Sequential Circuits Pro-One : Power supply

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Sequential Circuits Pro-One : Power supply

Postby Pushpull76 » Wed Oct 17, 2018 10:57 pm

Dear all
i'm Alex from Italy, a lucky owner of a Pro-One, one of the first units built.
I bought it on ebay from Usa, without the cable and some ic's; I started to repair/recap the power supply section.
Does anyone have a photo of the internal connections of the cable ?
The transformer is on the board and I see the 3 pin connector with the "earth" in the middle, is the only place where the earth is connected?

Thanks in advance for the help!
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Re: Sequential Circuits Pro-One : Power supply

Postby Mooger5 » Wed Oct 17, 2018 11:41 pm

Hello,
You better post some photos of the innards before doing anything, including powering it up. Didn´t the pro-one have a captive power cord? If there´s an IEC connector installed it is probably a modification. Most importantly if the transformer iswired for the US voltage you will roast it with the european mains.
There were two versions, one with the transformer soldered to the pcb and one withe trafo bolted to the chassis, that´s all I know. Now passing the ball to the pro-one experts.

Good luck.
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Re: Sequential Circuits Pro-One : Power supply

Postby Pushpull76 » Fri Oct 19, 2018 4:30 pm

Here is the back of the pro-one. Later I'll send you a photo of the board.


Image
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Re: Sequential Circuits Pro-One : Power supply

Postby Mooger5 » Fri Oct 19, 2018 7:19 pm

Well, I see a captive power cord. And what looks like an empty chassis. Did you install the cable yourself?
There is also a line voltage selector, which is good. Is it related to the 3 pin connector you mentioned?
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Re: Sequential Circuits Pro-One : Power supply

Postby Pushpull76 » Mon Oct 22, 2018 2:44 pm

Mooger5 wrote:Well, I see a captive power cord. And what looks like an empty chassis. Did you install the cable yourself?
There is also a line voltage selector, which is good. Is it related to the 3 pin connector you mentioned?


I installed the power cord (the unit was completely without it, even the piece inside the machine).
My doubt is related to the the ground connection, I'm not just sure it's only connected on the board but also on the chassis (I am aware about "hum" problems related to the ground connection). How was it made originally?
Here some photos of the board, with voltage regulators and tantalium caps already throw off the window.

Image

Image
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Re: Sequential Circuits Pro-One : Power supply

Postby Mooger5 » Mon Oct 22, 2018 7:45 pm

Hey Alex.
According to the schematic here on Version 1.1 the power cord green wire was connected to the chassis only. This should be "chassis, or safety ground". Usually it´s only a screw with self-locking washer near the mains wires. "Signal ground" is on the board and is provided by the center tap of the transformer secondary winding. Since the transformer is on-board, the signal ground is probably just PCB tracks.
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Re: Sequential Circuits Pro-One : Power supply

Postby Mooger5 » Mon Oct 22, 2018 11:26 pm

Image
Just noticed that 3-pin connector in the second photo.
In the schematic the central wire from the power cord leads to that connector and ends with a comb or trident like symbol. That´s the symbol for "chassis". See if there is a third copper track on the board leading to (or starting from) that connector. Pin nr.2. I believe there shouldn´t be any, just the two for live and neutral ( numbered 1 and 3) leading to the line selector and the transformer primaries.
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Re: Sequential Circuits Pro-One : Power supply

Postby Pushpull76 » Tue Oct 23, 2018 8:45 am

[quote="Mooger5"]Image

Many thanks for your screenshot, I see there are a chassis ground and a signal ground (not sure if, back in the 1980, symbols used for grounds were the same as today). Yes, there is a copper track starting from pin 2, I'll later post a detailed photo.
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Re: Sequential Circuits Pro-One : Power supply

Postby Mooger5 » Tue Oct 23, 2018 3:01 pm

See where it leads to. If it goes to one of the PCB´s mounting screws, and only that, there should be no trouble. It´s the only use I see for that third copper track (nr.2). It would be safer if the earth wire connected directly to the chassis via screw, nut and locking washer. But if the entire PSU is on-board, then why not use one the screws and mounting holes that are already on the chassis? Makes some sense. Provided it´s not in contact with anything but the chassis. If that copper trace also joins signal ground, then there will be hum.
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