Digital piano power repair - KORG concert 3500 - Page 1
Forum section: electronics / repair

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 03:15:57 pm
content: The manual for the piano is -http://cdn.korg.com/us/support/download/files/5f60949ee160fdd54043eaa66a427112.pdf?response-content-disposition=inline%3Bfilename%2A%3DUTF-8%27%27C2500_3500_5000_OM_E.pdf&response-content-type=application%2Fpdf%3BHi guys,I live in England and have just bought this digital piano - from the 80's I believe - and the power adapter at the back is missing with two wires sticking out. I usually confine myself to guitar pedal electronics so I do not have much experience with this side of things so I need a bit of help.Sticking out the back of the korg is two power wires (yellow and black), I am assuming yellow = positive, black = ground.With it I also have a NICEIG 3 prong power adapter. At the end of this there is two wires blue and maroon. This is where the problem lies as I'm confused as to which connection I should make with the KORG.(Another reason I am skeptical and slightly apprehensive of this is that I had assumed there would be 3 wires, one to be earthed?)After this is sorted out I was planning on just soldering each corresponding wire together and separating these with insulating tape.Appreciate any help, thanks alot and photos down below.Probably going to have to strip the black wire a bit more considering that those two strands of wire are not going to help much.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 31, 2017, 03:54:52 pm
content: The name-plate power rating for the instrument is 240V AC (mains power). There is no "positive" or "negative" with AC (Alternating Current). Since the unit appears to have been modified, you don't really know for sure that those black and yellow wires are the mains (240V) input connection.You COULD simply attach your mains power cord to the black and yellow wires and try it. However, if there is any internal modification, that experiment has a good chance of blowing up in your face (perhaps literally). I would NOT recommend doing that without further research.Even if the black and yellow wires ARE the 240V AC mains input connection, having exposed wires like that going through an unprotected hole in the metal panel is dangerous on several levels.Ideally, you would disassemble the instrument and confirm that the black and yellow wires still connect to the designed mains power 240V AC input. And then you would get a standard IEC power connector and install it into the hole to provide a proper mains power connection. You are correct that a proper power connection would provide for the green/yellow protective earth to be connected to the metal chassis of the instrument.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 04:00:02 pm
content: I would be careful as this method is one way to mark the device as defective. In some cases even dangerously defective --> Preventing someone try to plug it in and get hurt one way or another.Soldering this wires together and put some electrical tape on it is not a way you should go. You need to have al least a) a protection that none can pull on the cables and rip the ends out of the inner part of the instrument and b) a decent insulation of the repair place. Go get a fitting socket for the instrument and a recent cable. This way you actually get a look into the device and check that there's no obvious damage. For the first power-on I would be very careful.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 04:12:00 pm
content: Great thanks, will order an IEC connector and the plug.Will update this thread as I get them.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 04:13:48 pm
content: The device is not defective as far as i know, I saw it working and powering up okay.It was in transferring it that these have got caught on something and ripped off.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 04:27:18 pm
content: The screw holes look not like the screws are ripped off also the cables look cut off. And even if the socket was ripped off: Make sure the other end of the cables are at the place they belong to.And by the way, fixing a two or more pole cables: Having each wire at a different place is usually better than having all solder-joints at the same place. Especially if you use heat-shrink tubes over it. You shrink each individual wire and then you add a bigger one over the fixed section to cover it. Having all solder joints and heat-shrink tubes at one place you get a big blob at one place.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 04:34:17 pm
content: https://uk.rs-online.com/web/p/iec-connectors/0449310/I'll take it apart and follow the wires once I get a chance.Will that socket be okay, only problem being that it has three prongs which is confusing me a bit.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 31, 2017, 04:46:05 pm
content: Quote from: tombaxter97 on December 31, 2017, 04:34:17 pmThe connector in your reference is the WRONG gender. Please refer to most any appliance or electronic gadget that uses what you Brits calla "kettle cord". The connector on the back of the instrument should be the kind with exposed male pins. (Like the one I showed in the photo in my response.)QuoteGood. Recommend at least confirming that the internal wiring correlates with an undamaged mains voltage input.QuoteYou need the MALE connector for a power INPUT. For example:https://uk.rs-online.com/web/p/iec-connectors/0488191/The center pin is the protective earth (ground) connection. It should be connected to the metal chassis.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 04:55:24 pm
content: Audio equipment often comes with 2 pin connectors. From what I can see the original connector was called C9 or C10 (see:https://en.wikipedia.org/wiki/IEC_60320#Appliance_couplers). This is a 2 pole connector used for several synthesizers. That's the reason for just 2 cables. A ground wire would increase the chance to have a ground-loop (constant humming).So either you check if you can get the 'original' connector. RS seems not to have them, as they're uncommon and probably outdated. or you should see if you find an alternative which fits in the hole of the instrument.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 05:00:36 pm
content: Would it be possible to use the connector Richard poster but only use two of the lugs and leave the ground free.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 05:02:19 pm
content: That's not good as someone could rely on the grounding. But fear not there's the 2 pole alternative:https://uk.rs-online.com/web/p/iec-connectors/8702448/It prevents to use the 3-pin cable. So get the right cable with it.No, it's the other way around.Edit: Corrected my statement of the incomparability of the cables.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 05:12:05 pm
content: Can only buy in packs of 50. But at least I know what I am looking for now so I really appreciate it.Regarding when you said somebody might need the grounding. The keyboard is for my own use, not that it changes anything!

