnostic cart from Jamma Nation X (which, if I got it right, is you? ).
In the meantime I can go on checking connections. Fixing this machine is not urgent, I'm working on it to learn something.

Click to expand...

I sent you an email, did you get it?

Xian Xi said:

I sent you an email, did you get it?

Click to expand...

Nope, nothing in my INBOX, not even in spam... sending you a PM

smkdan said:

The SM1 with only 64KB is normal. On your board the A16 pin should be grounded so the second 64KB bank is never used.

Click to expand...

OK, I confirm that pin 24 (A16) is properly tied to ground.

So the SM1 is sane. I also did a binary difference of the first 64KB with the MAME rom, everything checks.

So I checked YM2610 A0/A1 lines mentioned here.
6 and 4Mhz clock output from LSPC2-A2 pins mentioned here.
Everything Z80 related in NEO-C1 mentioned here.
Also checked NEO-D0 connections to YM2610 and Z80 listed here. The only weird thing (but since it's the same on my working 1FZ I guess it's normal) is that NEO-D0 D0-D5 lines are tied to ground.

I guess I'll have to get a logic probe and/or wait for the diagnostic cart....

Bump. While the diagnostic cart is still far away, I received my logic probe: any advice on what I could check that can be indicative of the Z80 status?
For now, I probed the reset line, which seems to work: the machine starts with the line low, and half a second later the line is brought high.

I checked the lines for the Z80 and SRAM (In the Z80 error screen). P means the line is pulsing

Z80 (84C00AM-6)
1 (A11) - P
2 (A12) - P
3 (A13) - P
4 (A14) - P
5 (A15) - P
6 (CLK) - P
7 (D4) - P
8 (D3) - P
9 (D5) - P
10 (D6) - P
11 (VCC)
12 (D2) - P
13 (D7) - P
14 (D0) - P
15 (D1) - P
16 (/INT) - H
17 (/NMI) - H
18 (/HALT) - H
19 (/MREQ) - P
20 (/IORQ) - P
21 (/RD) - P
22 (/WR) - P
23 (/BUSACK) - H
24 (/WAIT) - H
25 (/BUSREQ) - H
26 (/RESET) - H
27 (/M1) - P
28 (/RFSH) - P
29 (GND)
30 (A0) to 40 (A10) - P

Sound RAM
1 (A7) to 8 (A0) - P
9 (IO0) - L
10 (IO1) - H
11 (IO2) - L
12 (GND)
13 (IO3) - H
14 (IO4) - L
15 (IO5) - L
16 (IO6) - L
17 (IO7) - L
18 (/CS) - H
19 (A10) - P
20 (/OE) - P
21 (/WE) - H
22 (A9) - P
23 (A8) - L
24 (Vcc)

I also checked pin 49 (2610CS), 48 (2610RD), 47 (2610WR), 43 (SDZ80W) and 41 (SDZ80R) on the NEO-D0, all were HIGH.

Check the IRQ line on the 2610.

OK, first thing tomorrow! I just put everything away (0:15 Here )

YM2610 logic status at "Z80 ERROR" screen.
NC means i've got no signal at all on that pin.

1 (GND)
2 (SDD0) to 9 (SDD7) - P
10 (RAD7) to 17 (RAD0) - L
18 (GND)
19 (VCC)
20 (RMPX) - P
21 (/ROE) - P
22 (RA9) - L
23 (RA8) - L
24, 25 - NC
26 (AGND)
27 - NC
28 (AVCC)
29 (SH1) - P
30 (SH2) - P
31 (OPO) - P
32 (GND)
33 (/IC) - H
34 (GND) - NC
35 (RA20) to 38 (RA23) - L
39 - NC
40 (/TEST) - H
41 (PA8) to 44 (PA11) - L
45 - NC
46 (/POE) - H
47 (PMPX) - L
48 (PAD0) to 55 (PAD7) - NC
56 (IRQ) - H
57 (/SCS) - H
58 (/WR) - H
59 (/RD) - H
60 (A0) - P
61 (A1) - P
62 (VCC)
63 (PhiM) - P
64 (PhiS) - P

I wonder if the /HALT line stuck high on the Z80 is normal: shouldn't the processor be stopped and waiting for an interrupt before starting a game?

