IIfx Tantalum Tantrum + Sound Chip Damage?

[willmurray461]

I recently bought a IIfx. At the time, I thought I was fortunate because mine came with tantalum caps from the factory for everything except C9 and C24. I assumed that Apple was being smart and kept electrolytics on C9 and C24 because they were needed due to higher voltage resilience requirements or something, but used tantalums everywhere else where electrolytics weren't necessary.

Turns out Apple was not smart, and actually put a 16v rated tantalum at C1 on the 12v line. This capacitor promptly shorted itself, caught fire, and exploded. It also caused a 12v trace on the motherboard leading from the PSU connector to burn itself and snap.

Apparently this is a known issue. Had I researched beforehand, it could have been prevented. However, since I've already recapped dozens of boards I never bothered to consult any guides beforehand. Anyway, I repaired the damage and replaced the capacitor at C1 with an electrolytic this time.

The machine seems to start fine, but the audio (at least the boot chime) sounds a bit "crunchy" and distorted. Does anyone know if the sound chips connect to the 12v line? Could they have been damaged? Could anything else have been damaged? I was under the impression that only the hard disk used the 12v rail. I took a very quick glance at the schematics, but nothing was immediately obvious to me.

Either way, I can salvage some parts from a donor board if necessary. Does anyone know if the damaged chips would be the pair of 343s0045's, or the 344s0063?

 

[Byrd]

I'd look at the basics - check speaker for degradation and 3.5" audio out if reproducible with external speakers. There is generally no problem using capacitors of a higher voltage rating 16V on a 12V rail. Also clean around the audio chips with electronic cleaning solvent and any other corrosion/dust/suspect ICs.

 

[Bolle]

12V goes to both Sony chips, the Nubus slots, PDS slot and the harddrive power connector. The ASC/DSC is not involved with 12V at all.
It's unlikely that the sound chips get damaged with the 12V line shorted to ground.

 

[willmurray461]

I'd look at the basics - check speaker for degradation and 3.5" audio out if reproducible with external speakers. There is generally no problem using capacitors of a higher voltage rating 16V on a 12V rail. Also clean around the audio chips with electronic cleaning solvent and any other corrosion/dust/suspect ICs.

The thing is, with solid tantalum capacitors, manufacturers often recommend derating the voltage by 50%. While they'll technically work at up to the rated voltage, it can significantly decrease their lifespan. A 25v rated part probably would have been more appropriate.

I'll definitely try swapping the speaker and using the headphone jack though.

12V goes to both Sony chips, the Nubus slots, PDS slot and the harddrive power connector. The ASC/DSC is not involved with 12V at all.
It's unlikely that the sound chips get damaged with the 12V line shorted to ground.

I was thinking the same. Maybe a trace/power plane got extremely hot underneath one of the chips? I'll try testing the speaker and headphone jack first, and if they're both fine I'll swap the chips and see what happens.

 

[willmurray461]

I think it was just the speaker. I swapped it out with a different one and it sounds better. Part of me thinks the chime is still slightly off with the other speaker, but it could just be placebo.

Interestingly though, the motherboard wouldn't read any floppies even with known good disks+drives. I swapped out the SWIM chip and that seems to have solved the issue. This is the second time I've had a Macintosh with a dead SWIM chip. Are they known to be fragile?