ARP Soloist meets 3D Printing

This is a ARP Soloisist Slider Fixrare original ARP Soloist (before the ProSoloist or DGX). A lot or restoration work was done on it, but one issue remained; the slider shafts were all broken at the point where they exited the case, and the slider caps were long gone.

The sliders are the same type as fitted to the ARP 2600 and moog SonicSix, amongst others. This is a hard to find part, and here we have 4 sliders with 3 different values. ARP Soloisist Slider FixSometimes you can find replacement shafts for these on Ebay, but nothing was listed at this time. After much head scratching I decidedARP Soloisist Slider Fix to raise the existing sliders by 8mm, and needed some precise flat blocks to do that with. I came up with a design and after many attempts (that’s 3D printing for you!) I produced a nicely printed set.

ARP Soloisist Slider FixThe sliders were removed, opened, cleaned, re-greased, then set on the blocks which I had super-glued to the ARP Soloisist Slider Fixcircuit board. There were locators in the blocks for the original slider mounting, so when the wiring was added using a heavy gauge wire extension, all was firm and true. I re-assembled the unit and added some Omni-2 style slider caps I had previously printed. All working and crackle free. Next move was to tune and scale it.

One item remains on the Soloist, the after-touch used conductive foam which was known to only last 8 months, and had all by disappeared on this 1975 keyboard. I would have liked to have got some modern strain/pressure sensors and tried to make a solution, but the owner decided to defer that to another time.

This Old Synth acquires the exclusive rights to Synthfools Opto-key for MiniMoog project

As a tech I’m increasingly frustrated with the chore of trying to restore the (oxidized) key contacts on the MiniMoog to full operation, and unless it is frequently played the issue of “squirelly keys” comes back. I have been installing Opto-key for my customers which removes the key cleaning cycle, AND adds simple note-on/off  MIDI.

I was concerned and saddened to hear that Kevin Lightner at Synthfool could not continue the Opto-key project  as it addresses the biggest reliability/playability issue with the MiniMoog, and have negotiated with him to transfer ownership here.

Once the on-hand materials such as circuit cards arrive I will commence building the product to Kevin’s spec and make it available for sale.

As I’m also a software developer with experience in embedded systems I plan to enhance and support Opto-key moving forward. I’m really excited by this!



MTG Pro One Turbo CPU

Turbo CPU

I’m generally against modifications.  I have seen too many vintage synths spoiled by badly drilled holes that hold fragile switches, connected through an “umbilical cord” to undocumented perf-board based mods made decades ago, and then more wires soldered to the circuit boards. Working on the gear becomes difficult as these wires are holding the case to the cards, and of course wires fatigue and come away, and you have a challenge as to where they once went, and how the mod is supposed to work.


CV WiringI do see the benefits of a good MIDI implementation, and I have come across a couple of MIDI kits where the designers have come up with good clean designs that can be unplugged for service. In this entry I talk about the Turbo CPU upgrade kit from MTG (Music Technologies Group). Grant runs the MTG and I have found him responsive, friendly, and helpful so recommend doing business with him.

In a different post I will talk about a nice MIDI solution for the Roland TR-808 from another vendor.

Installed MIDI interface cardAll of the instructions are available to download from MTG (, so you can see what is involved. I’m on the list of experienced installers and would be happy to implement this for you, but note that I am not a reseller so you have to purchase the kit from MTG. Turbo CPU a simple removal of the CPU and install of the Turbo CPU module.

Drilling holes for MIDII think most folks would be interested in the MIDI interface which for a solid installation does mean drilling the base of case to mount the MIDI interface card, and the back of the case for the MIDI connectors.

I prefer the look of the MIDI connectors squeezed between the lettering, and made some identification labels for the MIDI ports squeezed between logosMIDI ports. Both customers for this kit wanted the MIDI interface, plus the “DIY” CV interface which adds a tiny MCP4728 chip to the v2.00 MIDI board (I was fortunate that the MCP4728 was pre-installed on one unit, but had to order it from Mouser for the other.

Alternative MIDI connector mountingFine soldering skills are required to install this chip as it is surface mounted, so ask Grant if you can have a card with the MCP4728 pre-installed when you order :-). The DIY CV interface gives you MIDI control over 4 CV wires and is a great addition that requires no trace cutting or case butchering, so I like it!

MIDI interface with plug connected CVFor my implementation of DIY CV I installed a connector at the MIDI interface card for the 4 wires so it could be disconnected for service.