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 ๐Ÿ™‚

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 ๐Ÿ™‚

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


8 thoughts on “Sequential Circuits Pro-One – capacitor replacement

  1. Hello,

    I have a pro one too and plan to change the electrolytic and tantalum capacitors you change too. However, I can’t read properly the values on he schematics I got. Please can you give me these values.
    Thanks a lot. Have a good day.

    Liked by 1 person

    1. The 4 tantalum caps in the Power supply are 2.2 uF. Originally 10V on the 5V lines, 25V on the 15V lines. Replace with electrolytics, you could just go with 25 or 50V or something on all 4.
      The 2 big ones are 2200 uF 25V. Use 105 degree caps for the power supply, they last longer!

      There are 4 electrolytics in the audio path, also 2.2 uF. I used 85 degree “audio” caps but not sure how much difference it really makes. Opinions vary.
      Finally there is a 1uf 10V cap on the reset line of the CPU.


      1. Thanks a lot for your answer and your help. I finally open my pro one because I have problem with it : when the vco’s are on the lower scale, enverything’s right. When I increase the scale, vcos become very unstable and oscillate uncommonly. Maybe it’s really the time to change the caps. I don’t find radial capacitors to replace the tantalum ones (but I find the new tantalum with good value). Why tantalum’s not good on power supply circuit ? Thanks again. R.


      2. The problem with tantalums is that when they fail, they short. This can blow the expensive CEM chips and other important components. Radial electrolytics should be easy to find? Beware the height though! I realized the ones I mounted are slightly too tall, will redo them and bend them so they are flush with the PCB.


  2. I see your tantalums had beads on (some of) them. Mine also have some (but not on the same caps). Did you put those back in with the new caps? I’m not sure whether they serve as some kind of choke filter to keep noise down but that seems the most likely purpose.


    1. The beads are just a mechanical spacer, could be that the wires on some brand tantalums were thinner and they were afraid of them bending and shorting something.. not sure why they were only on the 5V caps.


  3. The Reset and Crystal circuits are not connected to the TurboCPU so there are a few components no longer essential (unless you go back to the factory CPU). If memory serves, there is that cap, a coil, and a diode.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s