Recapped my 840av

[beachycove]

I finally got around to recapping (the logic board of) my 840av today. It had been washed years ago in a dishwasher cycle, so I wasn’t especially paranoid about losing traces etc., but it had ceased to function and needed further TLC. Washing years ago did some good, because it then worked for some years, but then it gave up booting again.

Truth be told, there was fresh capacitor leakage in one or two places, and there was a fair amount of dried crud under many of those 47μF caps.... I’m glad I did the machine at last. 

Small  clearances for the soldering iron are a problem for working on the 840av, but much of the space limitation can be gotten around by cutting off the old caps where needed with pliers, which allows getting in with the iron once everything is removed. I also find it easier to solder in the new tantaliums than it is to remove the old electrolytics, for some reason, so space on reassembly is less of a problem, in my experience. That may be because I use one of those syringes with solder rather than wire. 

Anyway, the job is done now and it works great. Nice to see the old gal in action again.

On the whole, my 840av plastics haven’t gone brittle, by the way, nor has the machine yellowed. The only plastics lost in the entire machine is the little set of clips that hold down nubus cards at the rear. Everything else seems to be holding up pretty good.

 

[Microns]

Excellent to hear that your recap worked well and thanks for the heads-up about clearances.

I too have some 840av main boards that need recapping and your success gives me reason to get busy ;-)  

 

[0xABE]

I just replaced my first cap last night on a 660AV. Removing the old is harder than I expected. In theory you just heat the solder and remove it (I’m using wick) and the cap should be free, right? I found that much of each leg sits directly under the cap and its plastic base. I wound up carefully pulling the cap off it’s legs (this happened somewhat accidentally), carefully breaking the plastic base away to expose the legs and pads more, and finally heating and removing the solder to free the legs. Pads were fine.

Also, tinning the pads before putting the tantalum down was torturous because I always had a mound of solder with a pointy tip which made leveling, squaring, and setting the cap on both sides impossible.

That’s how the first one went anyways. 

 

[error1]

On a lot of the mac boards I've recapped there's a tiny dot of glue under each of the caps, probably to stop them from moving around during production before going into the reflow oven.

I've had some accidents with ripped pads trying to lift them off with hot tweezers, but now I've bought a hot air rework station after I saw someone use hot air to remove them in a youtube video with great results. It softens the glue and makes it really easy to lift the caps off, you just have to be careful not to melt other components in the vicinity of the caps.

 

[trag]

Also, tinning the pads before putting the tantalum down was torturous because I always had a mound of solder with a pointy tip which made leveling, squaring, and setting the cap on both sides impossible.

Don't tin both pads.  Lightly tin one pad per capacitor.   Set the capacitor in place.  Gently press down on the top of the capacitor with the eraser end of a pencil, or a flat screwdriver, or other blunt object, to hold it in place, while you work.   With your other hand, apply the soldering pencil to the tinned side.   The capacitor should sink into the solder down to the pad.

To get a good solder joint, you need to heat both the pad and the terminal on the capacitor to the melting point of the solder.   Don't just melt the solder and let it flow around.   Be sure to apply the pencil to the pad and the cap terminal as well.

After settling the cap on the soldered terminal, remove the soldering pencil.    Wait a few seconds.   Remove the object holding the cap down.   Now solder the other terminal the normal way.   Touch the pencil to the terminal and pad simultaneously, and apply solder.

 

[trag]

 now I've bought a hot air rework station

you just have to be careful not to melt other components in the vicinity of the caps.

I've found a block of modeling clay to be useful for this.    I cover nearby endangered components in a bit of clay to A)  keep them in place,   B ) protect them from heat.   Available at Hobby Lobby and similar.

 

[Fizzbinn]

Don't tin both pads.  Lightly tin one pad per capacitor.   Set the capacitor in place.  Gently press down on the top of the capacitor with the eraser end of a pencil, or a flat screwdriver, or other blunt object, to hold it in place, while you work.   With your other hand, apply the soldering pencil to the tinned side.   The capacitor should sink into the solder down to the pad.

To get a good solder joint, you need to heat both the pad and the terminal on the capacitor to the melting point of the solder.   Don't just melt the solder and let it flow around.   Be sure to apply the pencil to the pad and the cap terminal as well.

After settling the cap on the soldered terminal, remove the soldering pencil.    Wait a few seconds.   Remove the object holding the cap down.   Now solder the other terminal the normal way.   Touch the pencil to the terminal and pad simultaneously, and apply solder.

So I’ve already done two boards tinning both pads, and boy is this method so much easier! Ends up being so much neater compared to the others that do work but could in no way be mistaken for original.  thanks @trag (again). 

 

[Byrd]

Don't tin both pads.  Lightly tin one pad per capacitor.   Set the capacitor in place.  Gently press down on the top of the capacitor with the eraser end of a pencil, or a flat screwdriver, or other blunt object, to hold it in place, while you work.   With your other hand, apply the soldering pencil to the tinned side.   The capacitor should sink into the solder down to the pad.

To get a good solder joint, you need to heat both the pad and the terminal on the capacitor to the melting point of the solder.   Don't just melt the solder and let it flow around.   Be sure to apply the pencil to the pad and the cap terminal as well.

After settling the cap on the soldered terminal, remove the soldering pencil.    Wait a few seconds.   Remove the object holding the cap down.   Now solder the other terminal the normal way.   Touch the pencil to the terminal and pad simultaneously, and apply solder.

Trag, what do you find is the safest method to remove Mac caps?  I've been trialling with a rework hot air gun @ 360 deg C but finding this does results in pads lifting, but rarely.  Same goes with using wire cutters to cut off the old cap and remove remnants - the force required to clamp down on the old cap can result in some movement of the legs and pads and I tend to shy away from this method now.

 

[trag]

Trag, what do you find is the safest method to remove Mac caps?  I've been trialling with a rework hot air gun @ 360 deg C but finding this does results in pads lifting, but rarely.  Same goes with using wire cutters to cut off the old cap and remove remnants - the force required to clamp down on the old cap can result in some movement of the legs and pads and I tend to shy away from this method now.

There are many favorite methods out there but my personal favorite is still the two soldering pencil method.   I've gone from a pair of 15W pencils (which takes too long and encourages boring/prying) to using a pair of 35 - 45 watt pencils.

I liberally apply liquid rosin to both joints, then apply both pencils and wait for the capacitor to gently lift off.  This also seems to  quickly apply enough heat to melt any remaining glue point in the center.

Last year I de-capacitored a box of ~20   IIcx boards using this method without any lifted pads.   However, with all these different methods, skill/practive may play a part in avoiding lifted pads.     Certainly, patience does.

Regarding pencil power and the two pencil method.   If you're doing tiny surface mount resistors, like the work described in articles on Marc Schrier's Clock Chipping  Home Page, then a pair 15W pencils is good.   For these physically larger capacitors, which are often sitting on a trace with a short run to the ground or power planes, a pair of 35 - 45W pencils works much better.

Thinking about how I use the pencils during cap removal, I don't apply any downward or upward force with the pencils, but I do sometimes gently press inwards towards the center of the cap from both sides.    This helps it to lift off when the solder and glue are loose.

 

[trag]

So I’ve already done two boards tinning both pads, and boy is this method so much easier! Ends up being so much neater compared to the others that do work but could in no way be mistaken for original.  thanks @trag (again). 

You're welcome.  I'm glad it helped.

 

[Byrd]

There are many favorite methods out there but my personal favorite is still the two soldering pencil method.  

Thanks your tips, Jeff - it's a fine art and the more I can learn the better