Eurorack Implementation of RAKIT BABY8

Initial Thoughts

I first found Rakits after playing with some Korg Volcas for a bit and feeling I wanted to find out a bit more about sound/electronic theory. Starting from a level where I could solder a bit and could tell a resistor from a capacitor, I wanted to try my hand at some music (or at least sound) producing kits. I decided to get the Rakit Deluxe Atari Punk Console and Baby8 Sequencer with the idea that it would give me a highly simplified version of a Volca to play about with. I later added a Velleman Digital Delay kit, but that’s beyond the scope of this.

I contacted Rakit Support at the time I was building these kits, because the Deluxe version of the APC doesn’t have the direct link headers to attach to the Baby8 – I notice that’s now (Spring 2018) on the list of possible new features. Anyway, I remember Darren’s response being very helpful, and we got CV and power-sharing working in short order.

Fast forward to early 2018. I’d had an operation on my back just before the New Year and was laid up for six weeks. A friend of mine who makes panels for all kinds of synthesizers (www.thebeast.co.uk) decided I needed a recuperation project and provided me with a home-made Eurorack skiff case together with a few PCBs and matching panels. Thus began my introduction into BOMs, schematics and silkscreens – sometimes they even match up! Having started putting the modules together, and adding a filter to the LFO, Envelope Follower and Utilities PCBs I’d been given I quickly realised I’d need a CV source to make it do anything interesting – this time, I’m less keen on replicating what I can already do on the Volcas.

I recalled the Baby8 I’d built and on digging it out, found that it would fit with space to spare in the Eurorack size. In a proper Top Gear “How hard can it be?” moment, I decided to try and convert my Baby8 for use with the rest of the modules and fit it to the rack.

Gathering Data

For me, researching a new idea online always leads to the same result: Someone’s thought of it before, and in most cases documented it. In this case a chap called Dave.

Following the Facebook link to his page, I got in touch to ask about his conversion. He corrected an assumption of mine that I’d need to bring the +12V of the Eurorack standard down to +9V for the Baby8 – the 5V regulator already in the kit can do its job just as well at either voltage. He also suggested using Futaba servo connectors for the power – it’s standard for the 1U-sized Eurorack tiles.

It was also at this point that I found the post on the Rakit website about modifying the v2 iteration of the Baby8 so that the step switches skipped their step instead of resetting the sequencer. I was trying to keep the cost down and wanted to be able to do both so contacted Darren about how to modify the modification and apply it to my existing v2 board to allow a switch as exists in the current v3 board. In the end, we agreed that it would be best to start with a v3 board though.

The Build

Since I now knew roughly the direction I was heading, I started thinking about the face plate for the module. From Dave’s account, I knew I was working with a 10HP (50.8mm) wide panel and had the available height dictated by my case. Fortunately, I’d been introduced by my panel-creating friend to Front Panel Designer  which has pre-sets for these panels, including fixing holes. Without the PCB layout (I never even asked Darren for it – it’s his proprietary design) I was a bit stuck for getting accurate hole placement. At this point the new kit had not arrived, so I took the decision to scan my existing, built-up Baby8.

As you can see, it wasn’t ideal. However, the human brain is a marvellous thing and by measuring centres and averages, I managed to get a rough set of measurements of all the holes I needed for the board-mounted components. Obviously in a rack all the sockets need to be accessible from the front, so I knew I’d need space for at least three on the panel. It made sense to put them at the bottom, since two are already there. A bit of playing about in FPD led to a panel I thought might work, so I made use of my employer’s facilities and had a trial version laser cut in thick cardboard.

Pushed onto my existing v2 board it wasn’t too far out but parallax errors from the scan (where the board itself was held away from the surface by the potentiometers) meant most of the pots rubbed and only one mounting hole was correct. Not huge problems, and easily solved by scanning the new board as soon as it arrived. What it allowed me to see for the first time was the problem also faced by Dave – you can’t reach the skip switches.

Dave’s initial solution to this problem was to extend the switch lever with WD40 straw and countersink the holes to thin the face plate. I think he’s made a second version where the switch pins are extended, and the bodies sat on nylon blocks. I was scratching around for how to sort this issue, as I felt there must be a “production-ready” way – more on this fallacy later. Between Darren and I, we considered different switches (and wow, can’t you spend a lot on those!) before I remembered the innocuous set of Single In Line (SIL) sockets on the edge of the board used for hooking up to the APC. These are available in different heights, so although the 3mm ones used on the board were too tall, I found Rapid sold 1.8mm versions, but only in 20-pin lengths (seven switches, three pins each – total of 21 needed). Two rows duly turned up with my next order and the new version of the Baby8 kit arrived on the same day. After scanning the board “bare” for a more accurate faceplate design, I snipped the SIL lengths into threes and did a quick test-fit with the now slightly worn cardboard face plate – it worked perfectly (or so I thought at the time).

The rest of the build progressed as normal, with the small exception of the power connector (now a pair of header pins for the Eurorack power to connect to), the sockets (now three-core servo wire tails) and the mode selector switch (now three hook-up wires from the kit).

Testing

When power was applied, a problem immediately became apparent. The sequencer would not step! Without a schematic (again, not going to ask Darren for his – this is how he makes money!) I had to try troubleshooting using just the white traces on a white board. Fortunately, I had my original device still to hand and could get the schematics for the chips themselves online. With the aid of a cheap oscilloscope I made detailed notes on the signal at each pin of the three chips. Repeating this on the new board revealed a triangle wave signal getting to one chip, but not another. It looks like I’d knocked or scratched the board at some point and damaged a trace as a small piece of hook-up wire between those pins restored what appeared to be full function.

