Right speaker of Yamaha PSR-320 keyboard produces noise and crackling sounds

[tom_w]

Hi,

I have an old Yamaha PSR-320 keyboard that still works great. However, there's noise in the right channel. At low volume, you can barely hear it, but when you turn up the volume, the noise gets louder as well and becomes very annoying (s. the attached example). The noise is not just in the speaker, it's also in the headphone output, again, only in the right channel. It's always there, with every keyboard voice and also while you don't play at all, and it also remains ongoing with the same volume when you change the "pan" setting so that the sound comes only from the left speaker. To me, it sounds like white noise. It definitely is no power line hum.
When you turn up the volume to very high levels, you sometimes get a crackling sound when you play a loud note (last chord in the example). The crackling is only on the right side, too. It never happens when the volume is at mid-level, even though you can already hear the noise at that level.
The keyboard already had this problem when I first got it many years ago, so I don't know if something broke or if there was a manufacturing defect.
I recently found the service manual for the keyboard and I thought I could try to finally fix the problem. However, I don't really know enough to be able to make an informed guess what the problem might be.
I did open the keyboard, but the only thing I found out is that when I apply pressure to the area pictured below, the noise sometimes gets quieter and the crackling vanishes completely. (The noise never vanishes completely, it only gets quieter.) However, I could not pinpoint any exact location or component where I could reproduce this effect. It even worked sometimes when I pressed on the bottom side of the board (in the area pictured below).
According to the service manual, that area contains among other things an op amp for the right speaker (IC404, UPC4570HA).
If anyone has an idea what component I might have to replace, I'd be very grateful. I'm emotionally attached to that keyboard, as it was my very first one.

- Tom

I could not attach the mp3 directly, so I added it as a zip file. It's just a mono recording made with the smartphone microphone which was placed above the right speaker of the keyboard.

 

[Harald Kapp]

The opamp may be o.k., otherwise you'd have no tone or a very distorted tone. You can try to swap the opamp, the type you note is available online.
But first inspect the solder joints on the underside of the board. Also check the electrolytic capacitors (the little black cylinders) if you can (de-solder, check with a capacitance meter, if available).
Do you have access to an oscilloscope? In that case inspect the supply voltages for noise.

 

[tom_w]

Thank you very much! Usually, I could certainly borrow an oscilloscope at work, but I'm currently forced to work from home due to the pandemic, so it might be a little bit more complicated. I will get back to you when I have it.

 

[tom_w]

I finally got an oscilloscope, but they gave me a cheap one that's not suited for the task. At low voltages, its resolution is not fine enough, so I cannot discern a difference between the noisy channel and the other one. (It's possible after the signal went through the power amp, but I already know that the signal coming out of the power amp contains noise.)

So instead, I tried to find out where the signal gets noisy by simply shorting the left and right channel at different points and checking whether the noise stays on the right channel only or moves to both channels.

Here's the relevant part of the circuit diagram. The DAC outputs a negative current on OUTR and OUTL, which is then converted to a voltage by a two-channel op amp:




When I short the non-inverting inputs for both channels in the op amp like so,



the noise moves to both channels. My (uninformed) conjecture is that this shows that it's not the DAC output current for the right channel that's noisy. There's no reason why the noise should then also be in the left channel only because I use the same reference voltage on the non-inverting input for both channels.

So to me it looks like this reference voltage for the right channel is noisy. It originally comes from the COMR pin of the DAC, so I shorted COMR and COML as shown here:



This time, the noise stays in the right channel only, so the voltage at the DAC seems to be ok. I therefore believe the reason for the noise must be one of R403, R409 or C411.

Since I only know very little about electronics, I really have no idea if my analysis makes any sense. Would it be a good idea to swap these elements?

 

[Harald Kapp]

Actually you're not shortening out the signal paths.
The opamps in IC403 are used in inverting configuration. The signal paths are:
DAC pin 6 -> IC403 pin 3 for the right channel
and
DAC pin 4 ->IC403 pin 7 for the left channel.

By connecting pins 4 and 6 of IC403 you connect the reference inputs of the opamps. Anyway, your analysis seems to point in the right direction. Check the solder joints on the suspect components first, maybe re-solder them by applying a tad of solder and a soldering iron to reflow the connections. If that doesn't help, swap components to see whether the fault moves with the componnet:
R403 <> R404
R409 <> R410
C411 <> C412
Swap only one pair of components at a time and test in between. Otherwise you will not know which component is faulty.