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Jamma restoration

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Jamma restoration

Postby BJG145 » Wed Aug 12, 2020 7:50 pm

Back in the early 1980s, teenager Pete Jones invented the Dynacord Rhythm Stick...

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...aka the Jamma...a keytar-style rhythm controller that found its way into the hands of musicians including Michael Jackson and Billy Ocean.

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Here's Pete demoing it to Jools on The Tube

https://www.youtube.com/watch?v=X7rG6Kv ... =emb_title

Pete dropped into the forum a few years back to get ideas on how to update a Jamma he had, and took up the Megadrum suggestion as documented on his website.

I've been curious about this gadget, but they rarely come up, so when one surfaced on Reverb recently I couldn't resist putting in an offer. And...the seller recently came back to me with a sensible price, so this thing is currently winging its way over from Germany.

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This is the MIDI version. It's designed to connect to a PSU/adaptor box with a proprietary cable...

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...but these are missing (and the thing might not work anyway).

I think the MIDI electronics are housed in the guitar part though, so the missing box (which has MIDI out) might be more of just a PSU/connector thing. If not, I expect Arduino can come to the rescue.

Apparently the box also has an interface designed to connect the Jamma to other early Dynacord rhythm gadgets like the Percuter...

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I think it provides a set of trigger connections, which could be used with any trigger-to-MIDI interface.

I tried contacting Pete via his website, but no reply yet; I might also see if he replies to a SOS PM if I get stuck. In the meantime I've been digging out documents like the original patents, but I haven't found much info on the electronics so far. Look forward to pulling this apart. :D
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Re: Jamma restoration

Postby The Jamma » Tue Aug 18, 2020 7:54 pm

Hey Ben!

As per my PM, i'll dig around for a possible replacement PSU etc.

There is sooo much interest over the last couple of years that i'm seriously thinking about a new version!

I've still got a (very beaten up) Percuter and a case full of excellent Dynacord drum sample cartridges BTW, nothing quite sounds as good as those! If anyone would offer a loving home for them, please let me know, i'm trying to downsize!

Good to be back in the world of music, i kinda lost myself in the world of business for... humm a decade at least.

:) Pete
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Re: Jamma restoration

Postby BJG145 » Tue Aug 18, 2020 8:38 pm

Thanks for the reply Pete! Really cool to hear from you. Any info you did happen to find would be much appreciated.

* * * * *

This is my new gadget...

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I opened it up this afternoon and realised...oh, this is more complicated than I thought. ;)

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I've recently been experimenting with Arduino-based MIDI controllers which are fairly straightforward, but of course this was nearly 40 years ago.

The main playing mechanism revolves around this set of eight buttons...

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...which select rhythm sounds that are played using these pads.

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There are various different modes of operation where, for instance, the eight-button matrix can trigger sounds themselves, or select sounds to be played with the pads, and various other configuration options. These are selected in this panel, which also has an LED display.

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I wasn't sure how much of the MIDI circuitry was in the guitar body, and how much was in the missing adaptor box; but after taking a look I'm thinking that the box is mainly a PSU with different output options for MIDI or Trigger, so I'll let Pete dig through his archives and see if he can shed some light on that.

As you'll all probably have guessed by now, electronics is mainly just a fascinating and mysterious puzzle for me. For instance, that eight-button matrix is connected only by a single ribbon cable with eight wires, and the back of its circuit board is a maze. I wouldn't have guessed that. I'm interested in how the logic of that kind of thing works. (Is it based on binary or something?)
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Re: Jamma restoration

Postby Sam Spoons » Tue Aug 18, 2020 10:26 pm

A three button footswitch can control three functions down a single TRS cable, T+R=SW1, R+S=SW2 and S+T=SW3. Maybe this is an extension of the same principle?
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Re: Jamma restoration

Postby BJG145 » Wed Aug 19, 2020 1:35 pm

I posted up a request for pics of the box on a Vintage Dynacord Facebook group, and someone with one of these PSU/MIDI adaptors kindly opened it up for a couple of photos...

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This box plugs into the mains, and I guess the power supply back to the instrument is provided by those stripey wires marked +15 -15 +24.

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So, does that mean it needs a 24V supply? Seems a lot?
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Re: Jamma restoration

Postby BJG145 » Fri Aug 21, 2020 11:58 am

I reckon that PSU is going to be difficult to find or build, so I'm looking at the alternative of replacing the existing electronics with Arduino. First step is to find out how to connect up the various controls, starting with that eight-button keypad.

After a bit of research and trial-and-error, I've realised that it's a common kind of circuit called a keypad matrix. This has a set of switches and diodes arranged in a grid. A 3x3 layout would look like this:

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The Rhythm Stick follows a two column, four row layout. If you check out the ribbon cable connections you can see that two of the eight wires aren't connected:

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The six live wires represent the rows and columns. Counting from the blue one, it goes something like this:

row 1
x
row 2
column 1
row 3
x
row 4
column 2


The original circuit will have had some IC to decode the pattern (its HD6303RP MCU I think, looks incredibly complicated), but you can also use an Arduino, by connecting all the row and column connections up to the digital pins and using the Keypad Arduino library. After a bit of tinkering with the connections and the demo sketches, I ended up with this...

