Almost no sound from Korg C-15?

I have a Korg C15S and checking the unit with quite only a little bit experience in electronics. Transformer is getting full about 120+ V power and 5 wires coming out. 2 blue ones's have 10V coming out (same reading in Fuse 2 obviously). The black and 2 red wires show no voltage.
This doesn't seem right? I have the service manual but it is not giving voltage specs....should it be 50v or so? Why voltage only on blue wires? If transformer is done, is there a generic one available? Can't find anything with goodle when using numbers found on top of the transformer. Any help appreacited.

John
November 2021

How do i Open this piano

Joseph
June 2018

I just finished repairing a Korg C15S electric piano
that would power up but without any sound..
Also the preset switches would not show any of their leds lite.
Reading some forum posts I found that many of the sound board's electrolytic caps
would need to be replaced because of electrolyte leaks mostly probable.
So I changed all of them.. around 20 (10uf, 47uf, 0.33uf, etc..)
Some of them had started to leak on the pcb causing
some traces to be destroyed ! So I had a microscope soldering job
to do with some cleaning of the affected board areas.
Then I carefully soldered all the new caps in place.
And came the time to test.. Is-it gonna power up normaly now.. ?
No.. The preset switches were still dead.. All the leds OFF.
By luck after flicking many times the power switch
the keyboard went alive with all its sound playable !
But this was occasionaly that the keyboard would come alive
playing with the power switch.. Mostly it would be dead again.
But that gave me a clue that a component was working 'border line'..
So I checked all the cpu, memory side of the circuit
and found that when the presset switches were not working (no leds, no sound)
it was because of IC1 that was not scanning neither the keys, neither the preset switches.
I finally found that the supply pins of IC1 were at around 4 vdc instead of 5 volts.
A 'border line' supply voltage.. The real 5 volts supply rail
passed through a 3.3 ohms resistor (R17) that was almost open
causing this intermittent startup failure !!!
I replaced it with a good resistor then everything worked
fine ever since.

Hope these info would help anybody.
J-Pierre

J-Pierre
June 2017

Hi my name is Alex I have a korg c-36 electric piano, the power light comes on but there is no sound or life apart from the light, I have checked all the boards they are all like new there is no evidence of any damaged capacitors or dry joints. Can you help please I am desperate to get it working. Regards Alex

Alexander
December 2013

I have rebuilt a number of these and the problem is that all the surface mount electrolytic capacitors leak electrolyte all over the circuit board and it corrodes the board. it literally eats all the traces away!

To repair you have to be VERY skilled at SMD rework. you have to remove all the caps, CLEAN and CLEAN and SCRUB and CLEAN the board again to remove any trace of electrolyte and believe me you cannot clean it enough. If you touch the tip of your soldering iron to a pad and get a stinky fish like inner tube smell. you haven't cleaned enough!

Then inspect EVERY trace with a microscope, you will find many that are corroded and some that are no longer there. using very fine kynar wire wrap wire, you have to rebuild every trace. then replace all the caps and cross your fingers that it works when you are done!

and then, there is no telling how much longer it will last as inevitably there is still electrolyte under the parts you didn't remove and it will ended up eating other traces and the unit will stop working again.

The ONLY real solution is to replace the whole tone board assembly!

Overkill audio inc.
January 2012

I repaired a Korg Concert C25. All the small electrolytic capacitors had lost some or all of their capacitance.

Old Hand
September 2011

I have the same problem.
how have you repair ?

DIDIER
January 2010

Almost certain your problem is in the Pre-Amp Circuit which is a common failure on this range of keyboards. I currently have a C-25 with a similar problem that I am working towards resolving as a winter project. I recently purchased a copy of the service manual c/w fault diagnostics and schematics that helped. Other option is; if you have another keyboard c/w midi connections then you can still use it as a trigger through it if you want to avoid repair time/cost.

Marty
October 2009