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Post by unit7 on May 17, 2016 7:58:31 GMT -6
I just began recapping my Harrison console. I bought a desoldering gun a year ago and, even though it's not a Hakko, it seems to work very fine except in a few cases. On a few pads the solder refuse to melt. When I try with my iron on high temp, it still won't melt until I add a bit of new solder, probably thanks to the flux in it. But, from what I can see, the new led won't really mix with the old and when trying to suck the old led it still won't melt and stays on the pad. When I look closely at those joints I think they look grey'ish and grainy, so I was thinking perhaps there has been some kind of chemical reaction after all the years?
Is this a known issue? Any good workarounds? Perhaps I should just raise the temp? I'm at approx 420C currently, which I though was high already..
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Post by svart on May 17, 2016 8:11:56 GMT -6
that's about 790F.. That's about right, since it's the temp of the tip of the iron. Once you touch the trace/pad/wire the tip temp will drop drastically unless you have a good constant-temp iron like a Metcal or JBC. Most Weller/Haako and off-brands will have temp settings but do a pretty poor job moderating heat as you work. They're fine for small stuff but for production/fast work they are avoided like the plague, for this reason. The better irons monitor temps and keep them constant no matter how much heatsinking happens.
Adding more solder is something I do regularly to get old solder to budge. You might need to let it sit and boil for a while before using the solder sucker.
Older solder oxidizes, especially if a cap has leaked out on it. Once it does, it can be hard to get it to melt and wick. Scratch it a little with the iron tip to get shiny metal exposed and then add rosin or new rosin-solder to it and it should start to melt.
I'm also guessing that the through-holes don't have thermal reliefs around them, and are wicking all your heat away.. In that case, the best you can do is keep heating.
Frequently what I'll do is use a desoldering unit on the back of the board and an iron on the top, heating from both sides at once to get enough heat on the joints where I have problems.
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Post by unit7 on May 17, 2016 10:35:01 GMT -6
Thanks a bunch svart! Despite having a noisy cap somewhere at a specific PCB that I finally managed to pinpoint, I still haven't seen any obvious leaking caps yet. But whatever the reason is for those stubborn joints (approx 1 out of 15) to not melt, I'll definitely go for your double assault method. I know it will work! |
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Post by Deleted on May 18, 2016 14:40:17 GMT -6
Something you can also try:
If you can not desolder the old solder fully with a desoldering pump, try using solder wick afterwards, just for the rest. It does work in a lot of cases IME.
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Post by Deleted on May 18, 2016 16:05:07 GMT -6
Btw.: Did you measure the actual capacity of the desoldered caps?
Is it far off or nominal value +/-15%?
Just curious...
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Post by unit7 on May 18, 2016 17:57:00 GMT -6
Btw.: Did you measure the actual capacity of the desoldered caps? Is it far off or nominal value +/-15%? Just curious... Actually I didn't. I'm doing the recap because I had a cap making a frying sound. We have a small user group forum for these boards, and when I asked about this one guy told me he had the same problem a while ago and recapped electrolytics on all channels and the board got lower noise floor and better sound. Because I had the noisy cap the other day I started to listen (really loud) to the noise coming from all channels separately. I noticed some are more noisy than others which I believe shows that some caps are beginning to die. The board is 20+ years old and I heard the previous owner had some issues with the cooling in the studio. These need a lot of cooling.. Anyway, I'll start recapping only a few channels with better quality caps (Panasonic FM and Elna Silmic II), compare original/recapped channels and then decide. Btw the board is 72 channels so I could easily remove 4 at a time for recap and still be able to use it. |
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