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Replacing through-hole electrolytic capacitor from top of board

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R_C

Member
I have been able to revive some 20+ year old marine auto pilot components by replacing bad electrolytic capacitors. Some are through-hole and some are smd. One control head that was dead now works after replacing most of the caps, although it takes some time and struggles to start. I suspect the three remaining caps might be the problem. But an LCD that I can't remove covers the back of the board where these through-hole radial electrolytic caps are soldered so I can't test them with an ESR meter or easily remove them. Is there a good way to remove these caps without damaging the board? One that had leaked easily popped one lead when wiggled and the cap lifted right off the other lead. I then made solder pads for the new cap. I'd like to do the same for the others.
 

ronsimpson

Well-Known Member
Most Helpful Member
When replacing parts, it is common practice to destroy the old part getting it out.
I have cut parts into pieces. Crushed them. Do anything to get them out.
It would be nice to know it the part was really bad but.....
 

R_C

Member
Thanks. That's what I thought but I wanted to make sure there wasn't some simple elegant trick.
 

AnalogKid

Well-Known Member
Most Helpful Member
If the cap is not flush with the surface of the board you might be able to flex it back and forth until the leads break above the surface of the board. Or Rock side to side to loosen the leads internally and then pull it of. Other than that I agree with Ron, go in swinging.

ak
 
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R_C

Member
I think the through-holes will be the easier problem. I'm finding the smd caps are the real challenge. I have lifted a few pads off the board while desoldering the cap so now I need to find repair supplies. Any idea where I can find copper foil and epoxy to rebuild the pads? I have only found pcb repair kits running over $200.
 

R_C

Member
Good point. Fortunately, I have more than one of each board so I can look at another for reference.
 

Mosaic

Well-Known Member
Crushing electrolytics to get them out...is gonna spread electrolyte around and corrode the PCB. Worse...it's going to migrate thru vias to the other side of the board and continue corrosion there.

If u must do this, I'd suggest application of a good coating of Deoxit S5 shield spray on the PCB first.
Also plug all nearby vias with petroleum jelly.
 

Blueteeth

Well-Known Member
Desoldering SMD caps can be an arse. Smaller 0603/0805's are relatively easy - they are so small you can usually heat one side with a little extra solder, and flux on the board and the flux carries the heat to the other side, slide the SMD cap sideways to try to avoid lifting pads. Tantalum SMD's, the larger ones are a headache. My advice is to buy a 'cheap' hot-air station off ebay. They don't control airflow well, but are temperature controlled and make rework a breeze. I have removed SMD caps from laptop boards by preheating the powerplane it is on which gives more time to heat one side of it, and quickly switch to the next, but its a nasty way of doing it.

One more thing, at a push, using solder-braid to transfer heat to both sides at the same time can work.

This of course is if you want to minimise damage to the board *and* the part. As others have said, it is often better to sacrifice one - with the board if you want to salvage a component, or the component if it is to be replaced.
 

DerStrom8

Super Moderator
How do you plan to solder in a new TH electrolytic if you don't have access to the back of the board? :confused:
 

R_C

Member
How do you plan to solder in a new TH electrolytic if you don't have access to the back of the board
When the leads broke flush with the board, I cleaned the area, added flux, then made a solder pad. I then soldered the new cap to these solder pads. Not perfect but it has worked. If some of the lead remains I solder the new lead to the old. Three old caps remain and I am leaving them for now. They show no signs of leakage and using new needle probes for my Blue ESR I have been able to test them behind the LCD and they appear ok.
 
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