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Hi,

I recently bought a Korg SP250 on Ebay sold as parts only. The vendor said it simply didn't power up one day. When it arrived it looked like new, great condition but sure enough it did not power up. I opened it up and was surprised to find the power pcb was all split and components were hanging off the board. Looks to me like someone yanked the keyboard up or down while the audio cables were still plugged in. The damage has been caused all around the jack plug sockets. I thought about trying to repair but I'm pretty sure its a right off.

I've had a look for a replacement on line but so far no joy. Can anyone recommend a supplier or is anyone breaking a Korg SP250? It's the SP250 Amp Board.

Many thanks Alan

Last edited by Alan3666; 07/23/21 03:13 AM.

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This one?

https://syntaur.com/Items.php?Item=5647

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Out of stock right now. 😐

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Yes that the one. My one has a crack that runs in an arc around the two black Jack sockets.

I'm not sure how to add a photo?

Alan


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Cut your losses. It's time for a new piano.

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Hi Alan3666,

As you're UK based try Nigel at W D Greenhill & Co (https://www.wdgreenhill.co.uk) if Korg themselves are unable to help.

Good luck,

Tog

Last edited by Tog; 07/23/21 06:18 AM.

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Cheers Tog! I'll try them..

I did get a response from Korg but it was just to say the part was obsolete.

Alan


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I sent an email to WDGreenhill asking about the part.

I said I would take a faulty unit if they had one. I'm pretty sure the components on my damaged unit should be good to go.

I'd just need to brush up on my soldering skills.

sAlan


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Hello,

@Alan3666, Looking closely at @clothearednincompo's photo, together with your indication of where the crack is, I reckon this board is repairable if you can't find a replacement. Don't give up too early (which I get you aren't)!

Cheers and happy piano rescuing,

HZ

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Hello HZ,

thanks for the encouragement. If I knew how to post a picture I think you'd probably agree it is beyond repair. I thought about tracing etches and running wires but I'll park that idea for a bit. I removed one of the audio sockets and a big chunk of the pcb then fell off. It wouldn't be impossible to fit it all together like a jigsaw puzzle and then run wires to solder points but it would be a pain.


Not giving up!


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Wish I still had my old oscilloscope


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Maybe ask Korg support if they could share service manual (not user manual, but the thing they share with their repair services) or this amp board specification: I believe it can be exchanged for something else.

OT: Time flies so fast, I remember my excitement when my parents bought me Korg SP-250 (my first proper digital piano!) as a reward for good grades after first year of high school. It was 15 years ago.


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Thanks ABC,

I have the service manual. There's nothing to lose in trying to piece it all together but we're talking about a lot of patching/soldering. I cant believe how much damage has been done. If only those jack sockets had been secured by nuts. there are threads for nuts after all.

Oh joy!


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Hello,

@Alan3666, Imagine the sense of pride and accomplishment when you succeed... 😋.

At the moment I'm also restoring some 'old' but lovely equipment and the process can at times be a pain indeed. Not to mention the scare it presents to the insecure persona within ourselves. But when the last screw goes back in, lights come on, and everything functions once again for hopefully years of good enjoyable use to come, that feels sooo good.

Cheers and happy restoring,

HZ

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@Alan-with-three-sixes, great you got the service manual! If that was the damage board in #Post3139885 image, it would be fairly easy to trace since it has discrete components surrounding the jacks instead of ICs. Probably, it would be nice to start with the dc jack, labelled OJ1... if only the cracks where only nearby the jacks...
Hoping to join that adventure! smile


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Hello,

@Alan3666, Wow. Someone must have been really violent and/or angry at some point to create such a damage.

Still, a restoration doesn't seem undoable to me. Tool-wise, there's a start, 😀, I'm using that one a lot myself these days/weeks in restoring my three nice decades-old Onkyo amps (to be used in my piano setup and regular HiFi/TV use).

