The soundbox is an external device used to support peripherals on a NeXT computer. The internal circuit board is identical to the one found inside next MegaPixel monitor. As an important side note, if your MegaPixel has died DO NOT throw it out (if you must) without first removing this board. Two versions of the sound box were created. A non-ADB and an ADB version. ADB stands for Apple Desktop Bus and is fully compatible with apple peripherals of the same type.
The non-ADB came first and is compatible with all next hardware. The ADB version came later and is ONLY compatible with Turbo models which have a ROM revision greater than v74. Despite the limited compatibility, the ADB sound boxes are much easier to find. As such there have been many discussions about converting the ADB version to be compatible with non-ADB devices and systems. Another important thing to mention is that you must have a 983 model power supply to use a cube without further modification with a sound box. The 152 model needs a minimum load (previously provided by the monitor) to function.

The NeXT 19 pin connector:

It was previously thought that the ADB soundbox had the same connection to the 19 pin DSub connector as the non-adb version. This has been proven to be incorrect. Previously pin 7 was unused on this connector, but in the case of ADB is designated for the ADB bus. By providing 5V for the keyboard it is possible to bypass the sound box entirely and hook the ADB data line directly to the next system. You will of course lose the ability to produce sound with this configuration.

View from the back of a Next System
        10                         1
      \  o o o o o o o o o o  /
       \  o o o o o o o o o  /
        19                       11
    1.    +12V
    2.    -12V
    3.    MON CLK
    4.    MON DATA OUT
    5.    MON DATA IN
    6.    MON PWR SWITCH
    7.    ADB
    8.    VSYNC
    9.    HSYNC
    10.    VIDEO
    11.    +12V
    12.    -12V
    13.    GND
    14.    GND
    15.    GND
    16.    GND
    17.    GND
    18.    GND
    19.    GND

The non-ADB keyboard interface:

Figure 1: Non-ADB View From the Soundbox
non_ADB interface

Figure 2: The ADB Interface
ADB Interface

The ADB soundbox Modification:

The modification of the soundbox is not too difficult if you have a good soldering iron and a steady hand.

Figure 3: An Annotated View of the ADB Sound Board:

Sound Board

1) Locate the ASIC near the large capacitor. Use a utility knife to sever the annotated connection. We will be soldering to it later, so take care.

Figure 4: ASIC Trace to Cut
Trace to Cut

Figure 5: ASIC Trace Cut
cut connection

2) Use the schematic below to rework the board

Figure 6: Schematic

3) Connect VGA

Figure 7: VGA Pinout
vga pinout

Connect a 150ohm resistor across pins 1,2, and 3 (Red, Green, Blue). Use an RCA connector and connect signal to the 150ohm resistor and ground across pins 5 -10 on the vga connector. Connect V-SYNC to pin 2 on the SoundboxVGA connector. Connect H-SYNC to pin 4 on the soundbox VGA connector. As a side note it is important to use properly shielded cabling from the Next computer to the soundbox. Inferior cabling will produce poor image results.

4) (Optional) Create a dummy load for Revision 152 Cube Power Supplies:

The early version of cube power supplies (part number 152) requires a load of ~400mA on the -12V power rail in order to function. The easiest way to provide this is by using a power resistor ~30ohms from -12V to GND. This resistor will get very hot so we need a fan and heatsink to remove heat from the enclosure. I use two 5W, 68 ohm resistors in parallel to split the current between them, and lower the effective resistance to 34ohm. Using a pentium style cooler, remove some of the fins to allow the resistors to fit inside. Then bend the remaining fins over to secure the resistors in place. Connect the fan and the resistors across the -12V and GND connection on the soundboard header.

Resistors embedded in the heatsink:
Load config

Completed load:
Full Load

Finished Example:
Full Mod

Installed Conn


Modification tested on both a nextstation color turbo and an 030 cube with no problems in the firmware or OS. Use at your own risk. Please do not sell these to collectors without disclosing there has been a modification. Distribute this information freely, but my ego does enjoy credit. I can be reached at: archer174 at gmail if you have questions.