New knob caps for Prophet-5/10/600, Pro-One etc

Painted vs new
Ugly old painted Prophet-5 cap (left). My brand new cap (right)

I bought a Prophet-5 where the black caps were painted black… it looked absolutely awful. Searching the web for replacement knobs (like I’m sure many of you Prophet owners have) only turned up extremely expensive old stuff, which usually was “out of stock” anyway. I did get hold of some original replacement knobs, but these were pretty scratchy and worn too.

I decided to fix this! I had a company re-manufacture the aluminum cap to the exact original dimensions. (I also looked into making the plastic knob part, but then I realized most people already have these and they usually look ok).

The old caps are easy to replace, they basically pop off with the help of a knife blade. There is a little dot of glue in there that doesn’t provide much resistance 🙂

Now, only the more high end SCI gear (Prophet-5, Prophet-10 etc) had the fancy knobs with aluminum caps. The more low end stuff (Pro-One, Prophet-600, Split-Eight etc) only had the plastic part of the knob, with a simple painted white indicator line. The best part is that now you can pimp these models too! Many vintage (80s) Sequential Circuits gear used the same plastic knobs, with this cap being the only difference. The T8 and some other machines used different wider and shorter knobs, and unfortunately my caps will not fit those.

The pic below shows two old Prophet-5 knobs, one with a silver cap, the other with a removed black cap. Next to those a Pro-One knob. And in the bottom right corner: old knobs given a second life with my new caps!


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A Pro-One, Prophet-600 and Prophet-5 all recapped:

Sequential Circuits Pro-One – broken front panel spacer fix

STATUS: Done! (Nov 13, 2015)

The Pro-One has spacers welded to the metal front panel, onto which the main PCB is attached with screws.

On my Pro-One, one spacer had broken off. (It was still attached to the PCB though).

Spacer overview

I decided to glue it back using epoxy glue. I sanded both the spacer and the surrounding panel a bit.
sanded spacer

Now, to get a good result I needed to add some weight/pressure to the spacer once the glue was in place – but also make sure the spacer was glued at a perfectly straight angle.

I came up with a pretty clever solution if I may say so myself: I fastened the spacer to an old transformer (the one I replaced in the Pro-One).
so clever
This gave me the needed weight AND a straight angle.

curing epoxy
A little epoxy on both parts did the trick. The other end of the transformer rested on a piece of wood of the same thickness as the spacer.

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.