Halt is active low so it's working normal. Reset is also working properly. It's most likely bad ram or a missing connection unless you checked everything already.

I'm pretty sure I checked everything directly connected to the Z80, SM1, and the sound ram. I will recheck though. Anyway I can rewire another SRAM chip as a test, waiting for a proper replacement. Any advice on what else to check? YM2610 and D0 connections which could be at fault?

Check these:

http://www.jamma-nation-x.com/jammax/tutorials.html?show=mv1z80

OK, I'll recheck those connections. If nothing shows up I'll wire a temporary ram adapter on a breadboard, so I can see if replacing the ram changes anything..

Checked every connection listed at the link and everything is fine. Resistance is ranging between 0.8 and 0.4 OHM for every connection.
I'll try to wire up another SRAM chip and see what happens. I have some 6116s around, but I fear they might be too slow (150ns access time), and they're the fat variant. I also have other, bigger and newer, SRAM chips i can try (grounding the additional address lines).

Tried replacing the 6116 ram chip with another (rewired) one. Same Z80 ERROR. I hope the diagnostic cart will shed some light on this, because I'm at a loss of ideas.
Just to recap:

When I recovered the board, it showed a garbled screen at startup. Apparently stuck in watchdog.
I found and patched a dead trace between the cpu and bios (A8).

The board now shows a Z80 ERROR, even without any cart inserted.
1) Checked Z80 connections to SM1, 6116 ram, NEO-D0, NEO-C1, YM2610. OK.
2) Desoldered the SM1 bios and read on my GQ-4X. Dump is OK.
3) Desoldered and tried to replace the 6116 SRAM chip with one of a different kind, 20ns access time, rewired to match the 6116 pinout (additional address lines grounded)
4) Probed the Z80 reset lines. When the machine is powered up the line is LOW, it switches to HIGH half a second later.
5) Probed the YM2610, Z80, 6116. Results can be found above. Nothing obviously abnormal
6) Tested the board with diagnostic bios, without Z80 diagnostic cart. All test passed.
7) Tried the board with UNIBIOS 3.0 . Games run fine, but without any audio.

I finally received the diagnostic cart from JAMMA NATION/Xian Xi, and I used it to test the 1FZ.
The first screen I get when i start the board with cart inserted and pushing D says

Z80 SLOT SWITCH IGNORED (SM1)
SM1 OTHERWISE LOOKS UNRESPONSIVE

Click to expand...

When I push START to continue I get to a screen which says

Z80 DEAD / ERRORED / COMM. ISSUE

EXPECTED: C3
ACTUAL: 00

Click to expand...

I previously desoldered and read the SM1 rom with a programmer: it turned out OK.

Ideas? What are the chances that the Z80 is actually dead?

Wow you have already checked pretty much everything I would have suggested. Since you said you picked up two of them, maybe it would be worthwhile to swap out the SM1 for the one in the other 1FZ? Even though the programmer says it is okay, it may not be responding fast enough or may have some other weird glitch that the programmer does not pick up on. As for the Z80, your logic probe readings suggest it is not dead, but it is overclocked at 4MHz. Really it should be a Z80A, but SNK decided to use cheaper Z80's and overclock them instead. Given this overclocking, it is surprising how seldom the Z80's do go bad. Nonetheless it may be a good idea to try replacing it with a Z80A. One last idea is the 24MHz crystal. Crystals are one of the more fragile components and a good impact can damage them and cause them to oscillate at a frequency other than what it should be. If you don't have any way to check the frequency it is actually giving you, it might be worth replacing that 24MHz crystal.