Cheers and happy reclaiming,

HZ

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[Linked Image]
Originally Posted by Alan3666
[Linked Image]

Oh man! That is some wreck! I believe you can restore the board traces. Was their any jacks on the broken off piece?
Cause if there is, it won't be strong enough unless you pour some clear epoxy on the non-contact side to keep the board from cracking again since the jacks doesn't appear to be screwed on the back panel.

[Linked Image]

Last edited by josh_sounds; 07/27/21 06:35 AM.

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Ok so here is where I am. I don’t think the old Amp Board will be an easy fix. All the components are there but the cracks on the pcb run everywhere. I found a guy on eBay who was selling SP250 parts. He had a few listed but no Amp Board. So I contacted him and asked if he had even a faulty one. It arrived this morning and I fitted it this afternoon. Everything powered up fine. I had sound on both audio ports, both left and right headphone but only sound on right speaker. The guy said he had put it in for repair and the shop replaced a couple of parts but it came back with still only one working speaker. I’m pretty pleased with this progress but I’d like to get that left speaker working. From the disturbed solder it looks like they replaced IC2. I bought an old oscilloscope which should arrive in the next couple of days. Wish me luck. I started out working as a repair tech for digital equipment corp, so I know how to work the scope, but that was over forty years ago.


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Might have to read the manual


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http://forum.pianoworld.com/ubbthreads.php/galleries/3141224/sp250.html

Here is the schematic if anyone is interested.

The Audio left and right is shown top left, the head phones left and right is bottom right and the speaker outs are top right.

It looks like the power amp IC2 has been replaced. When the scope arrives I'll check on pin 7 for a signal. If there's nothing there I'll start working back. It might be a fault in the EQ, IC1.I hope not.

Anybody got any suggestions?

Alan


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Here is a audio out troubleshooting thread that you might find helpful.

https://groups.google.com/g/sci.electronics.repair/c/2LorckCy-cU?pli=1

Service manual for those interested

https://dealers.korgusa.com/svcfiles/SP-250_Service_Manual_Ver5.pdf

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Hello,

@Alan3666, Clever move, this already is a nice accomplishment!

The image of the schematic is not that readable, so I can't say much about that.

The audio outs and headphones now working is great news. In my Onkyo amplifiers, the signal path for the headphones out is largely shared with the speakers' path. If the same applies to your Korg board, that would mean a lot of it is already good.

Some thoughts:
-> Does the headphone out jack switch off the speakers? In that case, check if the switch in the jack is faulty, keeping the left speaker off all the time.

-> Find where the signal path splits into separate circuitry towards the headphone out vs speaker outs. Check all components and PCB traces from there to the left speaker out. If there are faulty components there, you may be able to source replacements from your cracked board.

-> If any component involving a heat sink is worked on, be sure to thoroughly clean the heat contact surfaces and carefully apply new heat transfer compound. But I guess you already know that.

Cheers and happy completion,

HZ

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HZ Piano might be on to something...

Regarding the left speaker... try using the left & right audio jacks (CN4 & CN5 on the schematic) and see if both have outputs. If both have outputs, then the issue might be in the mute circuit/switch (AMP_MUTE, pin 5 on IC2).
Or you can also hijack the outputs at CN9 (also near IC2) and check if there is really no output...


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Scope not yet arrived but will try all this

Thanks

Alan


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Just thinking out loud. Would there be any problems with simply wiring the left hand speaker in parallel with the right hand one?


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Hello,

@Alan3666, Wiring to one (the right) channel results in:

-> Your sound becoming mono.

-> Your sound lacking the stereo part that is only present in the left channel *unless* there is a switch or setting to make the piano produce a mono output.

-> Wiring in *parallel* presents a *danger!* of overloading and thus damaging the amplifier. Never do that *unless* you know the exact impedance specification of both the speakers and the amplifier output, and can calculate whether a parallel wiring is safe or isn't. If unsure, it *isn't*.

