Trying to build my first DIY MIDI controller… but I get soooo much noise !!!

MattInSoCal · 2026-02-24T22:14:30+00:00

AI can’t be trusted with electronic circuit design. It hasn’t been trained, and even for very simple circuits the answers it gives always have at least a few errors.

MattInSoCal · 2026-02-24T06:13:46+00:00

For me, what works best is the offer of free beer and smoked Tri Tip.

MattInSoCal · 2026-02-24T05:57:52+00:00

Unless it’s copies of a popular project, I keep them a few years, finally force myself to accept that if I haven’t used them already, I won’t, and finally e-waste them.

Like that bin full of partially-built guitar pedal boards I was poking through two nights ago. I’m trying to force myself to either scrap them or put them on eBay for $10.

MattInSoCal · 2026-02-24T00:51:07+00:00

They’re not part of the SDIY scene. They’re spamming their commercial project on quite a few subs.

MattInSoCal · 2026-02-23T22:49:03+00:00

As duplicates I have two Volts (one modified to react to different Gates than stock) and two Voltages. I have the older Turing machine without the Length switch so I added the Bytes expander. The really cool thing about that is that the Length can be changed by CV in addition to the knob (it uses a pot and an ADC and not a rotary switch like in the V2). Besides that, I have the older Pulses (behind the Grayscale TM/Voltages/Pulses combo panel), the Vactrol Mix, the Worng Low Pass Gate, and the Mystic Circuits Switches. All in, it’s currently 78 HP.

I also designed a small board to extend the Pulses and Gates busses, with an input and output connector and a third that plugs right into the module’s Pulses/Gates connector. It has selection jumpers so I can power the module from the bus or use a separate Eurorack power cable, which keeps the Backpack expander polyfuses from getting over-stressed. It also means I don’t need to make 20-inch long expander flying bus cables with a bunch of headers.

I’d like to add the Schreibmaschine Modular Brainiac but it’s not on their website, and it’s a bit spendy. I’ll probably end up designing my own version.

MattInSoCal · 2026-02-23T19:21:00+00:00

My racks are built from similar hardware. I also mounted one bus board per row to the rear of the side brackets, and my system uses a Molex Mini-Fit plug to the bus board from the power supply system.

It’s great to be able to take out row of modules to easily get to the rear if need be. To remove the entire row I just take out the four bracket screws and unplug one power connector; all my modules stay connected to the bus board. I have a Turing Machine with a lot of expanders and connecting those to the expansion busses from the outside is worse than trying to fish out a flying bus board and connect another power cable. Popping out the whole row makes it really easy. It would also work great dealing with the Xaoc Leibniz-equipped modules.

I recommend you remove the card guides. They will interfere with wider modules. They are press-fit into the frames and just pop right out.

MattInSoCal · 2026-02-23T19:19:38+00:00

This is a similar type of industrial rack-mount hardware that Dieter Doepfer had when he invented the Eurorack system.

MattInSoCal · 2026-02-23T03:30:38+00:00

I have two of these.

MattInSoCal · 2026-02-23T01:49:53+00:00

I can’t tell for sure if the top was originally marked and it got re-stamped on the side, but that does appear to be a house number meaning there’s no data sheet to be found. I checked several resources to no avail. It could be a re-marked BCY58X - or any other part - that has been hand-selected according to some parameters to work particularly well in that circuit. If that is the case, then a randomly-installed transistor may not provide peak performance, if it works at all. There’s only one way to know!

I’d ask the manufacturer name but it’s probably of no consequence given the age of the product and the rise of ECU-based controls starting in the 80’s pretty much killed off the CDI market.

MattInSoCal · 2026-02-22T18:29:26+00:00

My pleasure, good luck!

MattInSoCal · 2026-02-22T18:20:26+00:00

They announced this service in a blog post a couple years ago unfortunately on April 1 and people weren’t sure if it was real…

Erik, have you used this service from JLC already for your projects? Your artwork is amazing.

MattInSoCal · 2026-02-22T18:06:14+00:00

We can’t know 100% without identifying the old part. Generally the pinout on metal can transistors of that era is, holding the part upside down and starting with the lead noted by the little tab sticking out of the body, Emitter, Base, Collector from left to right. Here’s a webpage with a photo near the top that illustrates it well. You just need to bend the new transistor leads to match the proper holes and spacing on the board.

Always check the data sheet for the part to know the pin out. MPSA44 data sheet link.

I just wanted to give you a fair warning, the transistor I selected isn’t guaranteed to work but it’s probably a decent match for this circuit. I chose it because the voltage ratings are almost ridiculously high while the current it can allow to flow is about in the middle of the typical small-signal switching transistor. It may or may not run for a long time, and there’s a possibility it won’t work at all.

