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Korg SP-200 piano low volume

jnm

May 15, 2020
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You mean the sound ouput of a phone using a headphone cable and connect the jack's tip, sleeve and ring parts to the output boards two volume pins and ground? Or is it something more low level since I am not sure how a phone's sound output voltage could be tweaked. A tone emitter app?
 

Ylli

Jun 19, 2018
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You could use a tone output program on a computer, or yes, the headphone jack of a phone or ipod type device.

Durn, I forgot to attach the pictures in the previous post. You would not need to wire it as stereo, you could just feed a mono signal (or one channel of a stereo source) into both indicated points. You would be replacing the output of the CPU board with the external signal source. Cell phone headphone volume level should be sufficient.
Annotation 2020-05-28 142039.png Annotation 2020-05-28 142543.png
 

jnm

May 15, 2020
18
Joined
May 15, 2020
Messages
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You could use a tone output program on a computer, or yes, the headphone jack of a phone or ipod type device.

Durn, I forgot to attach the pictures in the previous post. You would not need to wire it as stereo, you could just feed a mono signal (or one channel of a stereo source) into both indicated points. You would be replacing the output of the CPU board with the external signal source. Cell phone headphone volume level should be sufficient.
View attachment 48416 View attachment 48417

Thanks, this was very instructive.
When feeding phone output (where DMM shows ~ 0.57 V with max phone volume) to pins 1 and 3 the sound is very loud indeed.
The main board looks a lot more intimidating and less likely to be fixable by soldering...
What to measure next? :)
 

Ylli

Jun 19, 2018
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So with an external audio source, you get good output from the audio section. Does the volume control behave normally, i.e. properly adjusts from minimum to maximum with no flattening?
The IN and OUT signals on these ICs are ~4.5 V .
The IC11 from the main board, the NJM4580 opamp, also has 9V between pins 4 and 8, but it only has ~2.77 on the IN and OUT pins. Could that be the problem?
While 2.77 volts is not 'centered', it is within range and will represent the DC output of the DAC. As a check on IC11, we can measure the voltages at all the pins:
Pins 1, 2, and 3 - all the same voltage, between +2 and +7.
Pin 4 - ground
Pin 5, 6, and 7 - all the same voltage, between +2 and +7. Should be close to the same voltages as on pins 1, 2, and 3, but does not need to be exactly the same.
Pin 8 - +9.0 volts.

Prior to that is digital circuitry, and while I will do digital on my own bench, I won't try to do it remotely. It's not really my area of expertise.
 

jnm

May 15, 2020
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May 15, 2020
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Yes, the volume slider works with external audio.

And all 6 IN and OUT pins of IC11 are around 2.77 V compared to pin 4. Pin 8 is ~9V.

Thanks for your help so far, I'll read some more about electronics and maybe give this another shot if I feel I understand this better and maybe have more tinkering experience :)
cheers
 

Ylli

Jun 19, 2018
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Strange thing here is that (if I understood correctly) as the volume control was advanced, the volume would increase up to a point, and then stop increasing. That *sounds* like the output section limiting. But the output section works fine with an external input.
 

jnm

May 15, 2020
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Right, with the external source the upper parts of the volume slider worked, unlike when the signal comes from the main board.
 

jnm

May 15, 2020
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I could measure a few of the DAC pins: VCC1(15) is 5V, VOUTL(16) is 2.5V, EXTL(14) is 2.5V, all from AGND1(14)
It's not easy to get at neighbouring pins with my DMM probes. I am afraid I could damage the chip by accidentally shorting GND and other pins hence no VOUTR and EXTR readings.
 
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