[Coco] Sort of OT: Recapping

[Dave Philipsen]

If the board was designed with the intention that the ground planes would sink heat away from the electrolytic capacitors then it should have also been designed with additional heat sinking or cooling around the actual body of the capacitor. Relying solely upon the ground plane for that would probably be a design mistake. 

-Dave Philipsen

> On Nov 27, 2020, at 4:06 PM, Gene Heskett <gheskett at shentel.net> wrote:
> 
> On Friday 27 November 2020 16:09:44 Salvador Garcia via Coco wrote:
> 
>> I am trying to recap a motherboard which is not from a CoCo, but I
>> thought this subject might be of interest to the CoCo community since
>> recapping seems to be a fairly common occurrence in the vintage
>> computer community.
>> 
>> I noticed three bad caps on the MB. I have done recapping before, so
>> no big deal, or so I thought. My soldering irons, 25, 30 watts, would
>> not melt the MB's solder. I got a Weller 1010 (70W) and this helped. I
>> managed to remove the caps, but I still had a hard time melting the
>> solder and two of them came apart and pieces of the pins (3 to be
>> exact) are still in the through hole. Suggestions included applying
>> clean solder and flux which I did. The clean solder helped, but the
>> flux only left a dark stain on the PCB, so I discontinued its use. I
>> can't melt the solder from three of the through holes where the pins
>> are located. I've provided links to pictures. One of the cap's pins
>> can clearly be seen sticking out of the MB for the cap in front of the
>> choke (or inductor, not sure). The fat cap behind the choke is the one
>> that I managed to successfully replace. I applied heat to the solder
>> pad and tried to push the stuck pin with a fabric pin (the kind used
>> in new clothing to hold the folds), was unsuccessful. I thought about
>> grasping the protruding pin on the solder side, but did not because I
>> figured that the pliers used would dissipate heat and make it harder
>> for the solder to melt. In one case, I ended up using a very fine hand
>> drill bit to remove the last remnants of solder from the through hole.
>> This won't work where the cap's pins are still stuck in the through
>> hole. At this point I've used up my options (and patience). Does
>> anyone have any recommendations on how to get the stuck pins out?
>> Hopefully this discussion will be fruitful to other recapping their
>> CoCos and other computers. Links to pics follow, thanks much, Salvador
>> 
>> Caps. I am replacing three caps. The fat one in the back (behind the
>> choke or inductor), which is already replaced, the smaller one in
>> front of the choke, and the front left. The MB did not have a cap at
>> the front right location. Note the cap's pin still soldered to the MB
>> (in front of
>> choke).https://drive.google.com/file/d/1heHQ3TfpMVcubUvvcUYxxFCOvEn9wo
>> Kl/view Solder side, The dark spots are from the flux that I applied to
>> determine if this would help to melt the
>> solder.https://drive.google.com/file/d/1bLeOGqPTrGhhm3Xiyt7DGKq3Av_4BZ
>> 9O/view
> 
> That is a hot air rework station job, and you will likely need to heat at 
> 375 to 400C in order to get in, do it and get back out before charring 
> the board. The problem is that in many cases, the board was used as a 
> heat sink to help cool the capacitors when the ripple current, 
> particularly in switch mode supplies runs to many amps. IBM boards are 
> some of the worst offenders, in that I charred the board getting the 
> caps out because they had a large expanse of heavier copper fanning out 
> on both sides of the board. Since it was my board, I said its junk and 
> bought another board.
> 
> Cheers, Gene Heskett
> -- 
> "There are four boxes to be used in defense of liberty:
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> -Ed Howdershelt (Author)
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> 
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