If I was going to change a chip blindly I would start with x9305, the digital potentiometer. Let me explain to you how to test that potentiometer. First locate W9 on the board. It is a simple wire jumper, and it's the output of the potientiometer going to the attenuator. Set your meter on DC voltage and measure from ground to W9 , on any side since it's a wire. What do you measure ? With the volume turned down to 0 on my Fish Tales, I read 3.58V With the volume turned to 8 I read 2.99V . So, as you see the higher the voltage read on W9 , the lower the volume will output. Every increase of the volume counter should lower the voltage you read here, increasing the volume. Now if the value is around 3.6 you can barely hear anything. Everytime you increase the volume , the voltage on W9 should drop by 0.06 to 0.09 approx.
If this voltage doesn't change at all , you'll have to check the logic driving the chip. It does seem to change a bit at least, since you mention that at max volume you hear a bit of sound. So it's probably not the circuit driving this chip that is at fault. The chip itself could very well be at fault.
I included in the picture the logic of how the chip is driven. You can test these points with you logic probe. Pin 1 should be driven low when you press any button, volume up or down. Pin 2 should be HIGH when you increase volume and LOW when you decrease it. Pin 3 , Chip select, should be driven low when changes to the volume are happening and should return high when you are not changing the volume. *** If you were to test this and the logic didn't work properly, your next move would be to remove the chip completely (without breaking it) and to do the test again. A broken chip *could* pull the voltages either HIGH or LOW and give bad results. If the results are the same without the chip , then we have to go look further and test the chip sending those signals.. But that's not for now. I've numbered the pins of X9503 if you want to test with your probe.
Now , if this chip is suspected bad, you can test the amplifying circuit. If you look closely at the schematic, J507 can be used as an "optional volume control" and if you follow the wire you can see it leads straight to W9. What you need to do here is to disconnect one side of W9 to remove the x9503 out of the circuit, and solder either a potentiometer to the header pins of J507 or just a resistor. The number 503 in x9503 means its a 50K potentiometer. If you have a 50k pot available you just use a pin from any edge and the center pin. Solder the edge to pin 4 and the center pin to pin 2. I have never done this myself so I would be extremely careful since the amplifier can be pretty loud. I would make sure that the measured resistance between the 2 pins are at 50k before I solder it and that the 2 pins don't measure a very low resistance, because the amplifier would probably blast extremely loud and could damage the speakers I guess.. There might be threads on mods using that optional pot. and tips on what to do. For a temporary solution I would just take extra care. The higher the resistance the less current will pass through, the safer everything is.
Before blindly changing amplifying chips , you really should test that pot.
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