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...all buttons recognised; pleased with that. :thumbup:

One of the Rhythm Stick modes would allow you to play sounds directly from those buttons, and that would be easy now.

Next step is to find out how those rhythm pads work and get those wired up...
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Re: Jamma restoration

Postby BJG145 » Sat Aug 22, 2020 11:12 am

These are the two pairs of wires for the rhythm pads.

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Referring back to the patents confirms that they're piezo.

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In the original they're connected in parallel so they both have exactly the same effect, though they could be separate in the redesign.

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At this point I decided to get the scissors out and commit to the Arduino makeover.

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So then it's a case of connecting up to an analog pin...

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...and reading the values.

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Simples, except that they're not good at returning to zero when released. I don't know if that's normal for piezos, or whether it's an effect of the aged rubber casing they're mounted in; maybe it's deforming and then not returning to exactly the same shape.

It's recommended to wire a 1Mohm resistor across piezos when connecting them to Arduino so as not to overload the 5V maximum input on the pins. I'm wondering about maybe using a lower value resistor, and with the right amount maybe the voltage would drain rapidly back down to zero after thumping one of the sensors. I'd prefer it if they could be persuaded to hold and drop a particular value when you press and release them, as I was wondering about using them for other things, like modulation. In the first instance though I think I'll get hold of a 1Mohm trimpot to put across it and see what happens.
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Re: Jamma restoration

Postby BJG145 » Mon Aug 24, 2020 12:32 pm

This circuit contains four momentary switches for the buttons, and the LED display...

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I traced the wiring for the four switches, and found they were connected in series with a diode in such a way that each completed a different circuit between two leads in the ribbon cable.

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(Although the top one looks like a blackened mess, that's just the result of some over-enthusiastic dodge/burn in Photoshop.)

After a bit of experimentation on a breadboard, it turned out that by connecting one wire to ground and the adjacent four to +3.3V through an LED, it was possible to get them working.

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Re: Jamma restoration

Postby BJG145 » Thu Aug 27, 2020 9:09 pm

That circuit also contains four seven-segment numerical LEDs like this one…

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The way these work is that you connect +5V to one of the pins, and you can then illuminate the various segments and the decimal point by earthing eight of the others.

They're controlled here by a couple of UAA2022 chips. These take a serial input (data and clock) and convert it to drive the LEDs.

I spent a while tracing the circuit and matching it to the example in the datasheet

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But although the connections made sense, I couldn't figure out how to get the Arduino to talk to it. There’s no other documentation, and certainly no easy tutorials for this thing. I got as far as some power-on garbage, but that was it.

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I’d like to have sussed them out, but I gave up at this point and picked up a mini 0.91” 128x32 OLED for £3.99. It fits neatly into the space occupied by the trio of old LEDs.

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This is a well-trodden path; tutorials abound, and I was able to get it up and running in about five minutes, as opposed to several hours failing to understand the UAA2022s. Four easy connections, and simple programming, with nice display commands instead of the incomprehensible stuff I'd seen in serial driver examples.

u8g2.drawStr(8,29,"JAMMA");

I’d like to preserve some of the original, so I’ll keep the fourth segmented numerical one and wire that directly to the Arduino. It’s fairly easy to control it that way, but it wasn’t worth the trouble for all of them and I’d have run out of outputs. Besides which, the OLED is nicer anyway.

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Re: Jamma restoration

Postby BJG145 » Sat Aug 29, 2020 10:48 pm

The next thing I wanted to do was wire up the numerical LED I was planning to keep, then fit this circuitboard and the new display back in the case. So, it was bye-bye UAA2022s and farewell triple-LED.

Years ago I tried some de-soldering ribbon that seemed to work pretty well, but the stuff I picked up this time was useless. It just didn’t absorb the solder at all; I might as well have been using dental floss. So I resorted to pliers and a hacksaw.

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I used to think circuits were delicate things that might go wrong if you breathed on them too heavily, but not at all. These chips put up a fight and it was pretty brutal, like pulling teeth in the 19th century.

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One of the buttons was fatally injured during the battle, but I only had three of the original caps anyways and hadn’t yet managed to source a fourth, so I figured, oh well. Never mind.

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Having cleared space for the new OLED, I checked what it looked like through the red plastic window of the case. Not amazingly bright, but, it’s OK. (The lettering for the buttons is upside-down relative to the logo as it’s for the player to read, so the display is the wrong way here.)

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I then wired up the remaining numerical LED direct to the Arduino. I looked at the holes where one of the UAA2022s was, worked out which ones corresponded to the LED pins, and soldered a bunch of wires into them, then experimented to see which Arduino pin controlled each segment when using the standard library for this kind of display.

