Generally, adding full foldback to a spinet requires the addition of some missing key contacts, and perhaps the replacement of some non-full-length buss bars.
GeoffWilliamson has written a great page about this modification. See 
http://www.sympac.com.au/~retrojet/foldback.htm.
?KarlFrick has added foldback to his ?PortaB, and he took a nice set of pictures while he was doing so.
If you don't have the nichrome resistance wires, MikeFulk suggests using 16 ohm, 1/8 watt resistors (which fit on the end of the key contacts) instead.
BullfrogMusic keeps foldback kits in stock (for M, M2, M3, M100, L100 and PortaB).
Also see CategoryPartsSources.
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