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 05:19:20 pm
content: Good catch. I don't use RS often. You can try to use the manufacturer number to find alternatives.The point is if the instrument is later sold the owner might think it's grounded. And in case you actually ground it you might get some hum. Especially if you start to use MIDI or an external amp this increases the risk of problems.

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 05:31:02 pm
content: https://www.ebay.co.uk/itm/Figure-8-Type-2-Pin-Power-Plug-Socket-Connector-IEC-AC-Figure-250V-2-5A-/171267671068found this, bit different from the one you posted but could just get a wall socket like this?https://www.google.co.uk/search?q=2+pin+ac+connector+ebay&rlz=1C5CHFA_enGB767GB767&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjp2ZXj4rTYAhUpB8AKHbb_ApUQ_AUICygC&biw=1280&bih=800#imgrc=rAiWMzcI7LfghM:Sorry for the messing about, just first time doing this don't want any disasters!

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 31, 2017, 05:39:46 pm
content: No problemo. But I'll probably go offline now as I need to prepare to greet the new yearThese are probably too small to fit in the hole. You should check the mounting hole dimensions. It's easier to make the hole a bit bigger than reducing the size. And the one from Richard and me were also screw in not snap in types (more reliable).For the 2- or 3-pole versions you can use a normal cable (e.g. for computers).

author: tombaxter97
author_rank: Contributor
timestamp: December 31, 2017, 05:41:49 pm
content: Cool, thanks for all your help.Hope you have a good new year!

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 01, 2018, 08:41:57 am
content: Quote from: Twoflower on December 31, 2017, 05:19:20 pmActually, MIDI is inherently immune from ground loops because it is optically isolated (at the destination end). It is analog audio signals that MAY be susceptible to ground loops depending on the nature of what you are connecting it to.My philosophy is safety first,potentialground loops second. IF there is a ground loop problem from properly grounding a mains-powered device, THEN I consider options for eliminating the problem (such as isolation transformers, etc.)

author: Twoflower
author_rank: Frequent Contributor
timestamp: January 01, 2018, 10:54:24 am
content: Something I learned today. I missed the insulation of the MIDI part.As the original deign of the instrument was 2-Pole I would expect that all measures are in place to prevent any problems on the safety. I would totally agree that safety must go first if required. In this case I would not see any problem as there is no modification to the original state.

author: tombaxter97
author_rank: Contributor
timestamp: January 05, 2018, 12:18:07 am
content: Hi guys, just got the parts through.I know it has been a few days so I don't know wether you will get notifications about this.As i mentioned I am used to the ol' simple 9v dc powering my breadboard for guitar pedals so I was wondering that for the cable end of.https://www.ebay.co.uk/itm/C7-2-pin-LAPTOP-AC-ADAPTER-CHARGER-POWER-LEAD-MAINS-CABLE-CORD-with-UK-PLUG-/202090236724it seems that can be put into the socket in either configuration - flipping the pins - so does this mean it does not matter which of the wires I solder from the keyboard onto the tags of.https://www.ebay.co.uk/itm/Figure-8-Type-2-Pin-Power-Plug-Socket-Connector-IEC-AC-Figure-250V-2-5A-/171267671068Thanks again

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2018, 12:40:57 am
content: ASSUMING that you have confirmed that the black and yellow wires really are the mains power input, then you are correct that it makes no difference which is connected to which pin because the connector is completely symmetrical.Note that if those black and yellow wires are some sort of hack that a previous owner made to the instrument, then you have an excellent chance of blowing the thing up in your face. It is imperative that you verify the black and yellow wires are connected to the mains power input of the internal power supply circuit.

author: james_s
author_rank: Super Contributor
timestamp: January 05, 2018, 12:46:54 am
content: Could you at least post a picture of the inside of the instrument before you try connecting random wires poking out to 240V mains? Ideally you'd want to find someone local with some electronics knowledge to take a look but if you post a picture of the inside at least we may be able to spot any serious issues.I see no problem with using a 3 pin IEC connector and not connecting the earth pin. If the equipment is in a plastic housing it shouldn't need to be grounded anyway.

