Thursday, December 26, 2013

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:
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.