KORG Piano Keyboard: Low Phones/Line Out help

[chinhang899]

Hey everyone!

My KORG iS50 has a volume issue where the internal speakers are only acceptable above ~50% on the master slider. What's worse is that the headphone (phones out) and line outputs are extremely quiet, even at max volume. Both left and right channels are equally affected.

Here's what I've checked:
Voltages: Vcc to DAC section measures ~5V, MUSB (mute pin on the DAC) measures ~4.xxv, and the 2 op-amps show ~9V
Master Volume Slider: Resistance ranges from single-digit Ω to ~11kΩ (seems functional).
Visual Inspection: Capacitors on the main/speaker boards look intact, no obvious damage.

Since the signal to the speaker amp taps from the headphone output circuit, I suspect the issue lies on the mainboard before this split?

Question:
What specific measurements or components should I check next to isolate the fault?


Any guidance on this would be greatly appreciated!

Thanks!

Attached: pic near the output area on the mainboard, speaker amp board and the schematic.

[CapLeaker]

Hmm… can you elaborate a bit more on how this happened? Did you loose volume slowly over time or was it an all of a sudden type deal? Are the left and right channels equally in volume? What happens if you feed audio into the input channel?
As for tools, I suggest you borrow an oscilloscope from a friend and then work you way backwards trough the amplifying stages.
All the left and right audio starts at IC30. I guess I would first establish if all voltages from various power rails are present and correct voltage reaches each IC involved in that audio chain. I see that each amplifier IC has each a series capacitor for their left and right channels. Now if these in series electrolytic capacitors go bad (high ESR) they will cut the volume down over time and you end up in low volume too.

[lotas]

Также проверьте, что происходит с сигналом «MUTE», возможно, транзистор протекает...

[stj]

c151 opamp damper,
or Q7 master MUTE control transistor are good places to start

[chinhang899]

Hmm… can you elaborate a bit more on how this happened? Did you loose volume slowly over time or was it an all of a sudden type deal? Are the left and right channels equally in volume? What happens if you feed audio into the input channel?

As for tools, I suggest you borrow an oscilloscope from a friend and then work you way backwards trough the amplifying stages.

All the left and right audio starts at IC30. I guess I would first establish if all voltages from various power rails are present and correct voltage reaches each IC involved in that audio chain. I see that each amplifier IC has each a series capacitor for their left and right channels. Now if these in series electrolytic capacitors go bad (high ESR) they will cut the volume down over time and you end up in low volume too.

Также проверьте, что происходит с сигналом «MUTE», возможно, транзистор протекает...

c151 opamp damper,

or Q7 master MUTE control transistor are good places to start

Thanks for the input! I picked up this keyboard at a flea market, so I'm not sure how or when this issue came up. This model (iS50) doesn't have an audio input—only the iS40 does—so those dotted lines in the schematic don't really apply to me. And yeah, both the left and right channels are definitely the same volume. I'll check the voltages again while I'm waiting for my ESR meter to arrive.
As for Q7 and the mute circuit: when I tested that transistor, it showed some weird values compared to Q1-Q6 (which are NPN), so I took it out to see what would happen. But the problem still happened, even without the mute circuit. I tested Q7 out of circuit and found it was normal (as PNP).

[CapLeaker]

With an oscilloscope problems should be easy to find, as you can see where it is amplifying or loosing gain.