author: tombaxter97
author_rank: Contributor
timestamp: January 05, 2018, 01:50:44 pm
content: Due to the weight and size of the thing it is currently still at my girlfriends mothers.Was waiting for the parts before I transport it home.Will send a gutshot once I can get it home and open the things up, may be a few days.Thanks again

author: tombaxter97
author_rank: Contributor
timestamp: February 01, 2018, 04:32:49 pm
content: Hey guys,it has been a month so I would understand if I don't get any replies back!Just finished exams and some other stuff so finally got around to opening the piano up!I will attach pictures below of where the two power cables lead to, considering where they are leading to it looks like a step down transformer so hopefully the original idea we had of the little c14 connector (or whatever number) and mains cable should be okay.If you need a different angle or another picture let me know.Thanks.P.S I have never really dealt with high power caps like those to the right. What is that orange stuff around it?

author: Zucca
author_rank: Unknown
timestamp: February 01, 2018, 04:50:14 pm
content: Looks ok to me, the orange stuff is for keeping them in place and prevent vibration. You can ignore that.Do you have a multimeter?

author: helius
author_rank: Super Contributor
timestamp: February 01, 2018, 05:04:43 pm
content: The orange stuff is glue to keep them from breaking off or vibrating. It is sometimes a big problem as it breaks down and becomes corrosive. Ideally, it should be removed and the capacitors (or replacements) remounted with silastic, but you don't have to do that until you're ready.The cut black & yellow wires are the mains input. They look to be a little bit short for mounting an IEC receptacle, so you may have to either extend them with splices, or replace those wires with longer ones and crimp new connectors to plug into the fuse board's CN1. I would follow Richard in suggesting that you do use a grounded IEC C14 receptacle and a grounded flex. The ground pin on the C14 should be connected to a green wire and spade terminal, and the spade hooked to one of the chassis screws with a star washer: the brass screw on the right side of the fuse board looks like it would be perfect, as long as it has electrical continuity to the chassis.The transformer is a worldwide type, with 100/120/220/240 V inputs along the right side. For 240V use, the outermost terminals are used. It says 36.2VAC output, but there look to be at least two separate secondaries, which are probably different voltages. The red/black/red output wires are probably the 36.2VAC output center-tapped, so 18.1VAC between each red and the black. This is normal for a linear power supply regulating down to +12/-12 VDC. I can't see what's going on with the yellow and orange wires disappearing behind, those could be even more secondaries.

author: tombaxter97
author_rank: Contributor
timestamp: February 01, 2018, 05:29:03 pm
content: Great thanks for the help.About the connector, what I got is the C7 so I don't have that third connection.

author: Zucca
author_rank: Unknown
timestamp: February 01, 2018, 10:24:12 pm
content: Quote from: zucca on February 01, 2018, 04:50:14 pmbecause the Fuse F702 looks open to my poor eyes.

author: tombaxter97
author_rank: Contributor
timestamp: February 01, 2018, 10:35:44 pm
content: Yes, I will check it tomorrow then. May be more down to the poor resolution than your poor eyes!

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 02, 2018, 03:00:20 am
content: If you acquired the gadget locally, it is probably already strapped for 240V.A better look at the far right side of the transformer would be helpful.Your photo cuts off the critical part that shows the primary connections.Of course, if it is strapped for 100V or 120V, and you plug it into your 240V mains, you will kill it.

author: tombaxter97
author_rank: Contributor
timestamp: February 20, 2018, 03:42:33 pm
content: Referring to Helius' message.After a long month of other things not relating to this - as usual - I am going to finally get this done.Decided to use the three pronged receptacle, so i will need to attach the green earth cable to that brass screw you mentioned (On attached imaged have i circled the correct screw? can confirm it does have continuity with chassis below)Do you mean these spade terminals?http://theinstallbay.com/subcategory/IB-CONNECTORS/IB-RING_TERM_STARJust solder earth to spade then screw this on?Thanks sorry for this ongoing nightmare.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 20, 2018, 05:08:39 pm
content: I see no "attached image"?Just open speculation: If you are referring to the screws that attach that AC entry circuit board, then no, those are NOT proper for connecting the PE terminal.The convention is to attach it to a bolt directly against the metal chassis. Not through a PC board and a spacer (which could be plastic for all we know?)It is also against the convention to solder to the lug. The convention calls for crimping, not soldering.OTOH, you could just leave the PE pin unconnected. Or just use an IEC C18 connector:

author: tombaxter97
author_rank: Contributor
timestamp: February 20, 2018, 09:02:51 pm
content: Oh sorry this.