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Reparing a Yamaha NP12

No sound on a Yamaha NP12

Introduction

The Yamaha NP12 piano is part of the Piaggero series. It uses a lightweight 61-key keyboard; the architecture is identical to the NP32 (76 keys). Fortunately, the service manual is available, as well as the datasheets for some key components.

Part 1 - Analysis

The symptom is obvious: no sound at all.

Going up the audio chain, it's easy to see that nothing is coming out of the stereo DAC, an AK4430ET from Asahi Kasei. The DAC-L and DAC-R test points remain at 0V.

As a first tentative to fix the issue, the DAC has been replaced, without success. I also replaced C363 and C364, capacitors serving as a charge pump for the DAC: no change.

Then I went into a deeper analysis, focusing mostly on the DAC

Power supply

All the DAC supply voltages are compliant with the datasheet:

On input:

  • VDD = CVDD = 3.3V
  • VSS1 = 2mV
  • VSS2 = 7mV
However, the following values are found at the output:
  • VREFH = 3.3V
  • VEE = 880mV (this value may be a bit high)

This gives a potential difference of 3.3 - 0.88 = 2.4V. If I understand the datasheet correctly, I would have expected a value between 1.85V and 2.15V, typically 2V.

Digital signals

I checked the digital input signals: they all seem normal.

The VH and VL values are 3.3V and 0V respectively, so they are compliant.

The clocks are:

  • MCLK = 11.3 MHz and LRCK = fsd = 44.2 KHz, which is consistent with the datasheet (256fs = double sampling speed).
  • BICK = 2.82 MHz which corresponds to 64 * fsd (double speed mode).

DIF is HIGH; the I2S protocol is being used.

The signal on SDTI follows the key presses on the keyboard; by observing the "communication density," you can clearly see the attack and release of the sound.

Since the SMUTE (Soft Mute) pin is used to disable the AOUTL and AOUTR outputs, it's logical to check the voltage at this pin; we find 146mV. This pin is driven by transistor TR361 (a DTC023J from ROHM). By shorting pins 2 and 3 of TR361, we lower SMUTE to 0V, but no improvement in behavior is observed; we can therefore consider the signal to be low (L), meaning the outputs are indeed enabled.

Next step

My guess is the new DAC could be a fake; the reputation of vendor is pretty low. So I decided to get a DAC from another source.

More to follow.

Part 2 - More analysis

Replacing the DAC a second time didn't solve the issue.

To make sure the issue is not generated by something after the DAC (preamp), I disconnected CN200 on board AM: no change. Therefore, after the DAC, each channel is followed by a RC low-pass filter with cutoff frequency around 31KHz.

I also replaced the capacitors C365 and C370; no change.

The remaining options are: a bad master clock signal, a wrong ROM.

The master clock signal appears "triangular" on my DSO, not clear "squares". However, it is 11.3MHz, which is a bit high for my DSO (bandwith 50MHz), so I used another DSO (Siglent 100MHz bandwidth):

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