"2phar" wrote in message
...
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.



Do you mean no audio from the inbuilt speakers, or line outs, or headphones,
or all?

Is the keyframe working? Check there is MIDI coming out of the MIDI out
socket using an LED inserted into the two pins either side of centre. (Try
both orientations, it will only work one way round)

The internal speakers are muted by a headphone jack into a headphone socket.
Could one of these sockets be faulty thus muting the audio?

If the OP-amp has failed it will have DC on one or both of its output pins.

The Master Volume control is a convenient place to scope for a signal, and
could be the problem.


In other words, look for the simpler things first, instead of thinking of
DSP's and DAC's and stuff.



Good luck.


Gareth.