Guide The Definitive DC Trimming Guide

[Nold]

Introduction

This guide and its graphics were created by me, to the best of my knowledge and certain. But without guarantee of correctness! (Even if everything has been tested by me)! So make sure that you know what you are doing, as I will not be responsible for any damage or harm you might do to your Dreamcast or yourself!

I used the BitBuilt-Logo in every graphic, but just as an advertisement and appreciation for this site and its community. But BitBuilt has nothing to do with this work

Special thanks to Aurelio for motivating me to trim further and pre-reading this guide & RDC for supplying scans of the DC Mainboard.
Also much thanks to the DC-Homebrew-Scene and the BitBuilt-Community!

If you have any questions/corrections/comments feel free to ask.


Prerequirements

GD-ROM Replacement

Spoiler

Since we do not want to use the original GD-ROM in our DCp, we will have to replace it with something smaller/better. There are basically two possible solutions for this:

1. USB-/SD-GDROM
An FPGA-Based board, enabling you to play games from a USB-Drive/SD-Card

Pros:
- Play ROMs from USB-Drive / SD-Card
- No custom BIOS Required
- Very good ROM compatibility

Cons:
- Expensive
- Requires extra hardware


2. G1-ATA-Mod
A relatively “simple” mod, for which you wire a standard ATA/IDE Hard-Drive/CompactFlash-Card to the DC’s GD-ROM ATA-interface. But that requires a custom BIOS-Chip and good soldering skills

Pros:
- Cheap / No special hardware required
- Play ROMs from IDE-/SATA-Harddrive or CompactFlash

Cons:
- Many wires to solder
- Requires custom BIOS/-Chip (“Dreamshell no-gdrom”)
- Requires AICA-Clock workaround
- Worse ROM compatibility than GDROM-Replacement


In this guide I will just cover the G1-ATA method!

BIOS-Mod

Spoiler

So before trimming your DC, you should have a custom BIOS-EEPROM installed and flashed with the Dreamshell No-GDROM Bios. I will not cover it in this guide, since there are already enough resources on the web.

BIOS-Mod: http://www.mmmonkey.co.uk/sega-dreamcast-region-free-bios/

After this mod, flash the Dreamshell Bootloader BIOS called "boot_loader_retail_nogdrom.bios". It will boot Dreamshell from SD-Card or G1-ATA.

Trimming

Spoiler

Here is the trim I figured out. Of course it should be possible to cut even further. But that’s up to you ;-)

When doing this trim, you will have to relocate the Audio-DAC Chip (IC303) before trimming and solder to small pads. Also you will have to reconnect two traces. But you will find all the needed information to do so in this guide.


Make sure that you cut OUTSIDE of the red lines and then carefully sand all the edges with at least 800 grid.

Reconnecting Traces
After trimming there will be two things to reconnect:

5V to IC401

Spoiler

Reconnect the shown trace to 5V.

Spoiler

RTC-Battery Voltage

Spoiler

Reconnect the shown traces.

Audio-DAC Relocation

Spoiler

Unsolder IC303 BEFORE trimming and reconnect it as shown. You can also reuse the 10uF capacitor CE301 or CE310 for pin 5 of the DAC-IC.

Power and Voltage

Spoiler

Main Voltage lines:
12v (Used by GD-ROM, not needed to boot and not covered in this guide)
5v (0.42A)
3.3v (2.8A when powering onboard-regulators, 0.4A without the)

Onboard Regulators:
2.5v (1.6a)
2.0v (0.75a)

All regulators should be replaced with custom regulators to reduce total power-consumption drastically to <= 10W.

No FAN Fix

Spoiler

Normally the Dreamcast would not boot if there’s no PWM signal coming from a fan. The following workaround removes the need of a fan capable of outputting the correct PWM-Signal.

AICA Audio-RAM-Clock

Spoiler

Normally the AICA (DCs audio IC) will get it’s RAM-Clock from the GD-ROM. If you are not using a GD-ROM-Replacement like USB-GDROM, you’ll want to attach the internal 33MHz PLL Clock to the AICA as shown. Otherwise the DC will be super slow and it will not work.

