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.
Hi Chris
ReplyDeleteMy wife brought home a Korg SP250 from the school where she teaches music. It has no sound or lights or anything when turned on. I read your post and removed the DTC114ESA. still nothing. Any other suggestions you might have or if you have a schematic you could forward wound be great. Thanks Dennis dkkoestler@wavedirect.net
@Mrs. Koestler kids, I have the same problem with my korg sp250, if Chris and you found a solution please help send me your guys diagnosis and solutions. thank you Jonas jchanh1@mail.csuchico.edu
ReplyDeleteKeep the power on for 5mins. It works for mine.
ReplyDeleteKeep the power on for 5mins. It works for mine.
ReplyDeleteDear All, I have the same problem. My KORG SP-250 is not switching on, only after 20 seconds it switches on and no sound is coming when playing keys or play in demo mode. Or after switching on and off a few times, you can play for some seconds and after this the piano gives strange sounds and freezes. Any one an idea? Service manual can be find here: https://www.mikrocontroller.net/attachment/279928/korg_sp-250.pdf
ReplyDeleteI either thought I had problem the mute circuit or with the output op amp LA 4628. As I traced signal all the way up to the 4556 & 4580 op amps , I went to pins 1 & 7 to check for output which I had when striking the keys. And I traced it all the way up to the LA 4628 main output op amp. There was no signal on pins 8 & 11. Where there should have been something there. I'm glad I didn't buy it. Because this Xtor will be a lot easier to place then that 14 pin op amp. Thanks so much for help .
ReplyDeleteI have an SP250 which someone had managed to destroy the Amp Board. Looks like they yanked the audio cables out and so cracked all the PCB around the jacks. I managed to find a replacement on Ebay but it has a channel fault. I now have sound on both audio channels, sound on headphones left and right but only sound on right speaker. Looks like someone has replaced the power amp chip. I'm waiting on delivery of a scope, but wanted to ask your opinion mean time. If its not the power amp, would a faulty equalizer IC (IC#1) be likely cause. I don't fancy desoldering that. Could I be lucky and have a failed capacitor? Any advice welcomed.
ReplyDeleteAlan
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