Thanks for the responses Gareth and N Cook!

Sorry.. I should have been a bit clearer to start with.

The output from the OpAmp on the main board goes to the volume pot, and
then on to the separate audio amp board which does all the jacks, line
outs, main amp, muting. The audio amp stage is working.. I can feed
audio into it ok.

I tested the output off the mainboard that goes to the volume pot, and
there is no audio at that point.

There is an on-board self test sequence which I can activate and the
memory/logic test passes, and the MIDI test passes when the midi in/out
ports are looped back, it fails when the MIDI ports are not looped (as
expected).. so I believe the MIDI and basic processor/memory etc part is
probably ok.

Here is the schematic of the main board:
"http://i.imgur.com/hVf4N6i.gif"

IC3=PCM1716E

So far all I know is there is no audio on CON8-1 and CON8-3 (right of
diagram).

Is it typical for both op-amps in the dual op-amp to die together?

I have to figure out how to get the boards running on the bench next so
i can get at the mainboard to test it in situ. I will check for DC
output on the amp.

Thanks again for the suggestions so far.


On 5/1/2014 9:23 AM, 2phar wrote:
Hi all..

I have a question about probabilities in troubleshooting a surface mount
mainboard in a Korg SP250 Digital Keyboard.

The mainboard uses a SAM9753 ('dream') DSP/controller, with digital
audio & clock signals going straight into a PCM1716E DAC, the stereo
output of which goes thru left/right low pass filters built on a
NJM4580M dual op-amp.

All power supply rails look good.

The mainboard passes its internal memory checks and MIDI loopback
checks, etc., which leads me to think the SAM9753 itself is ok.

I have seen reports of two other Kong instruments with similar
condition.. main DSP / power ok but no audio out.

I intend to report back here if I get to the bottom of it, but I was
wondering given the above what people would be inclined to suspect /
most likely failure of the following... solder, caps, the DAC chip, the
OpAmp, partial failure of the DSP chip? Unit is about 5-7 years old.