The Pictorial C64 Fault Guide

General tips - TTL (U8, U13-U16, U20, U25-U32)

With the smaller TTL type chips (the smaller ICs excluding RAM), the model makes a big difference. Normally they run cool and are very reliable, but the MOS versions of these chips are actually known to fail quite often. In case of a fault, check if your C64 has any smaller chips with "MOS" imprinted on them, and if so consider them suspect. Otherwise you may want to focus your testing on the larger chips and RAM first (unless the symptoms clearly point to one of the TTLs).

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