Controller

Spoiler

Even after trimming you should still be able to use these pins:

But you can also use these:

Serial-/SD-Card

Spoiler

Attaching an SD-Card to the DC is pretty much optional when using a GD-ROM replacement as it is much slower then the ATA-interface. But here is how to do it anyways:

RTC-Battery

Spoiler

The DC uses a rechargeable battery for its Real-Time-Clock which can be relocated as shown.

BUT DO NOT WIRE A NON-RECHARGEABLE BATTERY LIKE THIS!

Video Output And Forcing VGA-Mode

Spoiler

The DC supports multiple video output formats. As you will most likely want to use VGA (best quality) here is how to enable VGA-Mode and wire it up.


Note: You will still need some external components to make VGA work, but you can ignore the VGA-Switch, because we wouldn't need to switch off VGA-output anyways.

Source: http://www.mmmonkey.co.uk/dreamcast-internal-vga-mod/

G1-ATA

Spoiler

G1-ATA is a “simple” mod, where you connect a standard IDE-Hard-Drive or CompactFlash-Card to the Dreamcast’s GD-ROM Port. This is an advanced pinout, that you can use after trimming.

Note: CompactFlash to SD-Card Adapters will work, too. But only with a horrible performance.

More info: https://fnhack.wordpress.com/

Changelog
15.04.18: Updated Video-Output Pinout
16.04.18: Correction Video-Output Pinout(s)
09.04.24: (Wesk) Added Chroma and Luma pinouts to AV diagram

 

[Fly_5]

amazing work !!!! but I have a question, I see you have removed capacitors at the bottom, are they not necessary ?

 

[Wesk]

amazing work !!!! but I have a question, I see you have removed capacitors at the bottom, are they not necessary ?

It is best to keep the tantalum capacitors.

 

[Fly_5]

Thank you very much for the info, I'm going to start making a portable dreamcast, is it compatible with that plate cut the GDemu?

 

[Wesk]

Thank you very much for the info, I'm going to start making a portable dreamcast, is it compatible with that plate cut the GDemu?

This trim is compatible with the GDEMU but keep in mind it requires a lot of relocation wires.

 

[Fly_5]

many wires to solder ,do you mean GDemu ? or in general ?

 

[Wesk]

many wires to solder ,do you mean GDemu ? or in general ?

By performing this trim you'll be removing the optical drive connector, so the GDEMU will need to be manually wired which is approximately 30 wires.

 

[Fly_5]

Ok no problem I have a lot of patience

 

[Fly_5]

I don't quite understand this image, the tantalum capacitors are shown extracted and a circle with a specific voltage, I understand that the regulators must be replaced by custom ones but the capacitors indicated with a voltage? must be removed and give that voltage on a pad? or I don't understand.

 

[Wesk]

I don't quite understand this image, the tantalum capacitors are shown extracted and a circle with a specific voltage, I understand that the regulators must be replaced by custom ones but the capacitors indicated with a voltage? must be removed and give that voltage on a pad? or I don't understand.

Solder to the positive side of the cap, the image is just a clear indication of which side to solder to.

 

[Fly_5]

OK, I understand that, but do you have to remove the capacitors or just give voltage to that pad?

 

[Wesk]

 

[Fly_5]

ok sorry

last question, on which leg do you connect the 2V in this regulator?

 

[Stitches]

ok sorry

last question, on which leg do you connect the 2V in this regulator?

The big pad with the 2v designator on it, or the leg in the centre (pin 3)

 

[Fly_5]

Thank you very much, I thought the big pad was GND.

 

[Fly_5]

Cut and relocate components as in the guide, there is image on the screen but flickering and with image failures I have reviewed everything and I can not find the solution, could you help me.

 

[Fly_5]

Problem solved

 

[Wesk]

@Nold I updated OP to include the Chroma and Luma AV locations for S-video users.