Sequential Circuits Pro-One – Pics

Some hopefully nice/fun/interesting pics taken during the restoration process of my Sequential Circuits Pro-One.

My new power connector and MIDI ports
My new power connector and MIDI ports
Serial number
Serial number
Signature and date
Signature of inspector(?) and date: April 4, 1983.
Sanskrit on the PCB!
Some god
Vishnu? Gods are not really my specialty…

Sequential Circuits Pro-One – New ‘case’

There is (was?) a company called synthwood who sells kits to make your synths more woody and nice. They seem to have gone out of business now, and people seem to have different opinions on the quality of these kits. Anyway, I though this Pro-One case was pretty awesome. Thinking about if I could do something similar..

Sequential Circuits Pro-One – CPU replacement + MIDI

STATUS: Done! (Nov 14, 2015).

I bought the MTG Turbo CPU replacement, the extra MIDI board, and the CV option (to control filter and stuff via midi).

Check out Music Technologies Group Turbo CPU here!
  The Turbo CPU board replaces the original Pro-One CPU and fits right in the old CPU socket. The main reason for me to get it was that there was a MIDI option for it as well. There are other benefits such as faster performance (although this may border on unnoticable) and also completely new features such as an extra LFO (which I probably won’t use).

The new CPU board

The Turbo CPU attaches to the separate MIDI board with 6 wires. 4 for MIDI, 2 for the extra CV stuff that will control pitch bend, cutoff etc over MIDI.

Step 1 – soldering the 6 wires to the CPU board
CPU module with wires

Step 2 – removing the old CPU
Removing original CPU

Step 3 – drilling holes in PCB for cable ties
Drilling holes

Step 4 – mounting new CPU board and cable ties
CPU board and cables mounted

The MIDI/CV board
The MIDI and CV board

I added molex connectors to the board and the 6 wires so that the Pro-One lid (which has the PCB with the CPU) can be detached from the bottom (which has the MIDI board and connectors)

Step 1 – Molex Connectors on MIDI board
Molex connectors on PCB

Step 2 – Molex connectors on wires from main PCB
Molex on wires 1 Molex on wires 2

Step 3 – MIDI board wiring (MIDI connectors and LEDs)
MIDI board wires

Step 4 – Mounting MIDI connectors

Holes for MIDI connectors
Drilling with increasingly bigger drills, then filing the last bit, phew…

Step 5 – Mounting MIDI board and connecting it all
MIDI card mounted

Now I did a quick test of both MIDI In and Out – all working!

The CV connections
The remaining 5 pin connector is for the wires with CV signals going from the MIDI board and back to the main PCB (for pitch bend, filter control etc over MIDI).

Step 1 – soldering 2 wires to control Filter Cutoff and Resonance
CV Filter

Step 2 – soldering 2 wires to control Osc 1&2
CV Oscs

Step 3 – soldering molex connector in the other end
CV Molex

Another test. Now I could control pitch bend over MIDI! The range defaults to 2 semitones, nicer than the Pro-One’s “about a fifth” and much more playable (Looking at you, non-spring loaded pitch bend wheel). I also had control over filter resonance and cutoff (connected to mod wheel and using the extra LFO of the new CPU.. not sure how I like this, but it’s all configurable).

Finally I pushed the MIDI LEDs into the holes drilled in the chassis.

MIDI connectors

They are ‘modern’ bright blue LEDs – not a big fan.. Maybe I’ll replace them with something more retro like green or red in the future.

Here is an overview pic of the internals (the bigger brownish board is from my keybed replacement mod):
Overview final result

There – all done!

Sequential Circuits Pro-One – transformer replacement

STATUS: Everything working (Nov 12, 2015). All in all a pretty easy mod – but always be careful with high voltage stuff! There are some unused holes I need to plug, bu that is purely cosmetic.

The original transformer in my Pro-One hummed a LOT (I mean physically, not from the Audio jack).

I replaced it with a toroid, which is supposedly better for audio applications. I also ditched the 110/220 voltage selector, and installed a standard IEC power cable chassis connector so I could ditch the annoying and potentially dangerous permanent power cord.

Here is what it looks like before any mods:

(It seems someone has already done some work on it, the original fuse holder is not connected and there is a new additional one mounted).

Step 1 – ripping out the old stuff
Old stuff ripped out
I ditched the voltage selector and old unused square fuse holder, as my Pro-One had a new round one mounted.

Step 2 – mounting a standard IEC power connector
I modified the unused square hole from the old fuse holder.

