A firmware update for the OB-8 Synthesizer

    I recently noticed an ad on a popular auction site for an OB-8 which was missing its power transformer and CPU board, but looked OK otherwise.  I knew there was an OB-8 CPU board for sale on reverb so I bought it and the OB-8.   Luckily I was able to locate an OB-8 power transformer as well.   The DAC was missing from the CPU board, but I was able to fashion an equivalent replacement using a similar 14-bit chip that is still available.   Once everything was up and running with the A8 (non-MIDI) firmware, I decided to add MIDI, so I designed an interface board using the same exact circuit as the Oberheim factory MIDI board.  

   While I was waiting for the new pcb to arrive, I decided to study the B5 firmware and see if it might be possible to add some new features to it.  I created a Verilog simulation of the OB-8 and used it to learn about the different variables and routines.  Luckily, there was a fair amount of unused space in several of the EPROMs where I could put some new code.  I have now developed a new version of the OB-8 firmware.   

    This firmware is currently available.   The price is $110 for the five programmed EPROMs.  Here are the new features:

1) MIDI CC support for controlling all patch parameters.   (Does not work in Manual mode)

2) Four different MIDI Sysex dumps can be transmitted in page 2:

     a) Currently loaded patch (same as B5)

     b) All 120 patches, sent as 120 sysex packets

     c) Saved splits and doubles, sent as one sysex packet

     d) Saved envelope offset calibration settings, sent as one sysex packet

3) Non-volatile storage of MIDI channel setting and MIDI mode

4) MIDI clocking of the arpeggiator is now possible, notes can be configured as half, quarter, eighth, or sixteenth notes

5) A very subtle bug in the B5 firmware was discovered, and fixed. This can cause the OB-8 to crash (lock up) at seemingly random times if you pound it with MIDI messages, for example by sending it tons of MIDI clock messages every second.  (Of course the B5 firmware doesn't support MIDI clocking, so there would be no reason to do this)

   Here is a document that shows the CC mapping to parameters, and also describes how to use the other new features.

In order to have enough room for all of the new code, I removed the code involved with saving data to cassette.   The code for loading data from cassette was not touched and should work as before.

The new firmware is compatible with the Oberheim factory MIDI board, but will not work with the Encore MIDI interface, as the design is different.    A set of MIDI firmware for the OB-8 consists of 5 2732A or 27C32 EPROMs.   Please contact me if you are interested in upgrading.

    Here is some information about how to remove and install EPROMs.

OB-8 MIDI interface bare board now available:

    I have a few OB-8 MIDI interface bare boards available for purchase.  I don't plan to offer assembled boards.  All of the parts to populate it are available at Digi-Key, except the MC68B50 ACIA (UART) chip, which you can get elsewhere.  The circuit is identical to the Oberheim OB-8 factory MIDI interface, so you can use it with the B5 firmware, or with my new firmware.  Of course the connectors are a little different, so this is not a drop-in replacement for the Oberheim board.  Blank boards are $35 each plus shipping.  (I added a second set of mounting holes to shift the board to the left to allow the upper voice board to hinge all the way up.) Here is how it looks:  

OB-8 DAC replacement:

   The Sypex HS3140 14-bit DAC used in the OB-8 has been out of production for a number of years.  Some parts appear from time to time for sale, but I decided to see if I could adapt a chip that was easier to buy for this purpose.   I found a part that works well in the OB-8 (and has also been tested in the Xpander).  I made a small pcb to adapt the pinout to mimic the HS3140 chip.  Here is how it looks installed:

  The adapter boards are available for $5 each plus postage.   The DAC chip is Maxim part MX7538KN+, available at both Digi-key and Mouser. I used the Digi-key SAM1111-10 gold headers to allow the adapter to plug into the existing IC socket.   By the time you add postage, the cost is not that much less than what the HS3140 is going for if you are willing to buy one from China.   I prefer to use parts from my normal suppliers when possible.

  Please note: Taking an OB-8 apart and replacing the EPROMs must be done carefully, or damage to the synthesizer could result.  It should only be attempted by someone familiar with this type of work.  I will not be responsible for any damage to any instrument caused by either proper or improper use of the code offered here. 


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