Troubleshooting a high pitched buzz from an AKAI AM-M459 amplifier

[michalism]

Hello.

I am currently troubleshooting a high pitch buzz that is audible from the speakers or headphones connected to the AKAI AM-M459 amplifier. The noise is audible either with a source connected or without. Please note that this is not a 60/120Hz hum. I tried to compare it with several frequency samples from Youtube, and it seems to be in the range of 4000-7000Hz. Please take this estimate with a grain of salt, I’m just trying to give you a rough idea.
So, I’m well into my troubleshooting effort, but before I start with specific questions about what I see, I have some fundamental queries I’d like to sort out before that. This will help me to better understand the basic stuff instead of just poking around the circuit by guessing.

Here I go:

1.   How “Direct Source” works? I suppose that the STK module is taken completely off the signal path, and the input is fed directly to the speakers with minimum processing. From the block diagram in the service manual, I see that SW306 is used for selecting Source On/ Off, but I can’t figure out how the STK module is “disabled”. Any reference to an educational article would be highly appreciated 😊
2.   Why there’s a 3V battery in the circuitry around IC302 (A2054 or a.k.a LC6543C)? My guess is that it is there to provide for a better stability on the VDD line (pin2), but this sounds a lame explanation to me… The battery is completely drained, but I still get 5V on pin2.

As for the troubleshooting, I am focusing around IC302, since I am getting some erroneous voltage readings there, and also IC102 because switching from one source to another is not as “clean” as I would expected. All measurements on the STK module where according to specs.

Please note that I have checked for caps that are out of spec, and replaced all those that I found to be out of spec, or where on a path that I considered “suspicious”.

Thank you in advance for any support you may provide, and I promise I’ll be back for more 😊
Stay safe and healthy!
Michalis

ps. The service manual is about 13Mb in size, so I cannot upload it here. I will be glad to provice a link to my google drive or other source if requested.

[mzacharias]

These types of problems are usually caused by either power supply or ground problems, the higher frequency notwithstanding.

Bad grounding at screws to chassis, oscillation at the output of voltage regulators due to high esr caps, etc.

[coromonadalix]

The battery must be to maintain some functions memory / clock settings / channels settings  etc ... since ic302 is a mcu

To check where your hiss / hum came from  do you have a signal tracer, a scope ??


The stk4152 as 3 pins for the muting operation

Normally the pin 6 i used for the muting function, the others muste be for a timed / anti pop  function

https://www.petervis.com/electronics/stk4152ii/stk4152ii.html

TR112 and TR113  are the tansistors (mute 1 and mute 2 signals)  who mute the audio input of the STK amplifier, and they go to TR305 TR306


And the hiss     is not always a ground problem, it can be a "self oscillating" circuit / feedback in the amp or preamp etc ... a bad feedback return, it depend how the circuits are made, sometime it can be a cold joint a bad capacitor, broken resistor ...

I had in the past a Sony Pro logic STRAV xxx, it was missing a resistor on a preamp pcb,  "yes a missing part", over a long time period a dc component was added on the output and triggered the speakers protection.

[bob91343]

Sometimes it's just hum.  But hum that is coupled very lightly so as not to pass the fundamental frequency but the noise.  I have heard such sounds and if you couple the power line to an oscilloscope via a high pass filter, say a 1 pF or smaller capacitor, you can see the noise from various machines or radio stations, etc.  It's also called 'open grid hum' from the same situation in tube circuits.  High impedance, high gain, very light coupling.

Alternatively it might be a parasitic oscillation.

[michalism]

Thank you for your responces!

These types of problems are usually caused by either power supply or ground problems, the higher frequency notwithstanding.

Bad grounding at screws to chassis, oscillation at the output of voltage regulators due to high esr caps, etc.

Indeed. I do understand this, this is why I checked all caps and replaced almost all of them. What I have not checked for is for oscilations at the power supply of the mcu. The power supply to the STK is clean, or at least my scope did not pick anything up. I will take a more carefull look at this.

The battery must be to maintain some functions memory / clock settings / channels settings  etc ... since ic302 is a mcu

To check where your hiss / hum came from  do you have a signal tracer, a scope ??

The stk4152 as 3 pins for the muting operation

Normally the pin 6 i used for the muting function, the others muste be for a timed / anti pop  function

https://www.petervis.com/electronics/stk4152ii/stk4152ii.html

TR112 and TR113  are the tansistors (mute 1 and mute 2 signals)  who mute the audio input of the STK amplifier, and they go to TR305 TR306


And the hiss     is not always a ground problem, it can be a "self oscillating" circuit / feedback in the amp or preamp etc ... a bad feedback return, it depend how the circuits are made, sometime it can be a cold joint a bad capacitor, broken resistor ...

I had in the past a Sony Pro logic STRAV xxx, it was missing a resistor on a preamp pcb,  "yes a missing part", over a long time period a dc component was added on the output and triggered the speakers protection.

There's no obvious setting that would require a memory function, this is why I am confused. Then on the other hand, since the source selection is done via the mcu, the battery may be used just to "remember" the last source used...
Thank for the article, I have also seen it, very usefull indeed! But I still don't get how "source direct" works
I do understand how muting works, but I assume this is to mute the output completely, not to "bypass" the STK...am I wrong?

Sometimes it's just hum.  But hum that is coupled very lightly so as not to pass the fundamental frequency but the noise.  I have heard such sounds and if you couple the power line to an oscilloscope via a high pass filter, say a 1 pF or smaller capacitor, you can see the noise from various machines or radio stations, etc.  It's also called 'open grid hum' from the same situation in tube circuits.  High impedance, high gain, very light coupling.

Alternatively it might be a parasitic oscillation.

I will try the trick with the osciloscope!


Thank you all once again!

[coromonadalix]

The muting at pin 6  must  "kill" the amplifier section in the stk module, thats what i think,  the other transistors are muting the input signal and other left right signal(s) in different section(s) of the pre-amp / amp  since you have mute 1 and 2 signals and one from the mcu too

[michalism]

Hi again.

I'm coming back to the topic with some new findings...

I used a small audio signal amplifier based on LM386 to trace this buzz; started from the output of the STK module going backwards towards the unit inputs.
I can go through the details if needed, but the bottom line is that I do get a very loud buzz on the A2054 mcu chip. The 5V Vcc is "clean", but probing on other pins, I get a loud buzz from the signal tracer.
I also suspected the LC7822 IC, but I do not get any significant trace there. Also in my tests, sometimes pin1 on mcu was buzzing, sometimes it did not, and to me this is an indication that the MCU is unstable.

Please note that the "noisy" pins on the mcu, also give incorrect voltage measurements.

Would I be correct to say that the fault is at the MCU? What else would I need to check to add some extra re-assurance to this? I have already checked the surrounding components and everything is within specs.