Thanks for all the help! I took apart the irons and cleaned them both with steel wool. I then tried some silver solder and flux I got from Tower, and amazingly, got 6 good solder joints in a row! I then let the iron cool, re-heated it, and made 2 more good joints. Since I had already tried the cleaning once before and had been using flat tips all along, I can only assume that the solder or paste flux I had been using were bad in some way. Both are 5-6 years old and the solder is thicker (about 1/16"). They are both in the trash - I'll stick with what works! Thanks again.
Just to know: Silver solder is more "corrosive" on the old solder tips (the ones that are not specifically made for it) and you will have to replace them sooner (you'll see it).
My apologies for reading this thread a little late..
Use electrical solder ( i.e Kester 44 rosin core) and ONLY electrical flux ( usuallly a thin sticky liquid - made from essentially isopropanol & rosin) for electrical circuits. You shouldn't need flux anyway..it's typically used for surface mount prep.
First make sure your (new) tip is clean, heat it up & add some solder to it so it's shiny/dripping.
Have a wet sponge or rag around to wipe the tip ocassionally ( it only needs a stroke or 2 across the sponge).
There's often a "sweet spot" on the tip, try to use that for soldering.
Add a tiny blob of solder to the tip just prior to soldering, it helps the heat transfer to the object faster.
Make sure the object you are soldering is clean, with no grease/oil/oxides on it. A light going over with clean steel wool or sandpaper often works.
When you're done soldering, wipe the tip again and add a fresh "coat" of solder before turning off the iron, it helps protect the tip.
Kester Ultrapure tip tinner is also available for making the tips shiny again, but I use it very sparingly, as it tends to eat the metal plating off the tips.
Acid paste flux & plumbing solder are a definate no-no on electrical circuits.
An electrical joint made with acid flux may work today, but it'll invariably cause problems in the future.
I only use silver solder for mechanical, not electrical connections.
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