Sawing a little
Sawing a little
Filing a little
Filing a little
Mounting IEC power jack 3

Haven’t decided how to cover the remaining unused holes yet.

Step 3 – mounting new toroid transformer and connecting it
All connections done

All soldering done and heat shrink tubing in place.

The “middle tap” from the transformer should supposedly be connected to ground somewhere on the circuit board and NOT the metal case to avoid ground loops.

I measured the voltage once more to be sure everything was ok, and it seemed fine: around 20V on each pin against the middle tap/ground and 40 V between the two pins in the white connector. (The transformer is a 2*18V 30 VA model)

Hooked it up to the main PCB and fired it up… sweet music – no hum!


Transformer size: I was curious about the data for the original transformer and found that the company still exists. I emailed them and got a data sheet back in less than an hour – amazing! The transformer model is DP-241-7-36, and it is a 56 VA model. That’s a lot of juice.. but does the Pro-One need it?

No. Syntaur is selling Pro-One replacement transformers that are only 12 VA! I guess Sequential got a good deal on these bigger ones and just used them.Considering that the first 1500 Pro-Ones had a small transformer mounted on the PCB, there is no way that was a 56 VA model. The voltage regulators on the PCB also have very little heat sinking, and a seasoned tech told me the currents were then probably close to 200 mA or so. My 30 VA model can deliver 830 mA. So even that is bigger than needed.

Nov 16, 2015: Wrong fuse? My Pro-One didn’t have the original fuse holder, a new one had been added. There was also a sticker that said ‘250 mA’. Looking at the Pro-One schematics, it says “1/4A” (250 mA) when running it at 110V and “1/8A” (125 mA) when running at 220V. So I think the person who fixed this before missed that info.. Will change it to a 125 mA ASAP.

Sequential Circuits Pro-One – capacitor replacement

STATUS: Done! (Nov 14, 2015). Pretty easy mod. Now I can sleep at night – electrolytics dry out, and the tantalums in the power supply felt like an accident waiting to happen.

Replacing old capacitors is something people do with old synths, mixers, amps etc. Some people swear by it, other say it is often a lot of mumbo jumbo, especially if the claim is that the audio quality got much better.

Anyway. It seems most techs agree on the following:
– Electrolytic capacitors are the most sensitive to aging (they dry out). And my Pro-One is 32 years old after all.
– Tantalum capacitors are bad in power supplies since they tend to ‘short’ when/if they fail. (The Pro-One has this).

So I’m gonna replace:

4 small electrolytics in the audio path
4 small electrolytics in the audio path
1 small electrolytic connected to the CPU
1 small electrolytic connected to the CPU
4 tantalums in the power supply circuitry
4 tantalums in the power supply circuitry
2 big caps in the power supply
2 big electrolytics in the power supply circuitry

Desoldering & Replacing

I found this great tool
I found this great tool, a combination of soldering iron and solder sucker!

I replaced the caps from the power supply first.

4 new electrolytics in the power supply
4 small electrolytics close to the power regulators (replacing the tantalum ones)
2 big caps in the power supply - new
2 big electrolytics

I then started up the Pro-One to see that everything still worked. It did 🙂

Next up: One capacitor that is connected to the ‘Reset’ pin of the CPU.

1 cap connected to CPU - new

I thought the schematics said “10 uF” but the capacitor I removed read “1 uF”. Hmm. I read up on this and the capacitor is there to give a long enough pulse when the CPU is reset. This ‘resetting’ doesn’t seem to be something that is actually done, there is just a constant voltage connected to this pin. So I figured a 10 uF might work just as well as a 1 uF one.. Soldered it in, started it – all good. Fingers crossed.

Finally: The capacitors in the audio path.

4 caps in the audio path - new

These were originally ‘axial’ but I could only find ‘radial’ ones. Added some shrink tubing for insulation. They are also ‘audio grade’, whatever that means – can’t hurt 🙂

Final test – everything still works! (Nov 14, 2015)

Sequential Circuits Pro-One – keyboard replacement

UPDATE (April 2017): I’m in the process of creating a “plug’n’play” keyboard replacement kit using a modern keybed and all cabling and mounting accessories. Even a novice can replace their old unreliable keybed, and the kit uses the same screw holes and so on – no modification of the Pro-One is necessary. (You can sell the old keybed on ebay or keep it if you want to have the option of restoring it to original.)

The TM-2 kit will be available here: The Techsmechs Vintage Synth web page

I will also post info on my FB page:

The below post refers to my initial prototyping using a StudioLogic CMK-37. I strongly recommend getting my TM-2 kit instead 😉

STATUS: Done! (Nov 13, 2015).
This is not a very difficult mod if you have any experience with soldering and drilling and such. The tricky part was figuring everything out!


