[Test-Equipment] HP410B - Resistor Board Solder Question

[Barry]

Working on refurbishing an HP-410B and am replacing the old carbon composition resistors with more modern, higher wattage resistors.  I unsoldered the first 4 or them, but had a really hard time getting the solder to melt and getting the leads out.

Is it possible this is silver-bearing solder?  It seems a bit harder to melt and doesn't quite flow the same as good ole 60/40.  If it is silver-bearing, I wonder if I need to stick with that (I don't have any of that)?  I know that on other devices (Tektronix scopes come to mind) that the material used on the connection itself is silver/silver-plated and needs silver-bearing solder; however, I assume the traces and/or through-hole ferrules are not silver on this board (could be, though).

Anyone else run into this?  I think I've replaced some of these on another 410B I had and don't remember running into this problem.

Thanks,
Barry - N4BUQ