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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

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

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...

Scope not yet arrived but will try all this

Thanks

Alan

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

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

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?

Hello,


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

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

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.

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

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

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

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.

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

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.

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.

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