Building a MR9090 Mullet rhythm and DR55 sound card

mullet rhythm mr9090 drum machne with dr55 sound card


Having built a DR55 soundcard I wanted a Roland style drum machine interface to go with it.

I saw the Mullet Rhythm on the internet, apparently it was designed to complement a MR 9090 sound expander unit so it added the controller part to make it like a complete drum machine. It looked buildable with not too many components, I could use it to trigger analogue drum sounds and synthesiser modules, something I used to do in the days when I had lots of analogue stuff and used a Roland TR606. I also liked the fact that it had midi in and out and din sync in and out. I was going to have to program PIC’s so I learned about this and produced a separate article here if you need to know about that.

I started playing around with the design on strip boards and got it all working, but was running out of space for adding the din sync and midi interfaces so I decided to design a printed circuit board. I don’t know if anyone else has had this problem or maybe I’m looking in the wrong place but where do people buy those fancy push button switches from? There are some really nice ones with LED’s in that look like the type that synthesiser manufacturers use but where do you get them?

Cannibalising a computer keyboard for MX Cherry switches

If you’re lucky you may be able to find an old computer keyboard that is made of mechanical switches. That means that each switch is a pushbutton and usually a very high quality one. All of the mechanical ones I have stumbled upon have contained what are known as Cherry MX switches. It’s usually a case of the older the keyboard the more chance it will be a mechanical switch type. Almost all of the newer ones are membrane keyboards and they don’t have individual switches.

Most people aren’t aware that these older mechanical switch keyboards are worth a bit and quite sort after. I wasn’t until recently. It’s not easy to identify them without taking them to pieces, but you can do it by feel, it takes a bit of experience but if you have a mechanical keyboard and a membrane one next to each other, you can usually feel the difference.

I’m just getting them to harvest the switches from them. You can simple buy them but they do cost a bit if you need a few and then you will have to budget for the key caps as well.

Once you have the keyboard, you will find most of them are just screwed together, take out all the visible screws and then just pull apart the plastic top and bottom. You will then usually see a printed circuit board with a metal chassis on the component side with the Cherry MX switches pushed through square holes in the chassis and then soldered on the board.

First thing to do is to lever off the keycap, you can buy Cherry MX key cap removers, that’s an indication of how popular these switches are. But you can lever them off with a flat bladed screwdriver if you’re careful. You may want to get a tool if you’re going to be using theses switches though as it does make the removal easy and there’s les chance of damaging or marking a key cap. Once the key cap is off turn the keyboard over and desoldered the two terminals that mount the switch to the board. This isn’t too difficult as they have large pads and are usually single sided.

The black things are the Cherry MX switches on a metel plate.

Make sure that the connections are completely free which you can usually see and feel by wiggling the pin around. The plastic body of the switch is a very tight fit to the metal chassis and is held in place by some small indentations moulded on the side on the switches body. You have to lever them out with a flat bladed screwdriver. Do it carefully and they pop out with practice. Simply repeat for the other switches. You can see from the keyboard that some of the keys caps are bigger like the enter and space bar, so far I’ve only used the standard normal keyboard size.

Then you will have it plenty of Cherry MX key switches and key caps. I’m going to be experimenting with spray painting the keycaps and I’ll do an update after. One thing to note is that the keycaps are different shapes from different rows, something I didn’t realise until I looked at my finished drum machine and saw something didn’t look right.

cherry mx key caps salvaged from a keyboard

They are quite difficult for a hobbyist like me to mount though, they are PCB mounting and you can’t fit them to a front panel so I found the best way was to solder them to a PCB and then mount the board to the front panel with spacers and have the key tops through a hole in the panel. That does mean that if you mount a row together you need to cut a rectangular hole, again something I don’t find easy. They are excellent switches though for electronic music projects as they don’t need much pressure and excellent quality with changeable key caps.

mullet rhythm cherry mx pcb

I had to make separate PCBs to mount these so took advantage and also used the board to mount the LED’s as well.

