Roland SC-55mkII works with wrong AC polarity?? [SOLVED] \ VOGONS

Roland SC-55mkII works with wrong AC polarity?? [SOLVED]

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First post, by static-

Posted on 2018-11-29, 03:50

static- Offline

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Okay so, I bought a Roland SC-55mkII from a local seller who claimed the device turns on things seem to work but he didn't have hookups to really show me. He said the AC Adapter is not the original but it lights up and seems to function. While I was there I noticed the AC adapter had the WRONG POLARITY where the SC55 wants negative-center, while the adapter is positive-center. I didn't even bother trying, as I have a genuine adapter at home.

So I plug it in using my genuine AC adapter.. and it seems to work perfectly.

I'm too scared to try his adapter.. but how in the hell could it have worked if the polarity was wrong? Is the SC-55 really smart enough to support either polarity?

He didn't seem shady in any way, and I was at his house and had his phone number.

Last edited by static- on 2018-12-01, 03:52. Edited 1 time in total.

Reply 1 of 9, by keropi

Posted on 2018-11-29, 06:46

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Maybe he reversed the polarity on his psu and forgot about it - or he got it already modded. I don't remember if the sc-55 has a rectifier in it's power input , if it does have one then polarity is not an issue.
Better not test , you did the right thing. Maybe verify his pay polarity with a multimeter?

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Reply 2 of 9, by static-

Posted on 2018-11-29, 12:44

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Good idea! I will test it and report back.

Reply 3 of 9, by tpowell.ca

Posted on 2018-11-29, 16:58

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I too would be surprised that your unit supports both polarities.
I rescued an SC-55mkII from someone that used the wrong adapter, and by doing so the sacrificial D1 diode shorted out on the power input side, just after the L1 choke which I can only assume was placed there to prevent any damage to the rest of the unit beyond the diode itself.
With the diode replaced, the unit powered up without issue, using a center negative power supply of course.

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Reply 4 of 9, by tpowell.ca

Posted on 2018-11-29, 17:04

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keropi wrote:
Maybe he reversed the polarity on his psu and forgot about it - or he got it already modded. I don't remember if the sc-55 has a rectifier in it's power input , if it does have one then polarity is not an issue.
Better not test , you did the right thing. Maybe verify his pay polarity with a multimeter?

Yes it has multiple rectifiers including a L78MA05R, PQ05RF1, AN77L06, 78L05, etc, none of which are protected beyond the diode mentioned above, and have no rectifiers to correct polarity issues.

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Reply 5 of 9, by gdjacobs

Posted on 2018-11-29, 18:09

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Yup, polarity fix would require four diodes or a full bridge module.

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Reply 6 of 9, by tpowell.ca

Posted on 2018-11-29, 18:40

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gdjacobs wrote:
Yup, polarity fix would require four diodes or a full bridge module.

Not to mention that a full-wave rectifier would also have a non-negligible voltage drop of typically 0.7v per diode (2 diodes in series for either polarity).
If the unit requires 8v in some parts (after the AN77L08) and the power supply outputs 9v, a full wave rectifier may only allow 7.6v (0.7v drop for 2 diodes) which would be below the required supply voltage.

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Reply 7 of 9, by gdjacobs

Posted on 2018-11-30, 18:50

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0.7V for Si diodes. Schottky diodes are typically less.

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Reply 8 of 9, by static-

Posted on 2018-12-01, 03:51

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Cheers for the replies. keropi was right! I tested the AC adapter with a multimeter and it turns out that the PSU is indeed reversed and is actually center-negative. I fixed the label and tried it out and the SC55 works perfectly.

Nice to hear that there seems to be some basic protection by a replaceable diode!

Reply 9 of 9, by xjas

Posted on 2018-12-01, 05:46

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I've cut & spliced so many center-positive wall warts to be center-negative, thanks to Roland, Korg, and Casio. Usually I try to mark it with white-out or something, but sometimes I'm in a rush and forget. You sure the old one wasn't cut & flipped somewhere?

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