Sega CD has no bad memory, but still shows corrupted graphics

[fabiodl]

I have a model 1 japanese mega cd. It boots fine, but when I enter the options screen I get a completely black screen, and pressing start I get some blue rectangles.
Similarly, when I boot a CD I get completely garbled graphics.
I assumed I had some of the ram (either program ram or one of the two word ram) broken or with traces disconnected.
To identify which, I wrote this test program
https://github.com/fabiodl/megacdTest/
which I start using an everdrive. The program sets fills the memories with a known pattern,and then reads them back.
To my surprise, all the memories pass the test, both from the main cpu and from the sub cpu.

The mega cd 68K seems fine as well. As by program, it is able to communicate back to the genesis 68K, to blink the two LEDs and to receive interrupts (int2 from the main cpu and int 3 from the gate array)

I also tried to start the bios from the everdrive to see if there were problems with the ROM but the behavior is the same. Using the US bios I get no spinning SEGA logo at all.

What could be possibly wrong?

 

[Tokimemofan]

I would look at the main ASIC I’ve had several US boards go bad because of faulty solder joints, usually at the corner nearest to the save battery. Typical symptoms are failure to properly run the bios code, black screen or random crashes etc.

 

[fabiodl]

Thank you, I tried to reflow every single pin, but the problem persists. I will try to test the connectivity of all traces, but visual inspection under the miscroscope shows no signs of damage

 

[Druid II]

Could you post a bootable iso of that memory tester app? I have a Sega CD that might need it; once it has been running for a while it starts getting corrupted graphics and eventually crash - likely a memory issue.

 

[fabiodl]

I still didn't find a single issue on my mega cd, but the graphics are still corrupted.
I uploaded my checker rom to http://fabiodl.altervista.org/misc/megacdTest.zip
I will probably add more checks in the near future.
Reading the maintenance manual it seems there was an official checking cart + CD. Did it ever surface? I searched a lot, but I could not find any dumps.

 

[8bitplus]

I looked for the official test cart and CD before (well the data). never found it. I bet they were all destroyed long ago.....

 

[fabiodl]

I checked every single pin, and they are all connected. I also reflowed them just in case, but the problem still persists.

 

[John [FR]]

Hello !

Have you tested the MB with a Logic Analyzer ?

- John

 

[fabiodl]

I seem to have found the answer here
https://circuit-board.de/forum/inde...-MEGA-CD-1-Reparatur-PEEL-18CV8-PLD-auslesen/
unluckly, I cannot register to that forum

 

[fabiodl]

A quick update, it was a broken trace on the interconnect board, CAS2 going to pin41 of the 315-5540

 

[Vorde]

A quick update, it was a broken trace on the interconnect board, CAS2 going to pin41 of the 315-5540

Thanks for the update, I was curious as to what it was

 

[segasonicfan]

I still didn't find a single issue on my mega cd, but the graphics are still corrupted.
I uploaded my checker rom to http://fabiodl.altervista.org/misc/megacdTest.zip
I will probably add more checks in the near future.
Reading the maintenance manual it seems there was an official checking cart + CD. Did it ever surface? I searched a lot, but I could not find any dumps.

Thank you for this program! What a neat and useful tool. I would *love* to see any expanded testing options. I plan on developing a detailed repair guide on my website would love to share it and credit you of course.

I've never seen the official debug hardware, somehow I doubt it wil ever surface.

Congrats on the repair, cant believe it was in the enclosed connector PCB, that wouldve driven me nuts.

 

[kitahei88]

fabiodl, Thank you for your test prog.
It is very useful.
I want to know the detailed meaning of the displayed.
I have broken MegaCD v1 for Japan.
It freeze at start just showing sky image, no megacd panel.

I checked all board and repair bad capacitors. But didn't work.
Next, I have another work MegaCD v1, I change all boards one by one. I found bad board is main board.
So I checked all connection on main board, but no break or short on it.

I use your test prog, it shows like picture.
All "got bus" is bad, and stop testing.

Another working MegaCD shows all OK.
So I want to know what is that mean "got bus bad" and other details.

Thank you.

 

[fabiodl]

Thank you for this program! What a neat and useful tool. I would love to see any expanded testing options. I plan on developing a detailed repair guide on my website would love to share it and credit you of course.

I've never seen the official debug hardware, somehow I doubt it wil ever surface.

Congrats on the repair, cant believe it was in the enclosed connector PCB, that wouldve driven me nuts.

sorry for the unbelievably slow reply. I did not add much more to the testing, but if you have anything specific you'd like to be added let me know

 

[fabiodl]

fabiodl, Thank you for your test prog.
It is very useful.
I want to know the detailed meaning of the displayed.
I have broken MegaCD v1 for Japan.
It freeze at start just showing sky image, no megacd panel.

I checked all board and repair bad capacitors. But didn't work.
Next, I have another work MegaCD v1, I change all boards one by one. I found bad board is main board.
So I checked all connection on main board, but no break or short on it.

I use your test prog, it shows like picture.
All "got bus" is bad, and stop testing.

Another working MegaCD shows all OK.
So I want to know what is that mean "got bus bad" and other details.

Thank you.

If you get all BAD just after the first "got bus" I would say some of the address lines to the memory is broken.
For details on the how the code actually tests, have a look at

megacdTest/main.c at master · fabiodl/megacdTest

Contribute to fabiodl/megacdTest development by creating an account on GitHub.

github.com

 

[segasonicfan]

yeah, I definitely suspect bad address lines for the memory chips.
Do you have a working unit you can compare readings with? Have you replaced all the Leaky SMD capacitors that are known to fail on these?