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MV1F no graphics or sound, only text. No failures from Neo Diagnostics BIOS and cart. Possible bad L0 ROM?

Kujako

Kujako

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Seeing if I can clear out another Neo Geo repair. For this one, I get no/garbled graphics. Only text displays. So for example, the unibios boots but only shows the following...
1669679609188.jpeg

Likewise when the Neo Geo logo shows, or a game is loaded (metal slug bootleg I use for testing).
1669679654314.jpeg1669679680738.jpeg

I can go into the self test, and color bars etc all look fine. But I get no audio from the audio test. I've also run the Neo Diagnostics BIOS with the Z80 tests and everything passes. So thinking this is a bad L0 ROM (or is it Lo ROM?) with possibly unrelated sound issues. But, not sure where that even is on a MV1F. Any assistance is appreciated.
1669679953577.jpeg

Edit: found it... can't see anything wrong from a visual inspection.
1669680218230.jpeg
 
Removed the L0 ROM and added a socket. Put the old L0 ROM back in, and now I get audio (still no graphics). Which is "interesting". Question now is, where can I find a replacement L0 ROM?
IMG_1346.jpg
 
L0 in DIP format can be taken from any other board. MV1A/B/C have it in SMD format.
 
Ryoandr said:
L0 in DIP format can be taken from any other board. MV1A/B/C have it in SMD format.
Click to expand...

Yeah I have a 1FZ with issues I was thinking of grabbing it from. But would prefer to source another if possible. I can use a 27c010 as shown [here], but I'm lazy. Also a couple up on the ebays, but for exorbitant prices.
 
Swapped the TC531000CP L0 ROM between the two boards, and that wasn't it. The 1F still has the same issues and the 1FZ still works (outside of a known boot failure that UniBIOS bypasses).

So back to square one. Anybody have any ideas? May be a bad trace from the L0 ROM, will have to see if I can find a schematic and trace each pin.
 
L0 has a mask rom pinout, but is only 64k and 28 pins, the easiest eprom replacement is 27C1000 / 27C301(32 pins) , with rom doubled and remaining floating pins appropriately tied to vcc or ground.
L0 is also connected to a 74LS245 that can also go bad (1F has it in DIP, 1FZ is in SMD).
PBUS lines check would be a good start, it connects several chips and also goes to the cart port, but it's 24bit wide so it'll be long to check...
 
Ryoandr said:
L0 has a mask rom pinout, but is only 64k and 28 pins, the easiest eprom replacement is 27C1000 / 27C301(32 pins) , with rom doubled and remaining floating pins appropriately tied to vcc or ground.
L0 is also connected to a 74LS245 that can also go bad (1F has it in DIP, 1FZ is in SMD).
PBUS lines check would be a good start, it connects several chips and also goes to the cart port, but it's 24bit wide so it'll be long to check...
Click to expand...

May take some staring cross eyed at [this] before I can determine what pin should go where, but I'll give it a try.
 
I can program an EPROM for you for just the cost of the part and shipping. PM me if interested.
 
ShootTheCore said:
I can program an EPROM for you for just the cost of the part and shipping. PM me if interested.
Click to expand...

No need, but thanks. As I posted, I swapped a known good LO ROM over and the issue persisted. So the problem is elsewhere if still likely related to the L0 ROM.
 
Kujako said:
Seeing if I can clear out another Neo Geo repair. For this one, I get no/garbled graphics. Only text displays. So for example, the unibios boots but only shows the following...
1669679609188.jpeg

Likewise when the Neo Geo logo shows, or a game is loaded (metal slug bootleg I use for testing).
1669679654314.jpeg1669679680738.jpeg

I can go into the self test, and color bars etc all look fine. But I get no audio from the audio test. I've also run the Neo Diagnostics BIOS with the Z80 tests and everything passes. So thinking this is a bad L0 ROM (or is it Lo ROM?) with possibly unrelated sound issues. But, not sure where that even is on a MV1F. Any assistance is appreciated.
1669679953577.jpeg

Edit: found it... can't see anything wrong from a visual inspection.
1669680218230.jpeg
Click to expand...
It seems to me that it's a problem with the PCK1 clock from the LSPC for the C ROM latches. So check the traces from that pin.
 
