• Updated 2023-07-12: Hello, Guest! Welcome back, and be sure to check out this follow-up post about our outage a week or so ago.

Recapageddon: my 47-cap mission to restore a SuperMac S900

CircuitBored

Well-known member
Replacing dodgy old SMT capacitors is fast becoming one of the hallmarks of this hobby. Every year people are discovering new machines that have bad caps, whether they've leaked or simply fallen out of spec. I've even heard rumblings that G4s are starting to need new caps. To this end I decided that at 25 years old my SuperMac S900 was probably in need of a recap. Since I replaced the power supply in this machine it has been much more reliable but I was still getting errors when opening certain applications and control panels. As well as that, the S900 will often not boot first try and requires the power to be cycled once before it will actually boot. I was suspicious of this, hypothesizing that it could be evidence of flakey caps that don't work well when cold. There was no sign of leakage but that, of course, does not mean that all is well. Anyway, on to the task at hand.

The first thing you must know about the S900 is that it has a lot of electrolytic caps. In total there are 47. Here are some close up shots showing their locations.


boGKvEF.jpeg
qi3GB0Q.jpeg


gQhMQd4.jpeg
X2RFAfr.jpeg


If you've ever recapped a computer before then these pictures will likely have made you cringe. Look at all those caps buried in between slots, chips and other vital (largely irreplaceable) components. So much scope for untold damage caused by just the slightest bad soldering maneouvre. No matter how good at soldering you are this is undeniably a nightmare board.

Here's a breakdown of the caps with the parts I used to replace them in parentheses:

3 x 100uF 10V (TAJD107K010R)
7 x 10uF 16V (1206YC105K)
37 x 47uF 16V (TAJD476K016R)

My first obstacle was figuring out the safest way to remove the caps from this extremely rare board. We've already established that this is not a board where you can go fumbling around with a soldering iron. There simply isn't room. In a similar vein, most of the caps are so close to plastic parts that hot air would undoubtedly cause trouble too. With this in mind I settled on the unthinkable and decided to remove the caps the "low tech" way. I just twisted them off with pliers.

That said, I didn't go at it like a caveman. Holding the plastic base of the caps firmly with tweezers I very gently twisted each cap back and forth three or four times with pliers until they fell off. I kept a close eye on the pads to make sure nothing was moving that I didn't want to, and my ears were listening out for any unwanted cracking or snapping sounds. I did not apply any upward force at all, I just let work hardening do all the... work. Even using this method I ran up against trouble with physical clearance. I'm saying it again: this board really gives you almost no room to work with.

With the cans removed from the caps I was left with lots of plastic bases left over. To remove them I used an improvised paperclip tool to hold the plastic down while I snapped it off with pliers. I worked this way to mitigate the strain on the pads.

Following on from here I covered everything in the vicinity of where I'd be soldering with kapton tape and started cleaning the pads. This was the hardest part of all. The kapton tape doesn't insulate much of the heat of the soldering iron at all but it gives you a split second longer to react if you accidentally touch a plastic part. Thanks to a steady hand I was able to avoid any damage but I did have two close calls where the kapton saved me.

Caps C312 and C313 had both leaked but luckily the pads were salveagable. C313 was particularly bad and was just starting to lift off the board.

e0D6xxW.jpeg


Here is our completely decapped and cleaned board, ready to receive its lovely new tantalums.

Since this project is close to the electronic equivalent of keyhole surgery I decided to invest in a neat little gadget to help me attach the new caps. These Hakko FX-8804 soldering tweezers were a godsend. Using this tool you simply place the cap on the pad with some flux, give it a pinch and voila! It's attached uniformly!

DymokgX.jpeg


Once I got past the initial learning curve of essentially using two soldering irons at once I was installing caps at a rate of several per minute. Soldering tweezers get very mixed reviews but I really cannot sing their praises highly enough. I don't think I'll ever use anything else to fit SMT caps ever again. For removing SMT electrolytic caps they are almost useless but for everything else I have tried they are a game changer. In my opinion it is best to rely on hot air desoldering as little as you can.

After about an hour of soldering and testing connections the board was looking fantastic.

u7lyn3T.jpeg


Look at all those little yellow blocks of joy!

I rebuilt the system and apprehensively hit the power button.

hvMxsMA.jpeg


Success! After working through the normal S900 PRAM-reset panic mode it booted right up and worked perfectly! Mac OS 9.2.2 is now running better than ever before and the reproducible errors from pre-recap are gone (touch wood).

Sadly the issue of video not initialising on a cold boot remains. I now suspect that this is a Radeon 7000 issue rather than an S900 issue. For one reason or another the R7000 just doesn't seem to like the S900 very much. However, it could just be a case of "egrets, egrets". Next up, recapping a Radeon 7000?

And that's that! Finally I can stop fretting about this rare machine meeting a tragic end. With the power supply replaced and the board recapped I have just about exhausted the preventative measures I can take to make sure this machine lives as long a life as possible.

If you'd like to read more about my long journey with this wonderful, old oddity then there is a thread about it here.

For information on how to replace the power supply in your S900 I have written a guide here.

I hope this write-up is of use to my fellow S900 lovers - thanks for reading!
 

desertrout

Well-known member
Nice! I was audibly groaning looking at the first set of photos seeing so many (most?) caps in such close proximity to slots, but those tweezers sound like the perfect solution. A note on removing left over plastic bases... I find using a scalpel works great, gently pressing down usually cuts / snaps the base away from the legs (they're usually so brittle), no risky lifting motion required.

Thanks for sharing this. Congrats!
 

CircuitBored

Well-known member
A note on removing left over plastic bases... I find using a scalpel works great,

This is a great tip that I will definitely try out in the future - thanks!

I was audibly groaning looking at the first set of photos

This has been my response every time I've gone to recap this machine over the last month or so... until now! It truly was a tough job but I must say that thinking through my tool choices carefully made a big difference.
 

LaPorta

Well-known member
Everyone has different techniques. When I twist caps off, i purposely avoid the base until the end, because I feel the base can put torque on the leads, pulling on the pads. Who knows what really happens.
 

CircuitBored

Well-known member
Everyone has different techniques. When I twist caps off, i purposely avoid the base until the end, because I feel the base can put torque on the leads, pulling on the pads. Who knows what really happens.

I hold the base steady with tweezers so as to avoid twisting the pads. If the base is moving, you're doing it wrong!
 

Unknown_K

Well-known member
I am looking at getting a cheap Asian knockoff of that soldering tweezer. Thye are around $50 or so on ebay (model 938d).
 

desertrout

Well-known member
I hold the base steady with tweezers so as to avoid twisting the pads.
i purposely avoid the base until the end, because I feel the base can put torque on the leads, pulling on the pads.
That's very sensible... and not something I actually thought about. I've always just grasped the cylinder, not really thinking about the base moving... I'll have to start doing this too.
 
Top