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LC575 No sound after re-cap

rickrob

Well-known member
I'm working on a LC575 board-- recapped it and reworked an overclock section. It had been running at 43Mhz for about ten years.

It's running at 33Mhz right now and there's still no sound after the recap.  I found a bad trace from C4 to U1 was well. Had to re-flow a leg on U1.

If I plug in a set of ear buds, I can occasionally and barely hear some Alert sounds when I play them in sound control panel. But mostly I hear noise from the disk and mouse when I move it around.

Does anyone have schematic or drawing of the sound circuit? I'm wondering if the old caps took out the sound chip.

 

techknight

Well-known member
you need to get a good close up of the sound IC. there is probably a broken trace on the IC from the cap leakage, I have seen it once. You also need to make sure your -12V and 12V is good from the power supply or it wont get sound either. 

 
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rickrob

Well-known member
you need to get a good close up of the sound IC. there is probably a broken trace on the IC from the cap leakage, I have seen it once. You also need to make sure your -12V and 12V is good from the power supply or it wont get sound either. 
The board is in a Color Classic-- I have a good LC575 board that works. 

I've been looking at this board with a bright light and magnifying visor, but can't see any broken traces.

I checked continuity on every trace I can see coming off the chip.

Have you ever run into a new, but bad tantalum cap?

 
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techknight

Well-known member
I have run into a lot of new but bad caps. But usually the film type that internally short when used in HV circuitry. 

a new bad tant? not really. 

 

rickrob

Well-known member
Well, I can't find any broken traces.  I wonder if the 343S0129-01 DFAC chip failed.  The sound was failing before the recap-- original symptom was static and noise from the hardware-- when there is disk access, etc...

 

Bolle

Well-known member
I recently got a CC logicboard for recapping which also suffered from non-working sound. It was only giving me some static as well and some heavily distorted sounds here and there.

It turned out the soundchip was indeed broken. Swapped a new chip in and sound was working again.

What is interesting is that only the audio portion of the chip seems to fail. If you remove the IC the Mac won't work anymore as the soundchip is managing the initial resets in the bootup sequence. With the chip that would not give me any audio output it would still boot so it still had to work partially. 

 

rickrob

Well-known member
I swapped out the chip with another one from a donor board..  No luck and the same symptoms.

 

rickrob

Well-known member
There are two TL084 op amps on the back of the board, in the same area of the sound chip-- Does anyone know if those are part of the sound circuitry? 

My guess is that they are-- I've seen these  IC's used in audio applications before.

 

rickrob

Well-known member
Is anyone familiar with the sound amplifier on this board after the DFAC, or have any other suggestions on what to check?

Thanks

 
I recently got a CC logicboard for recapping which also suffered from non-working sound. It was only giving me some static as well and some heavily distorted sounds here and there.

It turned out the soundchip was indeed broken. Swapped a new chip in and sound was working again.

What is interesting is that only the audio portion of the chip seems to fail. If you remove the IC the Mac won't work anymore as the soundchip is managing the initial resets in the bootup sequence. With the chip that would not give me any audio output it would still boot so it still had to work partially. 
Where did you get the new chip from?

thanks a lot :)

 

Chopsticks

Well-known member
check the ground plane is connected, the lc575 (and likely others) use a split ground plane for the audio and the logic on the board, I imagine they did that to minimise noise on the audio circuitry but you can just connect the audio ground to the main ground of the logic board. a quick test would be to connect the audio jack ground to the chassis and see if sound comes back

 
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