When repinning a connector, the wire will not tin or bond to a new terminal, even with plenty of flux. The old pins may be so corroded they have rotted away.

Old wire won't take solder: corrosion has crept up the copper

Cause: Corrosion has spread along the copper strands inside the insulation; dull, silver or dark copper will not take solder.

Checks

• Strip a little more insulation and look for bright copper.

Fix

• Cut the wire back until clean copper appears (or splice in a new length), then crimp or solder the new terminal.
• Lightly scraping oxidised strands can help on mild cases, but badly corroded wire should be replaced, because the corrosion keeps spreading.

Systems

• Gottlieb System 1

Related tips

• Soldering wires onto System 1 replacement connectors: tape the parts to the glass
• Pins and tools for re-pinning System 1 connectors

Source: Pinball Repair help group!! (Facebook group)