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 Forum index » DIY Hardware and Software
Modifying an older EPROM drum machine
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bigtex



Joined: Mar 30, 2006
Posts: 321
Location: Cupertino, California

PostPosted: Mon Aug 21, 2006 6:38 pm    Post subject: Modifying an older EPROM drum machine
Subject description: The MXR 185 Drum Computer to be exact
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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.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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...
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ian-s



Joined: Apr 01, 2004
Posts: 2576
Location: Auckland, New Zealand
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PostPosted: Mon Aug 21, 2006 7:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

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
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bigtex



Joined: Mar 30, 2006
Posts: 321
Location: Cupertino, California

PostPosted: Tue Aug 22, 2006 12:49 am    Post subject: Reply with quote  Mark this post and the followings unread

g2ian wrote:
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.


Hey, neat trick. If these 2716 EPROMs weren't so expensive, I'd just buy a whole lot of new ones. Then I could stack them on the old ones and put on switches for the chip select/enable pins.


g2ian wrote:
From memory, each voice of the DX had it's own tunable oscillator, address counter and set/reset logic.


So my guess was right? A binary counter would be used on each ROM to generate the address, all controlled by a common playback rate clock, right? I need to look again at the board and start identifying chips.. and just answer these questions for myself. But I think this would work. New clock for each counter.

The voices with multiple ROMs must have more bits in their address to allow them to span chips like that. Oh, and how would the counter know to stop? Perhaps a specific bit would be wired to the reset pin or something so when it jumps form, say 0FFF to 1000, it would stop. Okay, enough with the speculation. Time to get my reverse engineering hat on. Twisted Evil


g2ian wrote:
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).


Which makes me think... I could just keep the voice board and replace the CPU and button board with a new one using a modern microcontroller. Hell, I could even generate addresses with a new CPU and do fancy playback tricks like arbitrary loop points, bending, stretching, etc.

Or put on bigger ROMs with multiple samples per channel and do wavetable scanning.... oh that's too much...

But I'm getting ahead of myself... I have this problem with coming up with too many ideas, making things too complex, and then never getting back to my original simple idea... never finishing anything...
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toppobrillo



Joined: Dec 10, 2005
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PostPosted: Tue Aug 22, 2006 1:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
But I'm getting ahead of myself... I have this problem with coming up with too many ideas, making things too complex, and then never getting back to my original simple idea... never finishing anything...


me too, my man.. me too.
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seraph
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Joined: Jun 21, 2003
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PostPosted: Tue Aug 22, 2006 5:02 am    Post subject: Reply with quote  Mark this post and the followings unread

topp wrote:
Quote:
But I'm getting ahead of myself... I have this problem with coming up with too many ideas, making things too complex, and then never getting back to my original simple idea... never finishing anything...


me too, my man.. me too.

Pierre de Coubertin wrote:
“The important thing in life is not to triumph but to compete.”

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