Korg SP-200 piano low volume

[jnm]

Hello,
I have noticed that the ouput volume of the electric piano is much lower than it used to be. It does not have built-in speakers. With a headphone it is bearly audible, while with external powered speakers it is ok, but low. It seems that moving the volume slider past its midpoint (between the 5-10 marks) does not produce a noticeable change in output volume.
I suspected that potentiometer to be faulty and opened up the keyboard. Luckily there is an official service manual with electrical schemes included. But my measurements with a DMM showed that the stereo volume slider changes are going across to the output board correctly via the 2x3 wires.
And this is where my skills sort of end. I am not sure where to look for a fault on the output board, with many capacitors, two amplifier chips, a couple voltage regulators, and the output ports.
I've found a similar question related to the SP-250 model answered on this forum, this is how I found this place
The Korg manual is https://data2.manualslib.com/pdf5/104/10340/1033931-korg/sp200.pdf?d5bab82b522550709438da31578d5f70
any pointers are appreciated
thanks

 

[Ylli]

I'd start with the assumption that the digital section is OK, and the fault is in the analog amplifiers. (that assumption may need to be revisited) So you are looking at the Output Board KIP-2052. Start by confirming the power supply voltages are all there and correct. I see + 12 volts in from the wall wort, regulated down to +9 and +5 volt lines. Check to be sure the voltage at "Vcc/2" is very near 4.5 volts. Then I would look at the 'Mute' signal and confirm it is low (<0.5 volts).

 

[jnm]

I'd start with the assumption that the digital section is OK, and the fault is in the analog amplifiers. (that assumption may need to be revisited) So you are looking at the Output Board KIP-2052. Start by confirming the power supply voltages are all there and correct. I see + 12 volts in from the wall wort, regulated down to +9 and +5 volt lines. Check to be sure the voltage at "Vcc/2" is very near 4.5 volts. Then I would look at the 'Mute' signal and confirm it is low (<0.5 volts).

thank you,
I'll try these next - will need to disassemble it again

 

[73's de Edd]

Sir jnm . . . . .

Considering that you are plugged into the units HEADPHONE jack for its outputtted audio.

Ditto on the checking of the bases of the to confirm that there is on onset of turning them on and a shunting of the audio signal to ground.

Although, realizing that you are experiencing the decline in audio on BOTH channels.
Remembering my past experience with an 'ole tyme circa 70's Tee Wee of mine, would you take a 220 ufd of the same voltage rating as C4 or C5 . . . . .closely observe its lead polarity. . . . and then shunt it across C4 or 5 and see what happens to the headphones volume ?


73"s de Edd . . . .

My doctor told me that jogging could add years to my life. He was exactly right ! — I feel ten years older already.

.

 

[jnm]

Sir jnm . . . . .

Considering that you are plugged into the units HEADPHONE jack for its outputtted audio.

Ditto on the checking of the bases of the to confirm that there is on onset of turning them on and a shunting of the audio signal to ground.

Although, realizing that you are experiencing the decline in audio on BOTH channels.
Remembering my past experience with an 'ole tyme circa 70's Tee Wee of mine, would you take a 220 ufd of the same voltage rating as C4 or C5 . . . . .closely observe its lead polarity. . . . and then shunt it across C4 or 5 and see what happens to the headphones volume ?


73"s de Edd . . . .

My doctor told me that jogging could add years to my life. He was exactly right ! — I feel ten years older already.

.

thank you,
I'll get some capacitors next week and will try this.

 

[jnm]

I'd start with the assumption that the digital section is OK, and the fault is in the analog amplifiers. (that assumption may need to be revisited) So you are looking at the Output Board KIP-2052. Start by confirming the power supply voltages are all there and correct. I see + 12 volts in from the wall wort, regulated down to +9 and +5 volt lines. Check to be sure the voltage at "Vcc/2" is very near 4.5 volts. Then I would look at the 'Mute' signal and confirm it is low (<0.5 volts).

I have measured the voltage across R17 and R18 and it's ~4.4 for both, so that suggests VCC/2 is ok?

