Hardware GBA Sound doesn't turn fully off

[driftme]

I've been working on a GBA that came to me with no sound. I've replaced the volume wheel and now the sound doesn't go fully to zero. It's way more noticeable since I installed a CleanAmp Pro since it amplifies the sound, but even without the amp its apparent. There's no way to play silently without headphones.

I've checked the volume pot and it seems to be working as intended - 0 voltage when turned all the way off. C36 (on the Vref input to the amp-agb chip) has ~1.7v going in and ~0.7v going out. Not sure if that's normal but seems like the vol or vref input to that chip might be a good place to start.

Any thoughts or suggestions?

 

[FAST6191]

My first thought would have been the replacement pot was not getting as low as it should go (don't know what the range normally is supposed to be though to see if another thing in series wants to be there) but if it does indeed go to zero volts then even with an amplifier eeking out what it can it should still be off.
Next step I would look at some kind of induction if the sound for the amp is being tapped off somewhere and a wire is sitting over say the input trace to the pot. If the wires have any length still then see about moving things around and whether that makes a difference.
Failing that if you can install all that you can probably make a hard cutoff as well.

 

[driftme]

I thought about a hard cutoff too, which I guess would just be a switch on the speaker wires at this point.
There are no stray wires anywhere around it, with or without the extra amp.

This is a weird one since the pot is definitely working. Thats why I'm thinking it has to be something related to how the amp chip is reading the signals for the volume and/or vref (surely not the ground?). Wonder also if the volume pot is a red herring. I don't know of anything else that might be controlling the sound output besides the headphone jack cutoff but I would think that would have the opposite problem (no sound vs always sound)

I suppose the chip could be bad, I haven't replaced it - interesting fault if that's the case!

 

[FAST6191]

Would be a first that was not water damage or something for such a chip to fail, though this many years in I suppose we get to start seeing the fun stuff.
Pots failing is every other minute at this point and common as you like in general audio fixing.
Phantom sound is shorts (I presume you did not blob some solder somewhere choice, not sure what, if anything, would fail short to possibly send something another way), induction, or leakage (and I don't think flash carts have ever been accused of that -- buzzing sound certainly*).

* https://gbatemp.net/threads/gba-speaker-noise-fixed.450093/ might possibly help if only by providing a better reference.

As far as hard pass then I would probably abuse the headphone cutoff already there -- if it is just a connected couple of pins inside the port that cause a shut off then might as well do that.

 

[driftme]

I read through that thread the other day before posing. Definitely a good resource. I also put together this reference for me to look at while im at the bench (attached).

One post on that linked thread says " C39(or 38?) resistor affects the volume. higher ohm does louder volume and lower ohm does quieter. or the other way."
except c39 is a capacitor, right? wondering if I put a resistor after that if I can drop the vref voltage and get a zeroed out sound...

Wondering what I should be expecting to see as a signal to vref - if the volume input indeed is going down to zero, and there are no shorts, I feel like that's something to look into further.

Any great references for that AGB AMP chip and how each pin functions/what inputs it expects?

 

[FAST6191]

Personally I was delighted to discover
https://circuit-board.de/forum/index.php/Thread/13913-STRIP-CLUB-PCB-Scans/?pageNo=1
some months back and pushed things forward considerably over what was otherwise known before.

If you are coming to us more from things with chip recreations, actual service manuals, recreated, leaked or straight released then... yeah. The gigaleak stuff had some CAD files for some things but nothing of particular note here that I have seen, though I don't know people would have looked at it or considered it as much.

To that end I have nothing you could not get searching on the likes of https://octopart.com/ , https://www.findchips.com/ or the usual suspects for chip vendors or whatever the part vendors themselves have (or have in a similar series if doing custom chip but basically different pins and some different tolerances to make it cheaper).
If you had seen that thread before then if there was any discussion from the groups doing various DAC replacements, amps and whatnot on what could be relevant here. However even those are usually more for the "these resistors limit volume because French regs/EU regs/common sense says/my particularly forward thinking injury lawyer consultant says for earbuds, replace with lower resistance to blow those eardrums faster", "here is the output according to my oscilloscope so tap this point for input in a further amp stage", "here they skimped on a capacitor in this version/revision* and used a noisy line, they even left a nice pad unpopulated" rather than going for deep understanding a la some of the hifi stuff I have seen over the years or old school radio stuff. If there is a particularly notable failure case (see some of the gamegear and megadrive fixes and improvements) then that might get known as well.

*even a decent breakdown of differences between board revisions would be a gigantic leap. Can do OK for shell colours though -- https://gameboy.fandom.com/wiki/List_of_Game_Boy_System_Colors_and_Variations .

Can do you a very nice line in terms of internal hardware descriptions as understood by the software though http://belogic.com/gba/ . That sort of thing is what most console homebrew scenes seem to focus on.

Even reading that referenced post I am not entirely sure what is going on with that one. Could be any number of things, meanings, wrong terms (derp moment or otherwise).

 

[CaptionFerengi]

Did you check R14 ?

 

[Carsor]

So glad to see this thread isn't completely dead yet. Guess what - I have the very same issue. Great contributions here so far they helped me a lot doing some measuring on my board (it has a AMP AGB IR3R60N, in case there is variety). So thank you all so far!

I removed the pot completly but the sound still plays quietly. There is ~0.9V between GND and Pin 10 of AMP which I would have expected to be 0. Even if I lift the leg it shows it. So if there is any leakage may it come from one of the other 17 pins, but which? I don't want to desolder all of them.. (but maybe not the worst plan ) Before that I reflowed the whole AMP but that had no effect. The volume goes to max if I short VCC and Pin10 but that was to be expected.

R14 is ~4.7k as it should be.

I had a clean amp and dehum kit installed but removed it later. Maybe some capacitor got lost or displaced on that journey but I checked them as best as I could.

So long.

 

[Red2503]

What worked for me is bypassing R14 by soldering a wire between the right pin of the potentiometer (VR2-1) and GND. I am not sure if this will break the audio chip over time, but if it does, I will just try to get a new motherboard. For me it is better than almost never using my GBA because I cannot turn off the volume.

 

[riddancegd]

Hey guys, I'm having the exact same issue here. Did you get to solve it? :/