-> Wiring in *series* would be safe, but *may* result in a lower overall volume. When doing so, the speakers are only in phase if you connect e.g. the red wire of one speaker to the black of the other.

Be careful!

Cheers and happy experimenting,

HZ

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Thanks HZ. I did wonder about wiring in serial. All this makes me aware of just how much I’ve forgotten. I have a higher national certificate in electronic and electrical engineering which I got when I was about 19 and I did go on to make use of the knowledge I gained as a technician and later a field service engineer. Can’t wait to reacquaint myself with this oscilloscope. How sad is that?


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Hello,

Originally Posted by Alan3666
How sad is that?

Not so much 🙂. As long as you're having fun with it and are getting some charming results, what's not to like?

I have no formal training in these matters whatsoever. My granddad had an electricians firm and my father did something in electronics, so somehow some wee bit of that must have trickled down the line...

Cheers and happy probing,

HZ

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Originally Posted by Alan3666
Scope not yet arrived but will try all this

Thanks

Alan
You can certainly wait for that or...
wire in some small speaker as your probe at CN9 outputs, presto Ear-o-scope... smile


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Sorry for no update. The scope still hasn’t arrived.
If it doesn’t arrive tomorrow I’ll need to leave everything till the end of next week. My wife and I are taking our dog off to Dunblane for a few days break.The weather here in Scotland has been unusually hot, even for summer. I’ve been playing the Korg through headphones and I think it sounds slightly better than my Roland. Piano is not my main instrument but I am improving.I’ve played guitar for about fifty years, just bought a mandolin which is really just a tiny, tiny guitar. I’m trying to play sax but it’s hard work.


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Eevblog forum has a lot of knowledgeable electronic engineers, might be worth posting there if your stuck.


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Hello,

@Alan3666, What a lucky dog (and wife, and Alan). I've been to the Highlands (and further north to Wick/Thurso/John o'Groats, and to the Edinburgh and Oban regions) several times and have a sweet spot for Scotland. It's been way, way too long, though.

I was about to ask if you'd made any progress -- playing the instrument is already a lot better than it being a write-off!

A tip: several good tips were shared in this thread so far that you can pursue without having your scope yet -- mine about the switches in the headphone out jack being one example.

Cheers and happy fault finding,

HZ

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Ow, .. come on now, don't spoil that dog too much, smile Enjoy... laugh

Last edited by josh_sounds; 08/05/21 02:55 PM.

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Here is what I have found so far with the oscilloscope.

The paths to left and right audio show a good signal.
The paths to the headphones also show good signal both left and right as expected.
The signal on R74 on schematic shows a good signal that then feeds into the EQ chip and on to the AMP chip and out to the right hand speaker.
The signal on R75 is zero volts, so there is nothing going into the EQ chip for the left channel.
It could be that the signal is being dragged down by the EQ chip but hoping for an easier solution I replaced each of the three capacitors in turn feeding into that chip; C24 (which looks like it had been replaced before) , C33 and finally C74. Each time I put everything back together and tried the keyboard. No change only sound from right hand speaker.
I know this is a piano/keyboards forum and not a technical forum, but if anyone has any suggestions I'd be glad to hear them.
One thing that might be significant, maybe not, is that when I power up the SP250 the left hand speaker, the one that isn't working pops quite loudly and you can see the speaker membrane expanding as it does this. The working speaker doesn't do the same.
I've found someone who is breaking an SP250, so I hope to have another Amp Board later this week but in the meantime I'll keep on trying to fix this one.


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Hello,

@Alan3666, Did you have a nice holiday week?

From the top of my head:

- Is R75 good?

- Any shorts or suspect connections/traces on the PCB in that area?

- If you'd unsolder the EQ chip and remove it, does the left signal come back up? If so, source the EQ chip from your cracked board as a replacement.