Across R3 and TR3 it's ~0.2 V, I hope that is a good indication of Mute.
I've only measured on the side of the PCB with the components mounted, I will probably need to take the board out for better access to the tracks for further measurements.

What is the mute signal used for? Is it an ON/OFF mute or does it affect volume more subtly?

 

[Ylli]

Looks like the voltages are OK. The mute signal normally would just turn the audio output on and off, but it can act as an attenuator if it is somewhere between on an off.

 

[jnm]

Looks like the voltages are OK. The mute signal normally would just turn the audio output on and off, but it can act as an attenuator if it is somewhere between on an off.

does this mean the assumption that the digital section is OK needs to be revisited? I see two of the master volume control lines go into the main board as well.

 

[Ylli]

The audio section could be tested further by lifting the injecting an external signal into the high side of the volume controls, and seeing if the gain with a different source seems normal. Could inject that audio here:

 

[jnm]

What sort of equipment is needed to inject (or what should I google for to understand the technique)? Is it something like connecting the left and right rings of an audio output jack to those points? In that case should the signal ground also be linked?

Does the fact that the upper half of the volume slider seems to have no effect on the volume, but the lower half may work as before(?) hint at any particular type of fault? This is why I thought it's the slider first. Overall the volume is low, but it may be only because it cannot go over 5. I do not remember how it sounded earlier, but I probably used the upper parts of the slider, which if not linear(?) would have quickly lead to noticeably higher volumes.

 

[jnm]

Sir jnm . . . . .

Considering that you are plugged into the units HEADPHONE jack for its outputtted audio.

Ditto on the checking of the bases of the to confirm that there is on onset of turning them on and a shunting of the audio signal to ground.

Although, realizing that you are experiencing the decline in audio on BOTH channels.
Remembering my past experience with an 'ole tyme circa 70's Tee Wee of mine, would you take a 220 ufd of the same voltage rating as C4 or C5 . . . . .closely observe its lead polarity. . . . and then shunt it across C4 or 5 and see what happens to the headphones volume ?


73"s de Edd . . . .

My doctor told me that jogging could add years to my life. He was exactly right ! — I feel ten years older already.

.

You mean I should touch the new capacitor's leads to the leads of corresponsing polarity of C4 or C5?
Since the volume is low on both phono and R/L outputs does it not indicate a problem earlier on the signal's path?

 

[Ylli]

The only thing that could affect both channels in the way you describe has to be something common in the audio section. It sounds like you are probably getting sufficient audio out of the DAC and into the volume control but the amplifier stage after that is limiting it's output. That translates to a bad +9 volt power supply or a defective IC3 *and* IC4.

Please measure and post the voltages on all 8 pins of both IC3 and IC4.

 

[jnm]

The only thing that could affect both channels in the way you describe has to be something common in the audio section. It sounds like you are probably getting sufficient audio out of the DAC and into the volume control but the amplifier stage after that is limiting it's output. That translates to a bad +9 volt power supply or a defective IC3 *and* IC4.

Please measure and post the voltages on all 8 pins of both IC3 and IC4.

Thanks, will do. Do the ~4.4 V measurements across R17 and R18 not rule out a bad 9V supply?

 

[Ylli]

Yes and no. You need to 9 volts to get Vcc/2, but would still kike to see an actual measurement of the +9 volt supply.

 

[jnm]

Yes and no. You need to 9 volts to get Vcc/2, but would still kike to see an actual measurement of the +9 volt supply.

Where would a good point of measurement be besides across R17+R18?

 

[Ylli]

IC3 pin 8 or IC4 pin 8.

 

[jnm]

IC3 pin 8 or IC4 pin 8.

Both ICs have 8.96V between pins 4 and 8.

 

[jnm]

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?

 

[jnm]

Anything else you think I should measure or try?

 

[Ylli]

Do you have another signal source of 1-2 volt p-p, such as the output of a cell phone or ipod that you could use? I would suggest you disconnect the CPU cable from the output board (CN5) and inject an external signal into the indicated points. Those points should be the inputs to the volume controls. See if the volume control exhibits the same 'flattening' as when providing the signal from the CPU. You will likely have to tweek the external source level to get things 'in range'. This test will confirm whether the issue is on the CPU board or the Output board.