Soldering

I'm going to assume that anyone reading this website has a pretty good grasp of soldering. That said, if just one of the things listed here makes someone go "Hey, I didn't know that!" then the whole thing will be worth it. I've been soldering stuff for a long time¹.

Copied from Wikipedia:

Common solder formulations based on tin and lead are listed below.
The fraction represent percentage of tin first, then lead, totaling 100%:

63/37: melts at 183 °C (361 °F) (eutectic: the only mixture that melts at a point, instead of over a range)
60/40: melts between 183–190 °C (361–374 °F)
50/50: melts between 183–215 °C (361–419 °F)

The 1st one is commonly called "Sn63" and it is best because it has no "plastic stage." It's either liquid or solid. The semi-liquid stage of other alloys will cause cold solder joints if the wires being soldered move even a little bit during the cooling period.

The SX-28s were made with globs of Sn60 and the wires were thoroughly wrapped around the terminals to prevent cold solder joints.

Here's some tips:

Heat rises. Put your iron under the joint if possible.

Get your soldering iron tip wet with just a touch of new solder to help pass heat from the iron to the joint.

Here's a technique that helps prevent damage to an 80 year old terminal. First, suck off as much solder as possible. For this piece, I just held it in a pair of needle nose pliers, got it good and melted, and flicked the loose stuff onto the bench. For terminals still in the radio, you'll need a solder sucker or wicking wire. The 1st pic shows the wires wrapped around the terminal. The 2nd pic shows me cutting off the loops around the outside of the terminal. Warning: the little bits really go flying. Guard your eyes. Be careful cutting the loops; you don't want to get part of the terminal too. The 3rd pic shows the terminal ready to be reheated again. Most of the wire pieces will just fall out now.

When working on a tube pin, remove the tube first. The tube's internal wires are soldered to the pins and if you inadvertently melt that solder, you could cause the solder to drain back into the tube and cause an open. This would be very difficult to detect or repair.

Footnotes:

I spent 2 weeks in NASA Solder School back in the 70s. Rockets are expensive and as they put it at the time, "We don't need to be spending $10,000 to put an extra pound of solder into space." Also "Service calls to fix a bad solder joint are even more expensive."

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Last modified: Friday, 24 Apr 2026