Cheers and happy problem solving,

HZ

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The headphone jack has a speaker mute scheme. I can't read the schematics so can't see how that works but worth a check as these can be flakey.

DEC recruited top-flight engineers and techs...that must have been a great job.

Korg already removed the high-resolution service manual...now need a password.

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Originally Posted by Alan3666
One thing that might be significant, maybe not, is that when I power up the SP250 the left hand speaker, the one that isn't working pops quite loudly and you can see the speaker membrane expanding as it does this. The working speaker doesn't do the same.

Could it be a DC spike, that is, some op amp with a faulty feedback such that the output pegs at the supply voltage? That would give a pop at power up and prevent any audio from passing.


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Here's a quick update.

Yesterday I tried to isolate the amp chip by removing R9. That is connected to pin 7 on the op amp which is the left channel input. I then had both channels showing a good signals coming out of the EQ chip. This looked promising but after a few minutes something went 'Phutt' and both speakers now not working, and worse still, no headphones signal. I didn't check the audio out signals but I expect they were dead too.

I left it alone for a while.

Today I put R9 back in circuit, still nothing, so I removed IC2 the audio amp chip. I checked the two regulators in IC3. These take the 12v supply and generate 5v and 9v. Both present and correct. I then scoped R8 and R9, left and right outputs from the EQ chip. Both showing a healthy signal when I play a note on the keyboard. Also the headphones working again. I didn't check the audio out channels but I'm pretty sure they'll be fine.

I noticed that some of the pads looked a bit iffy on the PCB under IC2. It might have been me that did it, but this chip had certainly been removed maybe replaced by someone. I'm guessing at the repair shop. Also the chip is a higher rating than the schematic shows. Its an LA4728 rather than an LA4708.
It provides 36W per channel instead of 20W so maybe they used this as the old chip was not available.

Having nothing better to do, I'm going to put it back in and check the continuity of the signals from pins to nearest components.

Alan


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Change of plan.

I found a replacement LA4728 on line. I'm going to hold off and fit the new one.

Alan


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The other Amp Board arrived this morning. I plugged it in and now have sound from both speakers although the left hand speaker is quite raspy. I suspect that it has been damaged by the faulty board. Something connected to the loud pop and the speaker membrane expanding on power up, I'm guessing.

I've found a replacement speaker on line and will replace when it gets here.

Meantime i'm waiting on the new LA4728 chip to arrive. When it does I'll install in the faulty amp board and see if I can get that one going. I'll leave the raspy speaker in till I've completed that task. I don't want to risk damaging another speaker.

The fellow who sold me the second Amp Board gave me the logic board that sits under the Amp Board, the one with the midi connections. I'll check that out too.

Alan


Korg SP250, Roland RD150, Novation K-Station, Novation Bass-Station, Akai Miniak, Akai MPK Mini, Kawai K1, Roland Pro E, Yamaha DD65, Gibson Les Paul Standard, Gibson J15, Line 6 JTV59, Esp EC-256, Columbus Les Paul, Yamaha Bass, Eko Ranger 6, Stagg mandolin, Kemble upright
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Final update.

LA4728 replaced and board now fully working. Also replaced raspy speaker. Now have good spare amp board and logic board which I also checked out.

Onwards and upwards

Alan


Korg SP250, Roland RD150, Novation K-Station, Novation Bass-Station, Akai Miniak, Akai MPK Mini, Kawai K1, Roland Pro E, Yamaha DD65, Gibson Les Paul Standard, Gibson J15, Line 6 JTV59, Esp EC-256, Columbus Les Paul, Yamaha Bass, Eko Ranger 6, Stagg mandolin, Kemble upright
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Hello,

@Alan3666, Lovely, well done!

Cheers and happy playing,

HZ

Joined: Apr 2021
Posts: 226
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Congratulations! Thank you for taking us along with your journey...
Oh wait..Are you still giving it to your son? grin #3141050


Hard at work while waiting for my dream DP....
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