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.