There were a couple more gotchas...the demo program just displayed garbage until I noticed the declaration:

hardwareConfig = COMMON_CATHODE

There are two varieties of these LEDs; one where you connect one pin to earth (common cathode), and another where you connect one pin to 5V (common anode). After changing the value in the code I was finally rewarded with sensible output, and packed everything back in the case.

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I'm trying an Arduino Pro Micro for this; it's like a miniature Leonardo, which has simple USB/MIDI support (supposedly). I cut the wires too short so the thing is trussed up like this at the moment...

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...but I've finally got the old LED working alongside the new OLED.

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Re: Jamma restoration

Postby BJG145 » Sun Sep 06, 2020 9:08 pm

At this point I was running out of connections, so I liberated the Micro Arduino and swapped it out for a Mega with a screw shield. These are brilliant for non-solderers like me.

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I ended up putting a 10Mohm resistor across the piezos, which tames the maximum voltage from a thump, and resets them to zero afterwards. The levels look a bit erratic, but I'll see what it's like when I get to the MIDI triggering.

I’ve ditched the breadboard and replaced it with a small circuit on stripboard, of my own design. I am extremely proud of this. There’s a couple of DIP sockets for the ribbon cables…and that’s about it. But stripboard is incredibly finicky stuff and it took me ages.

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The small one is for the 8-way keypad matrix, and the larger one is for the three remaining menu buttons.

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I may find a replacement 4th, or just a better way to blank off the space, like a darker shade of tape.

It took me a while to get those three simple buttons working because I didn't understand about pull-down and pull-up resistors.

(Basically, with momentary buttons like this, you can connect them between +5V and a digital pin on the Arduino, but you need to fit a "pull-down" resistor between the pin and ground to pull the pin down to 0V by default. Pressing the button then registers as "High". Alternatively, you can connect the buttons between ground and a digital pin, and use "pull-up" resistors to set the pin to +5V by default. Pressing the button registers as "Low". This method has the advantage that Arduino has the pull-up variety built in, and you can access them via software without wiring up more stuff, so I went that route.)

A slight annoyance with swapping the Micro for a Mega is that it makes USB MIDI more difficult.

From what I can gather, there are certain models of Arduino based an "ATmega32U4" processor, such as the Micro/Leonardo, and these seem to support USB MIDI. But the Mega doesn't support it natively. It's apparently possible to flash the firmware to add support, but this seems to interfere with the process of uploading programs so you'd need to flash it back to change the code. I can't be bothered with that, so I picked up a MIDI breakout board, though I haven't tried it yet.

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Pretty much everything else is in place now. I accidentally wiped out the simple all-in-one test program I’d been developing, not realising that the Arduino IDE saves automatically when you upload a program to the controller. But I have individual “sketches” for the OLED, LED, piezo sensors, volume pot, menu buttons, rhythm buttons etc which all work fine. Looking forward to securing the boards then being able to shut the lid on it and focus on the programming…
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Re: Jamma restoration

Postby BJG145 » Mon Sep 07, 2020 9:29 pm

...MIDI interface now working nicely, and I've got it to the point where I can mess around and play stuff on it. I decided to launch it upon the world with the riff from Tubular Bells, and I'll leave things there...at least until live music returns, when I'll be able to bring it to a wider audience. (Or a few bemused punters down at the local.)

https://youtu.be/HGymxM_366g
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Re: Jamma restoration

Postby blinddrew » Mon Sep 07, 2020 9:49 pm

Very impressive! :clap: :clap: :clap: :thumbup:
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Re: Jamma restoration

Postby The Jamma » Thu Sep 17, 2020 9:09 am

Hi Ben!

So sorry for the radio silence.

Can't find the power supply guy yet, but still digging! But I see you've got a long way down the road!

What the PSU did was power The Jamma, and split out 8 analog outputs that will trigger the Dynacord Perquter (or other analog drum unit), its also provided the MIDI… so if you can just power it up, you should be able to use analog really easily. BTW I’ve got a very old percuter and loads of sounds if anyone wants them.

Additionally, I have some spare Jamma main boards.... I've never tried it but they 'should/might' be replaceable with the dynacord board and controls. If you or anyone were up for trying that, I'd happily send one over.

I thought you might like to know that after what, say 20 years of waiting for tech to catch up, it finally has! Over the past year or so, I've managed to compile a formidable team of tech dudes that are just as keen as me to finally let The Jamma be free and do what it should have done all those years ago!

The Jamma can once again be imbued with internal voices - like my hand made ones did 30 years ago... Mind you, they were analog ‘booo, pooo’ drum sounds combining white noise with an eq sweep filter and ADSR oscillator, mostly built with discrete components by yours truly. But the point was, it was a self contained electronic instrument with sounds and power, just plug it in and make music…

Dynacord changed that part; something I never felt comfortable with...

Anyhoo, so now the team and I are steaming through a re-development and already have epic things to share with the world soon. The Jamma like never before, it’s going to mash people’s heads up!

Just thought it would be cool to share the news! Watch this space :D
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