However, I turned the board over and noticed that the sequence reset. Initially, I thought it was a suspect power connection, as I’d temporarily added a 9V battery snap to test with. But no, by tapping the table, it would reset at a different step, and always AFTER I’d tapped, not at the same time.

Look again at the image above with the switch fitted to the SIL socket. Notice the metal body of the switch? I didn’t. The SIL sockets are primarily designed for integrated circuits, which are fully insulated apart from their pins. The bodies of the switches were randomly connecting across the tops of the pins and causing a reset (or a skip). This is where my “production-ready” ethos went out of the window. I fixed the problem by cutting a thin strip of insulation tape, placing it over all the SIL sockets and then pushing each switch through it. Problem solved (badly).

Finalisation

With the board now built up, I redesigned the front panel using the measurements taken from the scan of the empty board. This time, I got a trial version cut in wood veneer. It’s not strong, but with some 10mm standoffs, everything fitted pretty well. I hand-marked the legend – again, this is temporary as I’ll be getting a metal panel cut.

The edge-mounted mode change switch needed mounting somewhere. Dave had mounted it to the existing pads but on the top face of the board instead of underneath. I decided (as mentioned above) to have it on fly-leads and mounted in the row with the sockets. That meant finding a switch (here we go again). Fortunately toggle switches are cheaper than slides, so I got a miniature toggle SPDT on-on switch for a few pence. However it only just fits between the board and the case.

Darren had been very kind when sending the v3 kit and included a set of panel-mount sockets (i.e. with a thread and nut) as well as the standard smooth ones since he knew what I was up to. Unfortunately, because the pins emerged at 90 degrees to the socket, there wasn’t enough room to fit them onto the face plate in the gap between the board and the edge of the case. I had two choices here: I had some Thonkiconn sockets waiting for other modules, and also some in-line panel mount sockets left over from a project to mount all the inputs and outputs for my Volcas in one panel. I decided to leave the Thonkiconn jacks for other modules and the in-line sockets fit with more room to spare too. Main thing that caught me out with the sockets is that the ones normally used on the Baby8 have the tip connected to the middle pin – I’d assumed they were ring-sleeve-tip (moving from the edge of the board). I just had to reverse sleeve and tip connections on my sockets.

In use

My Eurorack’s only current tone generator is the Humpback filter with its resonance turned right up. With one LFO providing some “wobble” and a second feeding the clock-in of the Baby8, I fed the CV out from the Baby8 into the second CV input of the filter. As you can see in this short clip it provides a spread of tones, as intended.

[vimeo 266665278 w=640 h=360]

There’s a longer clip, proving I could reach the switches here.

[vimeo 266665262 w=640 h=360]

What this proved is something Darren warned me might happen but had not shown up until it was in the rack and I could only reach the tops of the levers. The switches tilt under the leverage and try to pull themselves out of the SIL sockets because they’re not quite tight against the faceplate. Now that I’d already given up on quality fixes, I dropped a nylon washer over each switch to take up the slack. So here are all the bodges – bridging wire, insulation tape and nylon washers.

Controlling a resonating filter isn’t all that exciting though. However, controlling that filter when the filter’s being fed an audio signal from elsewhere makes more interesting things happen. In this case, my Volca Keys is first heard clean, then fed into the filter with the Baby8 controlling the cutoff via the CV.

Oh, and why did I call it Stiga Fram? I kind of like Scandanavian things, and this means “Step Forward”. Since the Baby8 is a nice, simple sequencer that doesn’t do reverse, ping-pong or random, I thought it quite appropriate.

Reflection

So, if I was doing this again (or “properly”) what would I change?

1. Ideally, change the skip/reset slide switches for ones that can be reached through the faceplate without modification. This might involve changing them for toggles, but there are space limitations due to the proximity of the step potentiometers – the smallest toggle I’ve found is 5.08mm wide, while the existing sliders are 4mm. Maybe extending the levers is the best way after all.
2. Swap my silly big “miniature” toggle switch for mode selection to a smaller, sub-miniature version.
3. Create a PCB for mounting the switch and sockets with header pins for connecting to the main PCB. Maybe on protoboard if it’s just for me.
4. Assuming point 3 is done, use Thonkiconn jacks – fractionally cheaper and seem to be more durable – repeated plugging is not a strong point of these in-line versions.
5. Add gauge indicators on the panel for the knobs.

Resources

Additional parts purchased:

PART	Code	Supplier	Price	Link
SPDT toggle switch	448-0747	RS Components	£1.50	https://uk.rs-online.com/web/p/toggle-switches/4480747/
2x SIP socket strips	50-8118	Rapid Electronics	£4.78	https://www.rapidonline.com/50-8118
7x nylon washers	33-5064	Rapid Electronics	£2.08 / 100	https://www.rapidonline.com/33-5064
SIP header (2 pins)	N/A	eBay – buyhere22	£0.99 / 40	http://r.ebay.com/eMzAKW
4x M3 x 10mm standoffs	33-3520	Rapid Electronics	£3.43 / 25	https://www.rapidonline.com/33-3520
8x M3 x 6mm screws	33-6549	Rapid Electronics	£1.68 / 100	https://www.rapidonline.com/33-6549
3x 3.5mm in-line jacks	005674	Kenable	£2.16	https://www.kenable.co.uk/product_info.php?products_id=5674
2x Futaba servo cables	N/A	eBay – LED-Essential	£2.99 / 5	http://r.ebay.com/9r7D7G
Face plate panel	N/A	The Beast	£35*	www.thebeast.co.uk

*£20 each in bulk

Face plate FPD file with legend and logos.
Front Panel Designer software.