My IIfx's tantalum C1 spectacularly released its magic smoke

[tattar8]

I recently acquired a IIfx that was sold as totally dead.  When it arrived, first thing I did was remove the old batteries and swap in new ones (since this machine won't power on without them).  I then tried to turn it on.  Clearly the batteries were the cause of the "totally dead" issue, since it immediately powered on.  However, within seconds (so don't know if it chimed yet) C1 exploded with a bright flash and released its magic smoke; this board was already populated with tantalums.  I immediately yanked the power cord and shut the machine off.

 

According to the schematics, C1 is supposed to be between the +12V rail and ground, so unless I'm mistaken the only thing it could have affected is the (probably already dead) hard drive and the floppy drive.  I had already ordered a full set of replacement capacitors, but those haven't arrived yet.  In the meantime, I have 2 left over from the last time I did a recapping project.  Can I safely just swap the blown one and try to power up the machine again, or should I wait until the rest come and recap the whole board first?

[MrFahrenheit]

To be honest I would wait and recap the whole board. I had a similar issue with a IIfx I bought earlier this summer. Interesting that mine was also C1. I ended up soldering all new tantalum caps and for C1 I instead used the other solder pads. You may want to do the same. 

Edited October 13, 2020 by MrFahrenheit
Added photos

[tattar8]

In that case I'll wait; the caps will probably show up this week, so I don't save a lot of time by trying to boot the machine now.  I did desolder C1 (far more easily than I expected) and found that the pads underneath were pristine.

 

Could it be because C1 is the only one on the 12V rail?  All the others are on 5V I believe.

[MrFahrenheit]

16 hours ago, tattar8 said:

In that case I'll wait; the caps will probably show up this week, so I don't save a lot of time by trying to boot the machine now.  I did desolder C1 (far more easily than I expected) and found that the pads underneath were pristine.

 

Could it be because C1 is the only one on the 12V rail?  All the others are on 5V I believe.

I'm not experienced enough with the IIfx to know the answer.  That could very well be.  Both of my IIfx had issues with C1.

[LaPorta]

The other issue could have been that C1 was installed backwards and exploded.

[MrFahrenheit]

16 hours ago, LaPorta said:

The other issue could have been that C1 was installed backwards and exploded.

Possible but I’ve had two that failed at C1 that were factory caps. One was repaired by Apple at one point in the far far past.  I think this cap undergoes a lot of stress for some reason, but again I don’t have a lot of IIfx experience. I have two machines and both had similar issues. Both were original factory tantalum caps. 
 

I would say this is good evidence to recap any and all Macs including those with factory tantalum caps. You never know when they’ll fail. 

[LaPorta]

Was the voltage rating below spec? Having a larger-than-spec voltage rating would help.

[MrFahrenheit]

17 hours ago, LaPorta said:

Was the voltage rating below spec? Having a larger-than-spec voltage rating would help.

It's a 16v cap, and the rail is 12v, so I'd say it should be fine.

[LaPorta]

I was not sure what rating was placed in there previously is all. If there is some sort of overvoltage i guess thats what would be blowing it. Perhaps a short.

[tattar8]

I’ll check to see if there was over voltage on the 12v line; I should be able to power on the supply without the logic board if I just provide +5V from USB or something, right?

 

These were the factory tantalums, so they shouldn’t be under specced.

[saybur]

The old school advice on tantalums was to derate by 50%, so maybe this is just an instance of failure from running too close to the margins, even from the factory?  It might be worth going to 20 or 25V on the replacements.

[tattar8]

The Mac now boots. Swapped all the capacitors and she fired right up.  This machine came with a Supermac video card (looks like a Spectrum/8 Series III), but I had to swap in a Mac II video card since the Supermac was having trouble with the monitor I'm using.  The hard drive (looks like an aftermarket Quantum) is totally dead, which is no surprise, but I have a spare SCSI2SD to throw into it.

 

The bigger issue is the floppy drive, which seems to be in bad shape.  It needs to be cleaned and lubricated, which I can do, but what I'm not sure about is the fact that it makes a constant hum when the machine is powered on.  If you insert a disk it does not read, but the disk does spin.  Any idea how to fix that?  Or another spare drive I have that doesn't seem to be able to eject?

[MrFahrenheit]

On 10/14/2020 at 12:18 AM, saybur said:

The old school advice on tantalums was to derate by 50%, so maybe this is just an instance of failure from running too close to the margins, even from the factory?  It might be worth going to 20 or 25V on the replacements.

The problem with higher voltage 47uf tantalum caps is they have different resistance ratings and their frequency “wobble” is different. This could cause issues. 
 

The Quadra 610 uses 10x 47uf 16v caps and nothing else, so I had wanted to see how the higher rated caps would affect it. 

 

I bought some 47uf 20v and 25v caps to redo a Quadra and after recap I found that turning it on did work, but the machine overall didn’t. SCSI would not work, mouse did but cursor operation was about 1/4 normal speed. Flashing disk question mark was 4x slower than normal. I removed all of the higher rated caps and replaced with the standard 47uf 16v 900mohm 100khz caps and it booted right up just fine. 
 

I had replaced a few caps on one Mac with higher rated caps, leaving the rest the same, and found the same issue. The caps I had done on that one were for the SCSI bus, and I found SCSI to not work (the 25v caps I bought had 3ohm resistance I think). I believe this higher resistance affects some parts of the Mac differently than other parts. 
 

I do prefer tantalum caps but this is the reason buying them is not as easy as just placing an order. Tantalum caps have these issues more so than other types of caps, and matching up proper specs can be difficult. I have no idea what Apple used for their 47uf 16v tantalum caps (resistance, wobble, etc), and I would love to figure that out. 

[Elfen]

Scary, as I recapped the few electrolytics on my IIfx Board years ago.

 

I would take the hard drives out and test a new C1 without it, putting a voltmeter on the yellow and black wires on the hard drive harness (12V line). It could be that the Cap was bad, or it could be the PSU as well. Seeing too many Youtube Vids on old PC repairs where the Tantalum Cap went bad. Maybe they do go bad with age? Who knows...