The Pro-One is a fantastic little synth, but the keyboards are not fantastic. They come in two varieties: “J-wire”, which is the best one, is found on serial numbers 1 to 8500. From serial 8500 upwards they used a “membrane” type, which really really sucks. That’s what mine has. Many keys don’t work at all. You can clean the membranes by taking everything apart, but it’s a lot of work and only helps for a while.

When googling around for replacement keybeds and MIDI kits, I stumbled upon Music Technologies Group. The guy who runs the show, Grant, makes a replacement CPU board for the Pro-One, as well as a MIDI kit. Great!

In his blog he also posted how he replaced the original Pro-One keybed with a modern one from Doepfer. The keybed inside is made by the Italian company Fatar, and it works pretty much the same as the original Pro-One keybed. All that is needed is some wiring and connector tweaks:
MTG Blog post about Pro-One keyboard replacement

Music Technologies Group keyboard replacement
Music Technologies Group keyboard replacement

In the comment section of that post other people describe doing the same thing, using keybeds from gutted StudioLogic CMK-137 midi keyboards, which has similar Fatar keybeds inside. A LOT of manufacturers (Doepfer, Novation, Access etc) use Fatar keybeds, they are pretty much the industry standard unless you are Roland or Yamaha.

Since my Pro-One was pretty much unplayable I decided that this would be my first modification of it. I searched the interwebz for these keybeds, but it seems 37 key versions are not popular at all anymore. Doepfer no longer sells them, Fatar is impossible to get hold of (and I hear that they don’t sell directly anyway). I basically couldn’t find any cheap 37 key MIDI keyboards in any of the usual channels (Thomann, Sweetwater etc).

Finally I found a store in France that had several “StudioLogic CMK-37” in stock. I bought 2 of them just in case. €59 a piece plus €20 shipping to Sweden.

I hoped these would prove similar to the one Grant used.
I was wrong 🙂

Opening up the keyboard, I did not find one red 20 pin ‘MicroMatch’ connector as expected, but rather two 16 pin ones. (A in picture)

They had flat cables attached, going to a small board which converts the signals from the keyboard into MIDI. (B in picture)

CMK-37 insides

I knew I didn’t need that little board, the signals I were after had to be somewhere in those two 16 pin connectors. I just needed to figure out which ones to use. (The Pro-One has a single 16 pin DIL connector on its main board).

Now, using a volt meter I tried to figure out which pins were which. I knew the basic system used in these Fatar keyboards, basically 5 ‘groups’ with 8 keys in each. The first group would handle the first 8 keys from lowest C to G, next group would be G# to D# etc). The last group only has 5 keys, and in total 8 + 8 + 8 + 8 + 5 makes 37 keys.

Here is the only Fatar schematic I could find, the same as Grant has:

Fatar 37 key schematics

After spending hours measuring each pin combination, I couldn’t get it to work as expected. Only like 3 keys gave me a connection.

After some head scratching I took a look at the small MIDI board that I had discarded (but not thrown away) and noticed that there were connections between all the even numbered pins on the two connectors. Pin 2 on connector A was connected to pin 2 on connector B. Pin 4 to Pin 4. And so on.

Up until this point I had just had the flat cables hanging loose with the stripped individual wires flying around, and it was just too fiddly. I decided to order a circuit experiment board so I could solder down the flat cables. This would certainly make everything tidier and make measuring a lot easier.

Flat cables attached

(I also removed all the keys so I could follow the connections and traces on the keybed circuit board).

Now I could easily connect the even numbered pins with jumper wires:

Initial wiring

Success! Now the even numbered pins matched the ‘keys’ (T0 to T7 in the Fatar schematic) and some of the odd numbered pins matched the ‘groups’.

‘Key shift’ issue
Unfortunately, the CMK-37 I have does NOT follow the key group arrangement described in Grant’s blog or the Fatar schematic. It is ‘shifted’ by 4 keys. So instead of starting with 8 keys, it only uses 4 keys in the first group. Which gives us 4 + 8 + 8 + 8 + 8 + 1 = 37. And that is 6 groups, not 5. Noooooooo! Did I just buy 2 unusable keyboards?

