Converting a VFX ROM Cartridge to EEPROM

Introduction

The only methods of storage for a vanilla VFX are sys-ex savings to another midi device (usually a computer) and storing to a EEPROM cartridge.

While sys-ex storage works fine, it can be quite inconvenient to move your computer with you if you ever play outside your home! Yet, the VFX on rare occasions does do a complete reset, wiping out the programs and presets you have stored in the INT (INTernal RAM) bank. For me, this happens maybe once or twice a year, but when it does, I discovered it was very important to me to be able to reload somehow...

The solution, of course, is an EEPROM cartridge. But EEPROM cartridges seem to be quite rare, and expensive when they can be found. As I recall, I paid over $70 for a used one.

Now, I have found an alternative: Modify a ROM cartridge to be a EEPROM cartridge!

Before You Read On

At this point, I think it's important that I should throw a few disclaimers in here.

I do not work for Ensoniq. I never have. I don't have access to schematics or maintenance manuals for the VFX or any of its accessories.

Although I may be comfortable with a soldering iron, I'm a software guy, not an electrical engineer. I have tried this, and it works for me, with no apparent ill effects to my VFX; or at least not so far. It may not be the same for you.

In other words, if you act on any of the contents of this document, you do so at your own risk. I cannot be held responsible for any damage that might occur to you or your equipment. Please be sure you are comfortable with this before you continue.

General Info - How I Got Here

The used EEPROM cartridge I bought was not an Ensoniq brand cartridge, but one made by Eye & I; I believe that's the Voice Crystal people. This cartridge has a case made of a completely transparent plastic. You can see the circuit board, traces, and components on it plain as day.

One of the more important things I could see was that the memory part was a 28C256, which is a fairly standard EEPROM part. I could also see that there weren't many more components on the board, only a handful of resistors and a capacitor.

This got me curious, so I decided I wanted to look inside my VPC-100 cartridge to see what was inside it. I found that my VPC cartridge contained a 27C256 EPROM, a couple of resistor packs, a few extra resistors and a capacitor. Hmm... a 27C256 EPROM vs. a 28C256 EEPROM.

(For those who might be new to this stuff, an EPROM is erasable, but is erased by shining UV light on the chip for a period of time; then it's programmed by a special programming device. An EEPROM is Electrically Erasable, and can be re-programmed in-circuit. In fact, for clarity, I'm going to refer to the EPROM as a ROM for the rest of this document; it may be technically inaccurate, but it won't be easily confused with EEPROM)

I checked the data sheets for the two memory parts, and between them, all but two of the pins have the same use. I traced those other two pins on the circuit board in my two cartridges, and sure enough, those two pins were wired differently between the two cartridges.

Even more interestingly, these two pins were routed through two resistors on the Ensoniq ROM board. Resistors with exactly one black band coding their value. That's a zero ohm resistor, more often known as a "wire". :-) (I asked some electrical engineers I work with why someone would ever use such a thing, and they said that some machine that automatically stuffed components into the board could probably handle resistors but not wires...)

Next to those two resistors were positions for two more resistors that were not installed; the positions for these missing resistors both had the number "28" labeling their position. And if the original two resistors were removed and identical ones placed in these empty positions, the wiring of the ROM cartridge would change to match that of the EEPROM cartridge.

AHA! Ensoniq must have set things up so they could use one circuit board for both kinds of cartridges, by just stuffing different components! This makes our desired task extremely easy.

What You Need

The parts you need are:

  • A VPC-100 ROM cartridge (or other in the series with the same circuit board).
  • A 28C256 EEPROM chip.
  • An optional 28-pin DIP Socket.

    For experimentation, I obtained a used VPC-100 over the net for about $25.

    I found the EEPROM at JDR Microdevices. They had this part available in three speeds, for about $8, $9, and $15. I don't have a clue as to what speed is required by the VFX. I chose the $9 150ns part and it seems to work for me.

    The tools you will need are:

  • A soldering iron.
  • Some solder.
  • Some sort of de-soldering equipment.
  • Possibly something to cut the original ROM's pins.

    Procedure

    1. Start by opening the cartridge. I was surprised to find out that the entire VPC cartridge assembly (at least for the two carts I have) is held together by friction. It's not welded. It's not glued. The plastic shell has a front half and a back half; four plastic pins on the back half fit snuggly into four plastic sleeves on the front half; and the circuit board is held in the back half, again by friction. You can just pull it apart, but of course, be careful! I find it easiest to start at the bottom, pulling the plastic "fingers" apart to open the case a tiny bit, then pull a bit with finger pressure at the top, then keep alternating between the top and bottom until the case finally separates.

    2. Remove the circuit board from the case. It's wedged pretty tight, but you can just wiggle it out.

    Remember that the component side of the board was facing you before you removed the board from the back half of the case, so you get it in right when you put the whole thing back together.

    3. Note the notch along the top edge of the 27C256 ROM chip. When you put the EEPROM in its place, the notch of the EEPROM should be in this same position.

    4. Use your de-soldering equipment to remove the ROM and the two 0-ohm resistors marked R3 and R6.

    If you don't have professional de-soldering equipment, you may find that you need to sacrifice the original ROM, by cutting its pins and removing the main body of the ROM, then individually de-soldering what's left of the pins from the circuit board. If you don't do this, you run the risk of damaging the board when trying to get all 28 pins free at one time.

    5. Clear the solder from the holes for R4 and R5, the ones that were "missing" when you started.

    6. Solder the zero ohm resistors you pulled from R3 and R6 back in as R4 and R5, or just use a piece of wire for each if you wish.

    7. Now you must make a choice. Either you solder the EEPROM directly on to the cartridge board, or you solder in a 28-pin socket and plug the EEPROM into the socket.

    I personally put in a socket, but then I found out the combined height of the EEPROM and socket is too much, and the cartridge case does not completely go back together. And as a result, it's a little tight plugging this new cartridge into my VFX. If I was to do it over again, I'd probably skip the socket.

    Whatever you do, remember to be certain that the notch on the EEPROM chip is a the top!

    8. Double check your work to make sure you don't have any bad solder connections or solder bridges.

    9. Re-assemble the cartridge. (Again, the component side of the circuit board should face you, not the solder side, when you put the cartridge back together.)

    10. Put the new cartridge in your VFX and try it out!

    Now, what does the VFX do with a blank EEPROM? Will it reject it? I found that when I tried to write a single preset to the "new" cartridge, the VFX found it was blank and decided to copy my whole INT bank out to the cartridge. How wonderful; I suddenly have a EEPROM cart with completely valid data!

    Or perhaps I shouldn't say suddenly. Those who haven't used an EEPROM cartridge before will probably need to be warned: writing data on these chips is a very slow operation! You will get a flashing "PLEASE WAIT" message that will last quite some time. When it's taken long enough that you start to get nervous that you did something wrong and your VFX is going to be damaged, you're about 70% done with the initial programming of the cartridge. At least, that's how it was for me. I didn't time the operation, but I would say it was maybe somewhere between 2 and 4 minutes. (Reading from them, on the other hand, is FAST, as you'd expect from a ROM chip.)

    Picture

    In closing, here's an inside picture of my two cartridges. The one on the left is the one I modified with the EEPROM part. The one on the right is an unmodified VPC-100 for comparison.

    Picture of Cartridges

    You can see in the upper right of each cartridge how R3 and R6 are present on the ROM cartridge on the right, but have been removed and R4 and R5 installed on the EEPROM cartridge on the left.