Japan MegaCD 1 Fix help!

Hi! I am writing to you from Spain, sorry if my English is bad.

I recently bought a MegaCD 1 JAP, first units, the ones that have a sub-board with oscillator into upper of the main board, but the interconnection board with the megadrive, where the two cables that go to this sub-board come from , it is already smd, without metal cover.

It was sold for parts, it didn't work. I've done some repairs but I can't get anything at all, no LEDs, no image, or cdrom movement.

Let me tell you. I first detected blackened TR4, and have replaced it with another darlington. The fuse was fine. I have also changed the regulator for a 7805, and on the power board I have changed the caps that affect only the power supply, I have not touched the sound caps. I don't know if any can also cause it not to start.

With this repair I did not get anything. On the main board I added a led with a resistor connected directly to 5V, and when I turn on the megadrive it lights up, then the power-board works well, and the signal from the megadrive, to start it, also reaches it.

Later I discovered that the flat-cable that connects the main board with the cd-unit was not the original, instead of 21 pins it was 20 pins, and at the end of the cd-unit it was destroyed, perhaps creating a short on some tracks . I ordered a new one. The unit does not work with the new cable either, nothing at all.

The next step has been to change all the caps of the main board and the connection-board for new ones, I have used smd tantalum capacitors.

Also not working, no new symptoms.

As the main board has a socket for the BIOS, from factory, I have programmed a new bios, and it does not start either.

If the cd-board is damaged, but not short-circuited, can it cause the megacd to no longer start? or would this just not work but the main board would boot?

The other caps of the power-board, which do not affect the power supply, can they cause the megacd to not start at all?

No cap had lost liquid, on any board.

The connection board with the megadrive looks in very good condition, but there is a square chip in the middle that manages some aspect. Could it be that the connection board gives a signal for the power-board to start, but prevents communication with the megadrive? If I put a cartridge, it runs normally, as it would with a functional megacd, it does not disable the megadrive.

I am totally lost. Can you help me?

Yesterday I did new tests, without result. I changed the remaining caps to change on the power-board, and I did not have any improvement. Then I did a test. I opened the CD tray manually, and then when turning on the mega drive, the tray closed itself, so I deduce that the cd-board still works. Could it be that the problem is in the connection board between the main-board and the mega-drive? What problem can there be? there is only a square chip and some capacitors. Nothing seems damaged there, the tracks look good.