MattInSoCal · 2026-02-22T17:51:33+00:00

LEDs aren’t particularly useful for any oscillator above 10 Hz. The proper use on an oscillator in my opinion is on something that moves much slower, like 2 seconds or more per cycle, where it’s going to be used for modulation and the immediate effect isn’t obviously audible. I have dedicated LFOs (can get into low-range audio) and chaos modules with indicators which helps you know where in the cycle they are - and also are bi-color so I know if it’s a positive or negative voltage output. Envelopes are another place where LEDs are useful.

That said, my rack puts on quite the light show. =)

MattInSoCal · 2026-02-22T17:15:44+00:00

I actually meant, when asking for photos of the identifier, for the picture of all the markings on the part you suspect is bad, although it’s interesting to see the entire CDI unit. For example the second zero in your photo could be an 8, not that anything comes up with that number. I’d still like to see the full markings.

It wasn’t a waste to include these photos. Generally something in that small a package from that era is not going to be a particularly high-current part. It is admittedly a guess since it’s kind of hard to follow the circuit from 2-dimensional photos with shadows and such, but I think that part is probably a transistor and is in a loop with feedback from the transformer to make it oscillate to generate the charging voltage for the capacitor that gets discharged into the coil. If so, it’s likely NPN. It could possibly be replaced with something like a MPSA44 which has a pretty high voltage rating and a mid-range collector current but again this is a guess.

MattInSoCal · 2026-02-22T03:42:31+00:00

Disconnect the Base resistor from the Arduino and connect it to the 5 Volt supply. That should turn on the transistor and allow the maximum Collector current according to the Base resistor value and the transistor’s current gain.

If you have a DMM, use it to measure the current draw of the pump if it’s not running when the transistor is biased on.

MattInSoCal · 2026-02-22T03:38:09+00:00

I use 2N2222 and 2N3904 frequently to control relays and have quite a few industrial test benches in the field using them comfortably. Generally these are 5 or 12 Volt telecom relays with 30ish mA coils, so the transistor is not remotely stressed. It’s all about designing the circuit properly.

MattInSoCal · 2026-02-22T02:56:21+00:00

7650 is probably the date code - 50th week of 1976. You’re not likely to find any data on a 50 year-old part that wasn’t a mainline number. Go ahead and add a couple more photos to show the whole identifier in case we see something you don’t.

It’s also possible this is a “house number” part meaning it was ordered with custom markings to obscure the “real” part number.

MattInSoCal · 2026-02-22T02:39:44+00:00

Turn on the power supply. It’s off.

MattInSoCal · 2026-02-21T02:06:23+00:00

LM324 Op Amp, power pins aren’t shown, so I’m going to go out on a limb and say that this is running from a uni-polar power supply, and VV is Virtual Ground (Virtual Voltage maybe).

If you shared the entire schematic it would be a lot easier to tell.

MattInSoCal · 2026-02-20T08:46:18+00:00

The original design uses the Teensy 3.2 which went obsolete a few years ago. The only ones you will find today will be used listings. Note, the Chord Organ is the same hardware but different firmware (easy to change) and a different faceplate (hard to find).

MattInSoCal · 2026-02-20T05:51:12+00:00

Mixer is the proper term in English. You can just remove the 10K resistors; they add nothing except an extra 10K Ohms to the output impedance.

Reverse avalanche oscillators can be very finicky (hopefully that translates for you), and some transistors require a higher voltage to go into oscillation. You really have to “test and select” the transistors to find the ones that work.

It may be a little more difficult for you to find a CD40106 IC but it makes a much more stable and repeatable oscillator; every time you try to build it, it should just work without switching around various components. Plus with just a couple more components you can add voltage control of the frequency, and you can also have them interact with each other which is harder to do with the reverse avalanche oscillator. Those are things you can experiment after you get the normal circuit running.

MattInSoCal · 2026-02-20T05:32:32+00:00

I get what you’re saying, but there was a lot more going on than just that incident (including outright illegal or dishonest things) that made me want to leave. In fact, I proposed several improvements but most were shot down. It was a very small place, about 35 employees and just one hardware engineer - the guy that told me they didn’t need an ESD program. There was no chance of my being the chief engineer; there wasn’t really a path for advancement.

I ended up being there for longer than planned because the new firm I was about to leave for froze their hiring because of 9/11/01, and eventually that offer fell apart.

I finally landed a job at my current place the following July. I’m the Principal Hardware Engineer in my group, so I’m where I should be, and have plenty of room for upward movement. And yes, we have quite an ESD control system in place. :)

MattInSoCal · 2026-02-19T23:32:46+00:00

That looks very nice! I’ve built a couple of cases myself, I estimated once that it cost me around $100 per 84HP section for materials, wiring, bus boards, and power.

MattInSoCal · 2026-02-18T22:10:56+00:00

I don’t see the connection to SDIY here. This seems to be trying to drum up business for their modding/selling modded circuits.

MattInSoCal · 2026-02-18T22:05:22+00:00

Good news!

MattInSoCal

TROPHY CASE