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MV-1A repair questions

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gorgyrip

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Hi. I managed to get the board to boot the neodiag bios.
Now i'm having the 3 following problems (among others):
1. backup sram is missing. In an online picture i can see that the sram is LH52B256N-70LL. Can i replace it with HSRM20256LM. I xould't find a datasheet, but on console5 it says it's a pseudo static sram with refrash on oe. If not, what's the slowest sram i can use?
2. CR4 and CR6 are dead. here's the schematic: https://wiki.neogeodev.org/index.php?title=CRE401
a) what value should I use for the caps? 100nf is ok?
b) About the 300R. I measured a working part and on my pcb is about 200R. Can I use 180R or 360R?
3. Use the retail bios and no cart i get this error: COLOR RAM BANK0 ERROR. address 00400000 write 0000 read 1F00
Bank 0 is RAM7 or RAM8? I hope it's not the neo-buf at GA4.
 
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Thank you so much! Those links are really useful.
I don't have a supergun. I just used wires on a jumma connector for video and power. And a few resistors to combine RGB so i could get a black and white image on my composite tv.
Since i didn't have bram and no controller, neodiag was stuck at the bram error. I used 100ns for bram. Now the neodiag says:
Palette address (A0-A7):
address: 400000
actual: F800
expected: 0000

I replaced RAM7 and the error is the same. I will also replace RAM8
I replaced the palette neo-buf with the one from GA11. It made no difference.

UPDATE: you were right. It was RAM8. Neodiag says: All tests passed.

But, with original bios (no cart, no cart slot pcb and no GA11): Z80 ERROR. Any tips?
With unibios, super sidekicks boots, but all the graphics are corrupted. Only text is displayed properly.
And when the game boots with the txt mx 330 mega, the big neo geo logo is missing. there's only the the C in a circle.

I replaced SM1 with 49f040. I hope i got the right file. No change.
I replaced Z80 with one that i believe it's from a game gear so it should be ok. No change.
 
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For the Z80 error I would check for activity on the z80 with a logic probe to make sure its alive and doing stuff. Since you replaced the z80 and sm1, that leaves the z80's ram and the ability for the z80 and 68k to talk to each other. I don't know what the data path is between the z80 and 68k on MV-1A boards.

For the corrupt graphics I would start here https://wiki.neogeodev.org/index.php?title=Graphic_glitches it should give you an idea of where to look, but for sure you are having issues with the C roms.
 
I will try tomorrow to find a 2k sram.
On the Croms I'm lost. In my case it looks more like in the lorom picture.
I made a neodiag cart. Cart or no cart inserted, I get this:
z80 slot switch ignored (SM1)
SM1 otherwise looks unresponsive.

SM1 OE an CE go to NEo-IO and NEO-D0. I hope it's not one of those chips faulty.
 
Z80 SLOT SWITCH IGNORED (SM1)
SM1 OTHERWISE LOOKS UNRESPONSIVE
IF MV-1B/1C: SOFT RESET & HOLD B
PRESS START TO CONTINUE

Did you press start? pressing start will force the switch from the SM1 rom on the motherboard to the M1 rom on the cart.
 
:)) No.
With no cart i get a solid color screen.
With the neodiag cart i get:
z80 dead / errored / com issue
expected: c3
actual: 00

There's a neo-buf and 2 x 74hc273 related to z80. i've used the neo-buf fromnear the cart connector. I don't have 74hc, so I used 74LS. I also replaced the 2k sram. Nothing changed.
I dumped the original sm1 rom using a so44 socket. I had to bend the pins of the rom to make it wider and after a few tries all the pins were connected. And the dump is good.
I did the same for the LO rom, but no matter how much i tried, the programmer said that A16 and CE are not connected. So I flashed 000-lo to a plcc32 and now i have spider looking ic and no more graphical errors!!! (as much as it can be seen in a black and white image)

So now the only issue it's the z80 error and it's driving me crazy. Most likely is the neo-d0.
 
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Congrats on getting the graphics issue resolved.

Do you have audio hooked up to the board? If the diag cart is able to run its z80 code it maybe trying to signal an error code via beep codes. Otherwise it could end up being the NEO-D0 or maybe a bad trace.
 
I don't have audio connected, but i will connect it. When/if i will find another board i will swap the NEO-D0. There's still a low chance that the neo-buf is not 100% functional. Or maybe the replaced z80 is dead too, but i doubt it.

I connected audio, but i've noticed that the pot on the pcb is missing. One pin is gnd so I bridged the other 2. I don''t get any sound or beep during testing.

UPDATE: I removed NEO-D0 and connected in a MV-1FZ and it passed all the tests.
 
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Update 2: I tested on 1FZ the z80 that was on 1A and it's good.
I tested the z80 from the game gear and it's dead.

UPDATE3: now i don't get any more errors in neodiag. I guess the problem was one of the neo-buf because i've swapped them around.
But i still don't get sound. Is my approach of the missing pot correct? On the audio line i have a 10K resistor
I believe it's the caps. I can get sound from where the pot was. I need to find some 470uf caps.
It was a cap with shorted legs :))
Btw, what is the value of the pot?
 
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