I've got video but no sound

[spilo]

I just did a recap of an original GBA. I lifted the positive pad of CP3, wouldn't turn on. So I ran a jumper wire to pin 8 of U4, now I got power and video. I have no sound coming from the speaker. What could be the problem?

 

[FAST6191]

Don't normally see recaps done but I suppose it could have perks. Likewise without checking I am not sure of the fix and what it does, or if you might have accidentally bypassed something.

Did it work before the recap (never seen a GBA fail to boot from bad caps so presumably a way to reduce noise for said sound system, see it more on the GB/GBC side of things but rounding error difference in ages and same general hardware means I guess it is applicable)? Do headphones work?

Traditionally fails tend to be ham fisted reassembly (doubtful here but I note it anyway), volume control pot broken in some manner (usually replaced but there have been some madmen that opened up for a clean, bypassing with suitable resistor/jumper wire is an option in this), headphone port getting stuck/losing spring tension and it thinking the thing is in (less common than it is in some phones and laptops that have it unplugged/plugged a few dozen times a day but who knows what its past was like).

There have been some other things that are solved by reflows but there is no great evidence that it is a common fault. If you have an oscilloscope then that might be handy to trace things through or poke certain points. You might be able to get a normal multimeter with a decent AC setup or similar to read something with one of the music homebrews spitting out a suitable sine wave but scope is best.

 

[spilo]

When I plug in headphones I get normal sound and volume control through it. Also when headphones are plugged in I get sound through the speaker but it's very quiet volume adjustment works for it too.

 

[RAHelllord]

Check the speaker for any damage, and check the headphone jack for any damage. There's a small switch in the jack to switch between speaker and headphones that might have dirt in it, or is rusted, or similar. The speaker can also burn out occasionally, but I haven't heard of that happening with a GBA before.
You could unsolder the speaker and check for continuity and correct resistance (should be printed on the back), if those are given and to spec the wiring should be fine, and barring any tears or similar leave the headphone jack as likely culprit.

But also, couldn't hurt to put some contact cleaner into the volume wheel and give that a good scrub. Those are contact potentiometers and can get dirty fairly easily as well.

 

[spilo]

The weird thing is that when the headphones are plugged in I get sound through the headphones and the speaker together. It's just that the speaker is really quiet.