After a deep breath, I took a look at the keybed circuit board. I could see that it was basically arranged in groups of 4 keys. So this:
4 + 8 + 8 + 8 + 8 + 1

…was actually more like this:
4 + (4+4) + (4+4) + (4+4) + (4+4) + 1

Now what if I could regroup it to:
(4+4) + (4+4) + (4+4) + (4+4) + (4+1)   ?
Key Groups

Hacking the keyboard
After some inspection of the circuit board, it looked like it could be possible. Time to cut some traces and solder some tiny wires! First purchase: magnification lamp thing:


Then I got to work. Here is an overview pic of the 9 ‘haxx’ I did:
keybed haxx

Green letters are jumper wires. (C is actually on the experiment board)
Red letters are trace cuts.

Here are 5 close-ups of the haxx:

Haxx A and B. Soldering a jumper wire (yellow). Trace cut (red).
Haxx A and B. Soldering a jumper wire (yellow). Trace cut (red).
Haxx C. Soldering a jumper wire on the experiment board to join two groups.
Haxx C. Soldering a jumper wire on the experiment board to join two groups.
Haxx D and E. Trace cut (red). Soldering a small piece of wire to join two traces (yellow).
Haxx D and E. Trace cut (red). Soldering a small piece of wire to join two traces (yellow).
Haxx F and G. Trace cut (red). Soldering a jumper wire (yellow).
Haxx F and G. Trace cut (red). Soldering a jumper wire (yellow).
Haxx H and I. Trace cut (red). Soldering a jumper wire (yellow).
Haxx H and I. Trace cut (red). Soldering a jumper wire (yellow).

And there we have it!
Using the volt meter I could verify that the signals I needed were present.

NOTE: “Make” and “Break” switches
Modern keybeds usually have two switches per key, “Break” and “Make”. When you start pressing the key down, the “Break” switch changes. When you reach the bottom of the keybed, the “Make” switch closes. By measuring the time between these two events, velocity can be determined. In an old synth like the Pro-One, velocity isn’t used, so in this project we only use the “Make” switches.

The Pro-One Cable
The Pro-One side of things uses a standard 16 pin DIL connector. I bought one of those and soldered a 16 pin flat cable to it.
DIL16 pinout

Pro-One DIL connector
I soldered the other end of the flat cable to the experiment board:
Pro-One cable connected
Of course I also needed to cut the traces:
trace cutting

I added Molex type pin connectors as well so I could easily test out the connections between the ‘Fatar side’ and ‘Pro-One side’.

Here is the pinout I have arrived at:
pinout sticker
Connecting the two sides
Using test cables on the Molar connector, I first connected “MK0” and “T0”, to see if I was anywhere close to getting this right. That should give me a working “lowest C” on the keyboard. And I guess the planets were all aligned- it worked! I then connected more test cables and got more and more keys working.

Test cables

Great success! I only had enough test cables to get 5 of the 8 keys in each octave working, but figured if I had it right this far, the rest should work as well. I removed the test cables and started soldering permanent cables.

The “Make” cables
All cables soldered
All cables soldered

Now I tested again. A slight problem, there was a hanging note, the G# in the lowest octave. It was as if that key was permanently pressed. I noticed that that key was the combination of “MK1” and “T0”, which are right next to eachother on the experiment board – short circuit? Yep. When cutting the traces between the two sides, I accidentally managed to create some thin strands of copper between the traces. After looking with my magnifying lamp I could see the problem and could clean up the traces with a knife. And then everything worked.

Out of curiosity I took a pic of the old keybed and the membranes. It seems like this keybed had been “fixed” with some rubber and electrical tape. Seems legit. Bye bye, membrane hell!

Membrane hell
Old membrane keybed
Old original keybed
Old original keybed
New keybed
New keybed

Mounting the new keybed inside the Pro-One
I mounted some wood rails under the Fatar keybed using the existing screw holes:
Wood railsI then placed the keybed in the Pro-One, put the lid on, and aligned the keybed. I then carefully removed the lid, and drew markings around the wood rails on the Pro-One bottom plate. Then removed the keybed and made marks where I wanted to drill 6 holes (being careful to avoid the existing screws in the wood):
Chassis markings
After drilling the holes in the bottom plate I put back the keybed, and put the lid on again. I put back the front screws to make sure it was in the correct position.

Now I could use a pen from under the Pro-One to make dots on the wood thru the holes. Then it was simply a matter of drilling holes in the wood rails and screw the keybed to the bottom plate.

ALL DONE! (2015-11-13)


UPDATE (Nov 23, 2015): I saw that the schematic for the Fatar 49 key model resembles the one I have very much: two flat cables, first group starting with 4 keys. I’m guessing if the world runs out of 37 key keybeds, one could buy a 49 key model and a saw and get to work on it! 🙂


If you need it, here are the schematics for the Pro-One:
Pro-One Schematics (pdf)