Lately I have had a Korg concert EC150 for service. This piano had one major issue, it was silent. Not even the slightest hum from the speakers nor on the headphones while the panel seemed to work just fine. So my first guess was this problem relates either to the power amp section or the corresponding power supply.
Sadly, there is no service manual nor schematics available on the internet for this digital piano so I thought it's a good idea to sum up what I came across. There is a good chance by the way that like with other digital piano series some of the information shared here may apply to all pianos of the EC series like EC100, EC120, EC300, EC310, EC320, EC350.
Disassembling the EC150
Open up the piano by removing three screws on the rear top, then slide back and lift the top cover. There are two PCBs mounted on a metal support standing upright in the piano, the power supply / amp board to the left and the mainboard to the right. Remove all screws keeping the support in place. Turn it by 90 degrees to make measuring easy.
There are three voltage regulators in the circuit, a 7805 attached to a large heat sink serves for all mainboard and panel board operations while a KIA78R12 (TO-220 with 4 pins, on same heat sink) followed by another 7805 cater for the amp section.
The external power adapter (center positive) delivers 24 V / 2 A. On the amp board there is a 1N5404 diode right after the power switch connector to protect from wrong polarity on the small cost of a 1 V voltage drop. The rest of the power distribution is pretty straight forward.
Plugged in and switched on I measured 23 V on pin 1 (input) of two of the regulators but on the output side (pin 3 on 7805, pin 2 on KIA78R12) only the heatsink-7805 gave me 5 V while both of the other regulators had a poor 0,83 Volts. Too low voltage in case of power supplies can e.g. have the root in bad capacitors or a failing regulator itself, among others.
Since the Amp Mosfet IC (Tripath TP2050, accompanied by a TC2000 controller) has some protection embedded against over-current, over-/under-voltage and heat I guessed the faulty regulator(s) was the only issue.
To get the board to the work desk remove all wiring by pulling off the connectors. The 5 pin connector in the board's center is of push down type, so press the collar down and pull up the flat cable easily. It's a good idea to mark both the right side of the connector and the rightmost wire (red) with a dot to prevent unwanted twisting before reassembling, see image.
Since most of the circuit is built on SMD technique and desoldering needs quite high temperatures and often a bit of leaded solder to bring down the melting point it was relieving there was enough reason to focus on some of the larger components first. Due to lack of schematics I did a quick sketch of the suspected area on the Korg KIP-2081 PCB, see image below.
EDIT: While trashing office papers I just found the quick sketch sheet again and decided to redraw this in KiCad, so here is the Korg EC150 PSU schematics PDF.psu_korg_ec150.pdf (40.44 KB) (40.44 KB)
While all caps before and after the regulators where within specs and all diodes functional, two of the regulators were gone, 4-pin-78R12 and the 7805 (II) in chain, the one without a heat sink. If your EC150 suffers from the same symptoms it could be enough to replace the 12 V regulator since its output feeds the input of the 7805 (II). In my case this was not enough, unfortunately.
So after replacing both all voltages came back to specs and the piano was good to be played again.
By the way it's a good idea to replace the 6800µ and two 100µ caps before the regulators with versions for 35 or more volts since the input voltage from (a bad) power adapter might already exceed the original 25V, not to speak of normal cap tolerance.
Note, the 12 V regulator's 4th pin is for enabling (high) and disabling (low). A blown heatsink-7805 (I) with no output would probably just switch it off.
Other issues and problems
Like many pianos of a certain age the EC150 had one key which could not be played soft. Instead, no matter how soft or strong you hit the key, the volume of the sound was always the highest possible. If your digital piano suffers from no dynamics or dead keys it's usually time to give the contacts beneath a cleaning. I described the procedure already here but for completeness, hey, once more.
Cleaning contacts of dead keys or keys with lost dynamics
The keybed is marked Fatar 1758, there are a variety of manufacturers using this for their digital pianos, even some chinese companies (sorry, probably to finally gain one star out of five in reviews). It's of good quality, the key mechanics is rock solid and the plastic does not tend to break or wear out. However, contacts catch some dust over time and start to fail, other than e.g. Yamaha keybeds that are more robust on this point. On the other hand I've had a lot of Yamahas with broken plastic.
Back to this one from Fatar. Remove the two keybed ribbon cables from the mainboard after you've marked them LEFT and RIGHT. Still, connecting them in the wrong order would do no harm except for the high notes appear on the lower keyboard and the low ones on the upper.
Remove the keybed's screws from underneath the piano. Note, three rear screws are a bit longer than the screws in the front. Lift up the keybed, turn it upside down and lay it on a desk or a piece of old carpet. Make a quick mark on the keybed PCBs where the metal support bars are. Then remove these bars. Remove all screws from the circuit boards and keep track of where which type of screw belongs. The wiring does not necessarily have to be removed.
Lift off the PCBs, turn them upside down and lay them on a desk. Remove the rubber contact mats and keep track of their position and orientation on the board. Be careful to get rid of all dust with a vacuum cleaner. Clean all contacts both on the boards and mats softly and properly with benzine or something comparable, make sure the liquid evaporates traceless. Use Q-tips or a soft cloth.
Put the contact mats back in place and make sure they lay really really tight on the board. Don't make it easy for future dust armadas! To push the nipples through the corresponding holes you can use a small Allen key. Be careful and use something that is not sharp, otherwise you will cut holes in the mat. You could also use tweezers to pull the larger nipples from the other side.
Reassemble the piano in reverse order.
If you have any questions or additions, feel free to write a comment.