DMX Files Frequently Asked Questions

Last update 2013-06-21

There is now a separate DX FAQ for questions specific to the Oberheim DX.

DMX Operation

question Can I tune the pitch of DMX sounds?
answer Yes, all DMX voices have a pitch adjustment pot inside the case, on the voice card. Remove two screws on the lower front edge of the DMX case and raise the hinged lid to access the inside of the DMX. Look for the trim pots near the back of the case, on the top corner of each voice card.

DMX Maintenance

question Can I add MIDI capability to my DMX?
answer Yes! The original factory MIDI upgrade is no longer manufactured but a new MIDI upgrade is now available for the DMX. More details can be found here.

question When I turn on my DMX I get a message that says "Data May Be Bad". What does this mean?
answer This message usually means that the battery in the DMX is in need of charging or replacement. This battery powers the memory when the main power is turned off. It is normally charged slowly while the AC power to the DMX is on. Older batteries lose the ability to hold a charge and must be replaced.
Until you get the battery replaced, you can reset the DMX memory by holding down the ERASE button while turning on the power.

question How do I replace the battery in my DMX?
answer Any 3.6 volt nickel-cadmium (NiCd) battery of the same approximate physical size should work.
Allied Electronics part number 621-1100 is essentially identical to the factory stock part. When installing this battery, be sure to clip the leads short enough that they won't touch the bottom of the chassis and short-circuit.

Or, you can take your old battery to Batteries Plus and they should be able to make you a replacement.
The battery is soldered to the main circuit board. Be sure to disconnect all power and audio cables from your DMX before soldering or unsoldering the battery, especially if your soldering iron is grounded!
If you're not confident about replacing the battery yourself, a local electronic repair shop should be able to do it for you.

question Some of the buttons are intermittent or don't work right. Do I need to replace them?
answer You may be able to fix this problem by cleaning the switches. See this link for more information.

question The display is starting to look weird. Can it be repaired?
answer Yes. For more information see this page.

question My DMX is broken. Where can I go to get it repaired?
answer Electrongate is currently offering repair service for Oberheim DMX and DX drum machines. The basic cost for a new battery, switch cleaning and any other minor repairs required is $89.00, including return postage in the U.S.A. Some repairs may cost extra if additional parts are required. Please e-mail Paul White for an estimate, and shipping instructions.

DMX General Information

question What's the difference between the Oberheim DMX and the DX?
answer See this chart for reference.

question How are sounds stored on the DMX/DX?
answer Sounds are stored as sampled PCM (Pulse Code Modulated) audio on individual EPROMs. Sound length depends on the capacity of the EPROM. Sample sizes range from 2048 (2k) bytes for some percussion sounds to 32768 (32k) bytes for cymbals. The playback time can be calculated by dividing the number of samples in the EPROM by the sample rate (playback seconds = number of samples / samples per second).
The data format is raw a-law companded. There are no headers or other non-audio data stored on the EPROM. The sample rate varies from about 16000 to 32000 samples per second, depending on the voice.

question Where can I get those cool easy-access thumbscrews for the front cover of the DMX?
answer These are now available from Electrongate! A set of two is $5.00, including postage. Send an email for more info.

Custom Sounds and EPROMs

question What are EPROMs?
answer Eraseable Programmable Read-Only Memory. This is a form of Integrated Circuit (IC) that is packaged in a ceramic Dual-Inline Package (DIP).
EPROMs are programmed on a special device called a device programmer, or PROM burner1. EPROMs are erased by shining an ultraviolet light through a transparent window in the part. This flips all the bits back to "1". EPROMs are programmed at the factory, or after erasure using a device programmer. Once programmed, they will hold their data for 5--10 years before data loss may start to occur.
One type of "EPROM" is the OTP (One-Time Programmable). These are electrically identical to EPROMs, but have no window through which to erase the chip. So, they can only be programmed once. You can also use EEPROM (Electrically Eraseable PROM) parts such as 28C64 in place of 2764 EPROMs, and erase them in just a few seconds using your device programmer.

question Where do I get EPROMs?
answer The DMX and DX drum machines use 2732 and 2764 EPROMs. These are getting harder to find these days, as larger capacity devices such as 27256 EPROMs are more common. But, they can still be found at many major electronic distributors and surplus houses. Search the web. Typical part numbers are similar to 27C32, 27C64, 2732A, etc.

question How do I program EPROMs? What hardware do I need?
answer A blank EPROM (Erasable Programmable Read-Only Memory) can be programmed with the aid of a computer peripheral called a "burner", "EPROM programmer", or "device programmer". Get yourself one of these, connect it to your computer, and follow the instructions that come with it. To convert a WAV file into a format that can be programmed onto an EPROM for use in the DMX or other companded format machine, use the WAV2DMX program on an appropriate WAV file. Then, program the resulting binary file onto the EPROM.

question How do I erase EPROMs?
answer With an EPROM eraser. This is a device that exposes the EPROM chip to ultraviolet light, which will erase it and make it able to accept a new sound. Note that "OTP" EPROMs are packaged in a solid plastic package, rather than the ceramic package with a window, and cannot be erased and reprogrammed.


Notes:

  1. Historically, PROMs were programmed by selectively burning out tiny fuses on the chip for each bit that was to read as zero. Fuses for bits that were to read as one were left un-burned. This is where the term "burning a PROM" comes from, even though modern EPROMs no longer use fuses for programming.

FAQ maintained by Paul J. White. Feel free to submit corrections and other DMX-related questions.