AXIOM 49 with dead midi output

I posted this unit on the blog because it's failure was caused by a most unlikely culprit: a failed ceramic capacitor which was used to filter out noise on the midi output. Although I encounter countless failed aluminum capacitors, I rarely (if ever) have encountered a failed ceramic capacitor.

Nevertheless, this was the case here.

The unit came to me because the midi out was failing, it was completely dead. Of course I checked the jack and the connections around it, and all seemed fine. I reset the software.

Just for a lark, I removed the tiny small value capacitor that filtered the midi out line to ground, and found that the midi out came right back.

Although the capacitor did not read as shorted, it's capacitance must have drifted and increased significantly, enough to actually be filtering out the midi signal, rather than just noise.

It was a 220 PF cap, shown below, and the unit worked fine when it was removed, and continued to work fine when a new one was installed. The second picture shows the area where the cap was located... I believe labelled C4 on the silkscreen.

Korg Triton LE Power Supply issues; heating up, distorted sound

This Triton came to me completely dead. The first thing I checked was the power supply, a 9v AC 3 amp wall supply with a 4 pin DIN connector... very much the same as some Alesis products... the DMPRO for example, which mentioned elsewhere in this blog.

I found a blown 2 amp inline fuse in the power supply, which can be disassembled (with the help of a drill). I am sure quite a few of these units have found their way to a trash can, with their only issue being this blown fuse. 2 amp fastblow and slow blow Pico fuses are available all over the internet.

After replacing the fuse, I turned the unit on, until to find that it was heating up, and only produced a horrid distorted tone.

I checked the voltage rails first, and noticed the 9 volt rail was substantially low. Through a process of elimination, and disconnecting one rail at a time, I came to find out that the 5 volt regulator had actually failed, and was sending excess voltage into the 5 volt rail, and consequently dragging down the other rails. It wasn't an extremely high voltage, at about 6 volts, and but it was enough to drag down the other rails.

This posed a substantial problem as this particular Korg uses a tiny surface mount switching regulator, and a tiny surface mount FET in conjunction with it. I found these parts in China, on ebay.

Once installed, the new regulator, fet, and a few caps (for good measure I replaced the filter capacitors around the regulator), the power rail came up and the unit turned on and worked. I also added a small 5.6 volt zener diode, which would short the rail to ground in case the voltage ever exceeds the specs again, in the hopes of protecting the chips.But this was too late...

Apparently, one of the PCM roms was damaged... probably by the previous over voltage, when the regulator first failed. There are four of these PCM roms on the Triton LE, and they provide the basis for all of the preset sounds and programs.

On this particular unit, after its resurrection,  some of the sounds, predominantly the organs and some brass sounds, sounded great. But others were distorted... primarily all of the piano patches and string patches. These must be looking for those damaged PCM samples.

Sadly, Korg no longer supplies these PCM roms, and so we had to settle for a partial repair, until such a time as I can scavenge these chips.

On the bright side, the sampling feature was restored, and many of the cool organ patches.. and probably about 50% of all the other patches sound perfect. But there was nowhere I could obtain those PCM roms.

However, if someone reading this has an old triton LE mainboard for sale, from which we could scavenge these chips, please let me know!

Korg CX3- old style- with Leaky Electrolytics

Dual CX-3!

I am always skeptical when I receive these Cx3 organs for repair because of the known issues with the Seimens chips.. at the organ's heart are a couple of divide down oscillator chips that create the oscillations and gate them for each note. Their failure is a serious issue, because they are very hard to find and replace, and can only be replaced with other used chips that are probably very close to failure themselves. It seems someone was offering replacements, but apparently the supply dried up and I suppose they found it was no longer worth it to sell them.

But here was an instrument that didn't have that particular problem, rather it suffered from intermittent failures due to these badly leaking electrolytic capacitors seen below. The acid from inside of them ate through several traces on the boards, which had to be repaired.

The good news is that replacing them brought the keyboard back to life... the bad news is that there are about a thousand 10uF capacitors of this type sprinkled all throughout this keyboard. In this instance, due to financial and time constraints of the customer, we did replace only those that were seriously leaking... and left until later a full recapping, which is certainly necessary.

Currently the organ is working again, and sounding great, but most likely it will need some more caps replaced... unless the chips fail first!.

The failing capacitors were all 10 uF 16 volt capacitors. I doubt that it is coincidence... but it seems to me that the most frequent capacitors, in all gear, that I have to replace, are those rated 16 volts and less. I am not sure what is different between the processes used to make 16 volt caps and 35 and 50 volt capacitors, but I seem to rarely find failed 50 volt capacitors, unless they are too close to a heat source.

For this reason, I always use 50 volt capacitors, where possible, to replace them.

Jupiter 8 Replacement Faders and Buttons

For those of you looking for working replacement tact buttons for your Jupiter 8, they can be obtained cheaply, and brand new, from Digikey, digikey part number CKN9100-ND . Check them out. They are the same size, pin spacing, etc as the original. You just have to be really careful when you put them in the "actuators", the button caps, as they are called. You must do it slowly using an exacto knife to pry the old button out of the actuator, carefuly spreading the housing away from the button, and then carefully putting the new button into the housing. But it does come out just fine!

As far as replacement faders, these are a bit harder to find. This is not because they are unusual electrically (10k linear is about as common as you can get), it's because they come in such a large housing. One workaround is to disassemble the old fader, if it is beyond cleaning, and then install a new (smaller packaged)  fader into the old housing. If you get one with the right lever height, travel distance, etc, and mount it in the old housing, when you assemble the housing you essentially have a fader that feels great and looks and works exactly the same as the original.

I used part number
BOURNS PTA3043-2015CPB103
for my replacement fader. It is exactly the same electrically, travel wise, taper wise, and has the same lever as the Jupiter original. It is just in a smaller housing, which fortunately mounts perfectly in the Jupiter 8 faders original housing.

A picture is worth a thousand words... see below:

The replacement fader sits atop the PCB from the original fader

From underneath, you can see where tiny holes were drilled to connect the pins from the replacement fader to the pins from the original fader. Of course, I made sure the carbon was severed completely on the orginal fader PCB so as not to be adding a parallel resistance.

When the housing is installed, the fader looks original again, except now it works! From the top, the replacement fader is exactly the same as the original... exact same travel distance and electrical taper.

Korg sp-250 repair: no sound on outputs

 

 

This Korg came to me with no output sound. Looking around the around the power amp IC, I could see the standby/mute function (pin 5) was not going high, as it is supposed to. I was able to trace the trace back to Transistor 4, which did not ring out as a transistor should. Suspecting that the transistor was the fault, I searched for a replacement or a cross, and found that the transistor is actually a DTC114ESA, known as a "digital transistor". I had not encountered one of these before, but found that it is actually a regular NPN transistor with the biasing resistors built in. It is called "digital" because it is biased to be used as a switch, as an inverting switch. It saves the designer from having to bias the transistor externally with resistors.

 

From its datasheet, here is a description of the circuit:

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And here is how it appears in the schematic for the SP-250:

 

 

 

And finally, here is where it sits in the circuit board:

 





 

 

It is useful to add that the keyboard works and sounds fine with this transistor removed. By removing the transistor, you cause the mute rail on the power amp IC to go high right away. You may get a bit of a "pop" when turning on the keyboard though, so it is better to have it installed. However, a handy way to test whether you are having a problem with this transistor (or somewhere upstream in the standby circuit) would be to pull it out and see if the keyboard starts working again.