Hi Can anyone offer some advice?

I bought a Korg SP250 on Ebay sold as parts only, no power. When it arrived it was in great condition but sure enough it did not power up. I opened it up and found that the power module was badly damaged. It looked to me like someone had yanked the audio cables out and the force had cracked all the PCB around the jacks. I managed to find a faulty Amp Module which the vendor said had only one working channel. I plugged this in and confirmed no left hand speaker output. However both Audio out channels are working and both left and right channels good on headphone socket.
So looking at the schematic below the most likely culprit would have been IC2, the power amp but it looks like someone has already replaced this judging by the solder on the PCB. I'm waiting on a Scope coming so I can have a proper look, but any thoughts anyone. If I find no signal on pin 7 on IC2 the next candidate would be the equalizer chip IC1. I don't fancy de-soldering that one. Could I be lucky and have a failed capacitor or a dry solder joint?
Any help appreciated.

 

Think lucky and you will be lucky (Old Neanderthal Proverb).
When you have your 'scope, I'm sure you will track it down.
You have a good channel to compare it with.
Good luck.....

 

I have now checked the signal paths with an oscilloscope and found that on R51 and R48 I can see the left and right signal as expected. These go on to feed the headphones. However scoping the speaker paths I have a clear signal on R74 which then passes through the EQ chip and into the amp chip and out to the right hand speaker but no signal on R75 which would also pass through the EQ and Amp to supply left speaker.
I replaced C24 then C33 and then C74 as these are all in the path feeding into the EQ chip bit no joy.

My logic being if any of these capacitors had gone they could block the signal.

Anyone got any views or opinions?

many thanks

Alan

 

Hi Alan,
I cannot see any components in the seventies.
Have you got any more diagrams/circuits/ schematics?

 

Hello,

I hope this is better readable:

https://www.synthxl.com/wp-content/uploads/2020/03/Korg-SP-250-Service-Manual.pdf

Bertus

 

 

Thanks Bertus,

that's a much clearer schematic. My diagram shows the good paths in green, the faulty path in red and the three componenets I have changes circled in blue.

Alan

 

Also noted that on power up the non working speaker pops really loudly and the speaker membrane stretches up like a balloon being inflated. The working speaker does not do this?

 

Hi Alan,
You need to check IC1 input & output.
BA3823LS Datasheet PDF - ROHM Semiconductor (datasheetq.com)

 

Hello,


Bertus

 

Hello,
View attachment 245296

Bertus

Hi Bertus,

There's nothing beyond R75 on the left channel. I can trace the audio from R74 all the way to the speaker connection CN3.

Alan

 

First thing to check - Is there sound from both Left and Right Phone output?

If there is, proceed as follows:

Trace and compare signals on both legs of R74 and R75.
Lift one leg of C74 (or remove) and trace again.

If still no signal, then loss of signal could be caused by:
1) C33 shorting
2) R16 shorting
3) R75 open

 

Something strange. I removed R9 which feeds the left channel into the amp IC2. I still had no left speaker as expected but I could now see the left channel signal beyond R75. I could read it on R14 which is at the other side of the EQ chip. Both channels were showing good signals on my scope. But then something went phut and everything stopped working. No left or right speaker. Headphones dead. I haven't tried the audio out channels but I expect they will be dead too. Oh joy!

I'll stop and take a break and think again.

Alan

 

First thing to check - Is there sound from both Left and Right Phone output?

If there is, proceed as follows:

Trace and compare signals on both legs of R74 and R75.
Lift one leg of C74 (or remove) and trace again.

If still no signal, then loss of signal could be caused by:
1) C33 shorting
2) R16 shorting
3) R75 open

View attachment 245329

Yes there was both left and right signals on both Audio out and headphones, but see my latest update.

 

From your comments in post #8 there is a severe DC offset on one or both outputs of the left channel. (It looks like a bridged configuration.) The DC level on both + and - outputs (Pins 12 and 14) should be close to zero. I suspect C15 could be leaky upsetting the DC state of the DC level on the input (pin 7) From your comments in post #13 I think the amplifier chip U2 has now failed totally shorting out the + 12 volt rail. I could not find a datasheet for U2 (LA6708) so I think it will be difficult to obtain a replacement.

Les.

 

Hello,

@LesJones , The LA4708 seems to be an automotive amplifier from origin.

Bertus

 

Hi Bertus, Thanks for finding the datasheet. From the block diagram it looks like the input pin should sit a DC level of ground.
When suspected C15 I was thinking it may was biased to some voltage other than ground and a leaky C15 could change the DC conditions on the input which would shift the output away from ground. I think this could only have been caused by a faulty LA4708. (This is using the evidence of the speeker cone being deflected from it's static position.)

Les.

 

Update:
I abandoned the keyboard for a while and went and cleared the roof gutters for a bit.

I put R9 back into the circuit but still no headphones or speakers. So I removed IC2 the Op Amp. The schematic shows an LA4708 but this module has an LA4728, which I think is a higher rated amp. The LA4708 is 20W per channel the LA4728 is 36W per channel. I mentioned in post #1 that this module had been in a repair shop but they hadn't managed to fix. It looks to me like they have replaced the LA4708 with the 4728. You can see the solder has been disturbed. When I got the chip out it looks like some of the pads have been damaged on the pcb. I found that pin 7's pad is definitely gone. It is possible that it might have been me that damaged it. But if that etch was open circuit it would explain the dead left speaker.

Anyway with IC2 removed I checked the two regulators on IC3. These take the 12v supply and generate 5V and 9V. Both voltages are present and correct. I then plugged in a set of headphones and both left and right sound good. I haven't checked the audio left and right but I'm pretty sure they'll be good too.
So I'm now more or less where I started. I have a working piano which I can plug into an amp or PA system or can play through headphones.
I don't see any harm in putting IC2 back in and using a meter to check the paths.

Here I go again.

 

Also with IC2 removed completely I have a good signal on both R8 and R9 which are the left and right feed from the EQ chip into the amp chip.

 

At least I found the datasheet for the LA4728 easily. Farnell , RS, or Mouser don't stock them but I found this one on ebay

Les.