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alexcom said:
It seems to me that it's a problem with the PK1 clock from the LSPC for the C ROM latches. So check the traces from that pin.
Click to expand...

It's early and I may be tired, any hint as to which pin?
 
alexcom said:
it is, some traces go under the asic. Should go to neo-b1 and as04.
Click to expand...

So should go to pin 96 on the Neo-B1, not sure about the AS04. Will check....

Update: Checked continuity from pin 103 on the LSPC to pin 96 on the Neo-B1 and I get some continuity, but not full. I compared it to a 1FZ and get the tell tale beep there. So, that does seem to be an issue... not sure where to check the traces. However looking at the general area I do see a via or two that are suspect.
1669823499148.png

(position F-4)
 
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Do you have a logic probe? They are super-cheap, a $10 one off aliexpress/amazon will do the job just fine

Pin 103 on the LSPC should be generating PCK1 and sending it on the p-bus to pin 96 on the Neo B1.

With a logic probe you can check this easily
Pin 103 on the LSPC should read PULSE
Pin 96 on the Neo-B1 should read PULSE

I suspect with the probe you will find that the LSPC 103 = pulse, but the NeoB1 96 = stuck low (rotten trace)

If you don't have a logic probe but do have an oscilloscope you could
compare the PCK1 signal with the PCK2 signal coming from pin 104 on the LSPC which goes to pin 97 on the NeoB1 (PCK2)

They should be (broadly) the same

I suspect that with the scope you'll see that LSPC 103 = LSPC 104 but that NeoB1 96 =/= NeoB1 97

If you don't have a logic probe or an oscilloscope then now would be a good time :)

IMG_2990 (1).jpeg
 
hatmoose said:
Do you have a logic probe? They are super-cheap, a $10 one off aliexpress/amazon will do the job just fine

Pin 103 on the LSPC should be generating PCK1 and sending it on the p-bus to pin 96 on the Neo B1.

With a logic probe you can check this easily
Pin 103 on the LSPC should read PULSE
Pin 96 on the Neo-B1 should read PULSE

I suspect with the probe you will find that the LSPC 103 = pulse, but the NeoB1 96 = stuck low (rotten trace)

If you don't have a logic probe but do have an oscilloscope you could
compare the PCK1 signal with the PCK2 signal coming from pin 104 on the LSPC which goes to pin 97 on the NeoB1 (PCK2)

They should be (broadly) the same

I suspect that with the scope you'll see that LSPC 103 = LSPC 104 but that NeoB1 96 =/= NeoB1 97

If you don't have a logic probe or an oscilloscope then now would be a good time :)

IMG_2990 (1).jpeg
Click to expand...
I have both, as well as a massive pile of other diagnostics tools. Will take a look at your test points. Also thinking that the partial continuity I got from pin 103 on the LSPC to pin 96 on the Neo-B1 might have just been oxidation. Will give it a clean and try that again as well. Annoying part is I suddenly got old (need glasses now) and this board doesn't fit under my microscope... need to figure out some magnification.
 
Kujako said:
Annoying part is I suddenly got old (need glasses now) and this board doesn't fit under my microscope... need to figure out some magnification.
Click to expand...
Omg this is my life now. I have a biology student microscope which is great, but a lot of big boards I’m restricted to about 4 inches around each edge of the board. There is a chunk in the middle that’s a mystery every time.
 
hatmoose said:
Omg this is my life now. I have a biology student microscope which is great, but a lot of big boards I’m restricted to about 4 inches around each edge of the board. There is a chunk in the middle that’s a mystery every time.
Click to expand...

I've got an LCD microscope, but I really need a boom-arm for it... think it's a fairly standard mount type, but not 100% sure.

Edit: thinking something akin to this https://www.aliexpress.us/item/3256804169943394.html
 
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