Power Circuit (Sega Mega CD)
The Mega CD / Sega CD has a power circuit that is enabled when the Mega Drive / Genesis provides 9V power through the side connector.
So without a Mega Drive / Genesis attached, the Sega CD / Mega CD will not turn on.
The DC jack is a positive outer style 5.5mm x 2.1mm DC jack.
The ground passes through FB3, and the 9V power passes through FB2.
On the Mega CD 2 this is often labelled FL301 as a single combined common mode choke.
The purpose of these inductors is for noise suppression. If they go fautly the insulation can break down and short 9V to ground. In such a case you can remove the inductors or filter and replace their original path with just a solid wire.
After this the 9V is fused through F1 (or F301 in Mega CD 2). If this blows, replace it with a 1 or 2 Amp fuse of any kind.
The 9V input from the power plug that you plug into the Mega CD / Sega CD is the power coming in on the CN2 DC Jack on the schematic.
This power flows through the choke and fuse, and then hits the PNP transistor TR4 (or Q301 on Mega CD 2).
This PNP transistor has the 9V input going into the emitter, and it will go out of the collector to the 5V regulator once the base is pulled low by a second NPN transistor TR3 (or Q302 on Mega CD 2).
The base of the NPN transistor, once pulled high will sink the base of the PNP transistor enabling it and passing power to the next stage (the 5V LDO regulator) turning on the console.
This "power on" signal gets pulled high by pins A26 / B26 of the connector where the Mega Drive / Genesis connects (which is just the VCC1 5V voltage that appears only when the console turns on).
This voltage comes through A26 / B36, through the side connector, into the Mega CD on A26 / B26, into pin 11 of the ribbon connector, then over to a voltage divider on the power board, which lowers the 5V to 1.26V via a 4.6k and 1.6k voltage divider.
Due to the current draw on the NPN transistor it will appear slightly less, about 0.7V.
This ultimately pulls TR4 base low as mentioned, which passes the 9V power through TR4 into the LDO 5V regulator, and passes the 5V output up pin 19 of the power ribbon to the Mega CD.
Once power is enabled, the 5V LDO regulator IC6 (or IC301 on Mega CD 2) should receive 9V power and then regulate it down to 5V, which goes to the rest of the system.
The output of the 5V goes to pin 19 of the power board ribbon, onto the Mega CD motherboard, through the fuse near it, and out to the rest of the system.
Make sure to check the fuse on the power board near the power input, and the fuse on the Mega CD by the power ribbon connector.
A great way to visually test if the Mega CD is receiving 5V power, and so testing the power inputs to the systems, the side connector, ribbons, and all of the power board work, is to simply solder an LED on the 5V rail using around 1k to 5k resistor.
Here I solder on a UV LED with a 4.7k resistor. If I turn on the Mega Drive and this LED does not come on, I know there is an issue with the power board, ribbons, or circuit to getting the Mega CD to receive power.
If you do not have a Mega Drive / Genesis or just want to test the power on part of the circuit independently, it is as simple as either enabling the PNP/NPN transistor chain by applying 9V to the base of TR3/Q302, or bypass those transistors all together and just apply 9V to the input of the 5V LDO regulator IC6 (or IC301 on Mega CD 2).
When powering directly from 5V nothing will visually boot, no LEDs will come on, but you should expect around 300mA of power to pull.
If you don't have a power supply, solder on a USB A cable to the GND and 5V pin above, and you can plug in the USB, then turn on the Mega Drive and it should fully work, with the exception of no audio (as the audio IC is on the power board) and the CD drive won't open or run but the indicator lights (green and red) will work.
I typically solder the USB on the right of the fuse and the ground by it, so the system still has the fuse protection if something is wrong. Remember to check if your fuse works also.
This is a great way to test the Mega CD motherboard in general works without needing a working CD drive, power board or ribbons.
Without a CD drive attached you will get to the BIOS screen just without the text or graphics in the middle.
If you connect the CD drive it will fully work but without audio or open/close of the disc drive.
The most common components to fail are:
- Fuse
- TR3
- TR4
- Traces from 5V Mega Drive (power ribbon pin 11) to voltage divider network.
- Bad 21 pin power ribbon
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