Prototyping the MR9090

I’d got it all up and running on the bench with loose PCBs scattered around so not the most practical thing to use. I wasn’t sure how to mount it or what case to use or if I should put by DR55 drum sound module in the same box. All of this indecision basically stopped the project for months even though it was all basically working. I got too hung up on trying to make the whole thing look good and ended up not having a usable machine. Does that sound familiar to you? While working for Marconi I was surprised that the boards that went into some of the high tech devices weren’t production standard they usual had modifications to implement and I later found out that if they did another run, then more mods then another run, by the time the project was finished it was out of date! While having s shower one evening (that’s where I make most of my decisions) I decided I was going to forgo the cosmetics and get a functioning unit that I could modify and change as I was going along, this was a light bulb moment for me.

prototyping the mullet rhythm and dr55 sound card

As you can see the problem with working like this is it can get messy, you can't move it around and wires can break off.

I did get it working though!

Multi storage box

I found a perfect case to start building it into. It may look a bit strange but I decided to use one of those plastic multi storage boxes. Size was it was just about perfect and it cost less than £3. I bought a couple more to use on future projects. The front translucent plastic is soft and easy to drill and cut with a Dremel, as is the black plastic base and the sections can be trimmed away with a sharp knife if required.

In a rush to start I forgot to take a picture of the box before I started. Imagine it without the stuff on top.

The first thing I did was to mount the Perspex front panel by cutting a slightly smaller square hole in the plastic box front and bolting the two together using black nylon M3 nuts and bolts these have quickly become my favourite mounting devices along with black nylon threaded stackable 6mm and 12mm spacers. I now keep a stock of these as they are tremendously useful in projects.

mullet rhythm front panel

This is the perspex front panel. I now realise it's difficult to photograph perspex it's like trying to get a picture of the preditor (the creature not the football boot).

By this time I had decide to mount the DR55 sound module in the same box. You can just cut away some of the plastic compartments with a knife for the various boards. One of the compartments was ok for the power supply and it was easy to drill a hole for the socket. Cutting through a partition made a section for the main MR9090 control board and another for two DR55 sound module boards, one for the standard sounds and one for DR55 mods.

multi storage box dr55 case

The compartments form useful mounting and holding for components and PCB's

Using this plastic box was certainly speeding up the project. Having things in place made the wiring up between the boards more straight forward and I could shorten and rewire some of the connections. More importantly everything was now in one movable base unit and the hinged front panel was perfect to build a prototype unit in. I drilled holes along the edge for the sound outputs and trigger outputs again very easy with the soft plastic certainly easier than steel or aluminium.

multi storage box dr55 lid

The lid also hinges so you can work inside and outside to your hearts content.

My enthusiasm was back now as I was making quick progress. I was being too precious with decisions and not wanting to get it wrong but this way I was just getting a working unit. This is how I’m going to build future projects, by getting them working in these boxes I can always rebuild them later if I want something more professional, at the end of the day it’s only a few pounds even if it eventually gets thrown away if you rebuild it, but in the end I want something I can use quickly not wait till I eventually get round to a proper case.

DR55 mouted in the corner of the multi storage box

The first Dr55 board is mouted in the corner and the second Dr55 board with the modifications is mounted on top with the controls along side on the front panel.

Functionally the MR9090 Mullet Rhythm is like a Roland drum machine interface with sixteen buttons and LED’s that you use for each instrument, you can store your patterns into the banks A to D and link them together to form tracks. Stuff can be named and you can shuffle and flam and do pretty much anything programming wise that you can with a Roland drum machine. I used two DR55 sound boards to give me 8 analogue sounds with a four external triggers for my other analogue synthesiser stuff.

Almost finished with knobs and labels, just need to fit the key caps.

Midi sync -Din sync conversion

I’ve not yet added the midi or din sync connections yet although I did include provisions for them on the PCB. It will be easy enough to add them by just soldering a few more components and drilling a few holes. It will be interesting to see if the MR9090 Mullet Rhythm functions as a sync converter, that is, if running it to be synced by external midi in, will it output din sync at the same time to interface other stuff? That’s going to be something for another day.