Menu Select BIOS (Sega Mega Tech)
The first part of the board you should look at bringing up / testing is the Menu BIOS.
There is sometimes a daughterboard containing a Z80, RAM and some ROM. It's job is to bring up the game list on the second monitor. It handles game selection, coinage, time keeping and so on.
This daughterboard is based on Sega Master System.
The board has the Z80 CPU, and everything else appears to be used to simply output diagnostic information over the DB port, so is not necessary for operation.
You can remove the Z80 and install it directly in the socket below where the daughterboard connected, to have it directly on the main motherboard, like so.
The main menu is generated by a combination of:
- BIOS (IC20)
- Z80 SMS CPU (IC17)
- Z80 SMS RAM (IC27 IC28)
- SMS 2 VDP (IC14)
- SMS VDP (IC16)
- SMS VDP RAM (IC2)
- I/O Expander (IC7)
There are likely other components involved but those are all key.
Make sure you have working clock signals by probing with an oscilloscope the 10.7386MHz and 53.693MHz crystals.
In order for anything to happen the Z80 must come out of reset.
Check pin 26 is 5V to make sure the Z80 is out of reset.
Check then for general activity on the address and data pins also using an oscilloscope as a quick check that the Z80 seems operational.
I presume the !RESET pin is kept high (not pulled low) by IC14 to the right of the Z80, which is a general purpose programmable logic device with custom code.
After this I would check your I/O expanders for general activity (pulsing 5V). The important thing is that you see activity on the pins, not necessarily the actual data being sent.
The I/O expanders are CXD1095Q.
You should see !RESET pin high, and Dx and Ax pins pulsing with data for example. This is a good sign the chips are generally functional.
If you see no activity, but !RESET is high, then the chips could be faulty.
If !RESET is low, the chips are held in reset so the reset circuit needs to be fixed first (I suspect its related to the PLD IC14 to the top left of the IC7 I/O Expander.
If all the above checks are ok, you can probe the CN9 connector (RGB2) for the top CRT output video data.
The video output has RGB output, using CSYNC as the VSYNC/HSYNC timing.
The RGB is standard, so is CSYNC.
CSYNC is basically a combined HSYNC and VSYNC signal.
HSYNC is small pulses every 16.63us
The full frame is 59.9Hz refresh rate.
The VSYNC is signalled by three long low pulses of 64us (instead of the usual 4.8us).