I use a big (45 Watt) non-temp-controlled Weller iron with a big wedge tip to do tough desoldering (and less commonly, soldering) jobs like this. The high thermal capacity and heat transfer of that tip make all the difference.
If you don't have such a beast, then you can do quite well with pre-heating. Every degree hotter to make the joint/component prior to applying the iron is one degree less that the iron needs to heat it. Try heating the board with a hot air gun, even the paint-stripping hardware store kind, prior to desoldering. Or here's another idea: take a rectangular ceramic power resistor like 10 W, 10 ohm, strap it directly to the rectifier, and crank the power. After a while, the component will be very hot, meaning that it won't draw nearly as much heat away from the joint you are trying to melt.
I also agree with others that you will need some good quality lead based solder. Always generously apply fresh solder when melting old nasty joints. A eutectic 63/37 solder is a good choice as it has a lower melting point and solidifies instantly. Get a solder with
lots of
good flux.
Lots means a "66 core" ... this means that the flux core is 66% of the diameter of the solder wire, which is the most you can get. The only time you would want a lesser core is in production where you know for fact you can always get away with less, and you want to have less residue to deal with. On the bench, and especially if you are new-ish to soldering, more flux == better joints
, and you can just deal with residue if necessary. Kester 44 is a
good one, the flux is a highly active rosin ("RA") so it can cut through all kinds of old corrosion and wets easily to almost anything, but is still generally considered a no-clean type. When desired it cleans very easily with isopropyl. Of course there are
many opinions about flux and solder but it's really hard to argue against having a roll of Kester 44 in 63/37 on any bench. The only valid argument is that's it's a strictly RoHS compliant bench.
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