Modifying an older EPROM drum machine

[bigtex]

I just got an MXR 185 Drum Computer for a mere $15! Check out the sexy wooden end-cheeks. They just don't make stuff like this anymore. It's a serious solid steel tank inside.



It's got an EPROM per voice, and for some voices it has multiple ROMs.



There are a few different things I could do here. I don't have an EPROM reader/burner, but I could make/get one. Then I could make new voice ROMs. This was apparently done with some amazing results to an Alesis HR-16, so why not this one, right?

My other idea is to try to adjust the pitch clock on these more drastically than the pitch knob on the front panel allows. Dave Wright did this to a Roland TR-505 (scroll down a little bit) and it had two different oscillators. I guess there were two voice ROMs, so each one has it's own clock. If that worked with two clocks, why not a seperate clock for each voice? Look at all these preeeety ROMssss:



I'm hoping I can pitch the voices individually. This should be possible, right? I'm assuming that the traces that feed the master clock to the respective voices could be cut, and I could put a new square wave oscillator in for each voice. I could then install pots, or even have CV control over each voice's pitch. There's already a trigger input jack on the back (some kind of crazy square 24 pin molex dealie) so I could really integrate this into a modular setup once I get some nice step sequencer action going on.

So to persue that second option, I'd need a bunch of simple square VCOs. Proper 1V/oct would not be necessary, but it would be nice. The simplest method I can think of would be an AVR, but I don't know if they make one with 12 analog inputs. I suppose I could try another microcontroller if not...

Or just a simple logic gate oscillator would be nice. Is there an easy way to make these voltage controlled without using a vactrol or other voltage controlled resistor? Then I'd only need two hex inverters' worth (since the drum machine has 12 voices).

Now these EPROM drum machines, do they have a binary counter to cycle through the memory when the drum sample is to be played? Does the onboard OS have to spit out the addresses to the ROMs? Do the EPROMs just "know" to dump out all of their memory? How is this handled? I've never messed with these old digital drum machines before.

I'm a chronic project starter. I keep coming up with ideas before I finish previous projects...

[ian-s]

I modified an OB DX drum machine in the early 80's. I burned the 4 kick/snare options which were in separate EPROMS, into one larger EPROM and added a couple of switches to select.
From memory, each voice of the DX had it's own tunable oscillator, address counter and set/reset logic. All the cpu did was supply the trigger and accent signals. I think microcontrollers of that era would struggle to generate all the address lines (but I could be wrong).

The DX used companding, 8 bit DACs which would slightly complicate the process of making new EPROMs

Cool looking unit BTW

[bigtex]

[toppobrillo]

[seraph]