faulty motherboard.

shirreff · Post by **shirreff** » Sat Apr 07, 2012 9:19 pm

I have a problem with the motherboard on my korg M1. It just has a loud hissing sound with the real sounds just barely noticeable in the background.
Any Ideas?

synthjoe · Post by **synthjoe** » Sun Apr 08, 2012 6:31 pm

Does it hiss only when notes are played, or even after power up, without playing anything? What happens if you route sounds to outputs C+D? Do those have the hissing, too? Is this the same on the headphone out?

shirreff · Post by **shirreff** » Sun Apr 08, 2012 6:57 pm

same whatever route, headphones too.

synthjoe · Post by **synthjoe** » Tue Apr 10, 2012 10:44 pm

Only when playing, or even when idle?

shirreff · Post by **shirreff** » Tue Apr 10, 2012 11:09 pm

Only when playing,

synthjoe · Post by **synthjoe** » Tue Apr 10, 2012 11:50 pm

If you are experienced in electronics, you could program a single monoaural signal (e.g. sine panned to the center, no effects) on the synth and test pin 1 of IC 12 on the KLM-1261 board to see what signal comes out there (e.g. via a small headphone amp, or line input to an amp). Should the signal be clean at that point, then you have some trouble in the analogue demultiplexing circuitry.

Should the signal be corrupted on pin 1, then you could have a fault in the digital part (unlikely) , the D/A converter chip, or the opamp you're taking the signal from (pin 2 of the same IC 12 is directly wired to the D/A, but a current output - you can test that carefully, if you know how to do it). Make sure you check voltages as well - the problem you describe migh also be due to a missing power supply rail voltage.

Maybe a short recording of the noise would be more revealing than a description - but the method above is still valid for checking, regardless. Just in case you want to avoid opening the synth at this stage.

probani · Post by **probani** » Tue Apr 24, 2012 6:52 am

I test the PIN 1 of IC12 on the KLM-1261 board and the wav form is square, this is correct?
with a sound Init

synthjoe · Post by **synthjoe** » Wed Apr 25, 2012 10:09 pm

No, I'm afraid it is not. It should be similar to the waveform expected on the line output of the synth - which in case of a freshly initialised program should be something pianolike. However, looking at it on an oscilloscope might be deceiving as all four output channels (L+R+3+4) are present in multiplexed form at this point and switching between the channels might look like square wave. Therefore it is better to listen than to look on an oscilloscope. If it is indeed a square signal, it should result in a quite distorted sound - is that what you're experiencing?

You can check the signal on pin2 of the same IC, if the signal is OK (i.e. different from pin1) then you'll have to check R85 (between pins 1 and 2 of IC12) or maybe replace IC12.

Otherwise the problem is likely in the digital part of the synth, or the D/A converter has to be checked.

woodynat2000 · Post by **woodynat2000** » Wed Oct 11, 2017 4:03 pm

This post is old however I have the same issue and just wondered if you ever found a fix? Cheers, Nathan

shirreff · Post by **shirreff** » Wed Oct 11, 2017 4:08 pm

No, sorry. I still have the faulty board

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