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Casselfornia

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Hi,

I recently acquired this board with Dynamite Deka. Previous Owner already resoldered the sh2 chips. The system passes all checks when I press the test button while powering up.

Problem is that it doesn't boot. No animation, no nothing. With or without cartridge, just a black screen. Any idea what could possibly cause this?

Cap c62 also seems to have blown one day. It's definitely dead. And I found that something is weird with r100. It seems there is an transistor soldered to the two pads where a resistor should be. Can any of those cause the system to not boot?

The LED lights up of course.
 

Casselfornia

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Did that. I used the multibios. Did not make a difference. Without the bios I cannot access the diagnostics menu. So the bios I burned must work.
 

ic3b4ll

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If you can boot into the factory diagnostics menu but not the normal menu and run the tests, then I'd check the address lines. Maybe one is bad. I believe the address lines go to the 74LS245 buffers next to one of the connectors. Check if you have continuity between the bios address pins and those.
If you have a logic probe check the activity on the pins in factory test and normal menu. If the problem is that it should be easy to pinpoint.
 

Casselfornia

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Thanks for the suggestion. I'll look into it. Probe is here. I just didn't know where to start with this.
 

Casselfornia

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Okay. I tried to probe it but it's really not clear to me where the pins of the bios end up. They go through vias in the surrounding areas and it's not visible to me where the vias they come out of end. There are no visible traces on the board.

One of the pins of the bios surely is connected to pin 1 of the ls245s (ic5,6,7) but doesn't have connection to pin one on the other ls245s (ic3,4) which seems a bit odd. That's vcc btw. Ic3,4 aren't connected to vcc. Can anyone who has a working ST-V check if that's normal and are there schematics out there?

It ain't VCC. It's the direction!
 
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ic3b4ll

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The STV has inner traces sandwiched between the power planes so you can't actually see where they are. I think it's awesome :D

As Rom1 stated you can use the Saturn schematics to check where everything is supposed to go. You want the schematics for the first model, VA0.5 I believe.

Also, the STV uses some form of conformal coating. You have to push down or scratch (gently!) the pins you are checking with the probe or you won't get continuity. Yes, really.
 

Casselfornia

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That whole thing is so damn flawed... Why would you hide the traces? Why would you apply that coating? Why would you build a cart slot like this without enough support to keep the board from flexing? Stuff for many nightmares.

I recognised the coating as I had to push the probes harder than normal.

Thanks for the tip which Saturn revision has the same schematics ✌️
 

ic3b4ll

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That whole thing is so damn flawed... Why would you hide the traces? Why would you apply that coating? Why would you build a cart slot like this without enough support to keep the board from flexing? Stuff for many nightmares.
Hiding the traces helps against physical damage and the coating against oxidation.

The cart slot decision seems strange but makes sense when you look at the bios screen and realize there should be support for multiple cartridges. I believe SEGA thought about doing something like the MVS by using an expansion board. There are in fact expansion boards that take the space where a hypothetical horizontal cartridge slot would be.

Support is actually very reasonable. If you look at the board you'll notice many places where to add support. If you find a board with all PCB feet intact you'll find it well supported.

The slave SH2 CPU is very exposed but I don't think SEGA's hardware engineers ever expected people to still be using the board after 25 years or so.
Thanks for the tip which Saturn revision has the same schematics ✌️
No problem. Pay attention to the type of BIOS EPROM your board has. Some are JEDEC pinout while others have Mask ROM pinout.
 

Casselfornia

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I probed the shit outta it today and spent ages reading the saturn schematics and making my own notes. I can trace all of the biosses pins back to the ls245's which are IC 3,4,5,6,7.

So the address lines here aren't the problem. Any suggestions? If I don't get any I'll try resoldering the SH2 chips.

I realised another thing when I am in the Factory Diagnostics Menu. If I select Sound, CRT or Clock Change the system hangs/freezes.
 
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ic3b4ll

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I probed the shit outta it today and spent ages reading the saturn schematics and making my own notes. I can trace all of the biosses pins back to the ls245's which are IC 3,4,5,6,7.

So the address lines here aren't the problem. Any suggestions? If I don't get any I'll try resoldering the SH2 chips.

I realised another thing when I am in the Factory Diagnostics Menu. If I select Sound, CRT or Clock Change the system hangs/freezes.
I saw your post yesterday and was going to ask you to ask you the results of the factory test so I'm glad I waited.
This is actually a very interesting result and a great pointer towards a solution.
I'd focus on sound section first. I hope you're comfortable with SMD rework...

My *hunch* is that the sound chip is toast and is causing some address lines to get stuck high or low.

Check the connections between this chip's address pins and whatever it's connected to in the schematics.

Use the logic probe and check if any is stuck (high or low).

Repair tip #57: if you have a tiny pin connected to a large pin or IC leg, probe it there.
 

ic3b4ll

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Actually... The sound CPU and clock are handled by the SMPC. If both tests are getting stuck then I'd look into this first as it's a common point of failure.

I *think* you can salvage this part from a Saturn and replace it if needed.
 

Casselfornia

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Actually... The sound CPU and clock are handled by the SMPC. If both tests are getting stuck then I'd look into this first as it's a common point of failure.

I *think* you can salvage this part from a Saturn and replace it if needed.
Nevermind. I wanted to resolder the SH2 chips with my iron and of course the new solder I have is leadfree and I didn't realise that. Of course I made bridges which I am not able to remove, even with the IC desoldered with hot air. I read that the EU managed to ban solder with lead from private use in 2018. Didn't knew that until yesterday and got really really mad.

I managed to buy a 100% working one from Italy and will not bother with this board anymore.

If anyone is interested, drop me a line!

Thanks for the assistance @ic3b4ll :)
 
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