The original C64 board is different to those found in later C64C and C64G machines. It is larger and contains a bit different set of chips. The ones that were troublesome in my C64s are:
- MOS VIC-II (PAL: 6569,71,72, NTSC: 6566,67), best to replace with another piece
- MOS 7708 – it is actually 74LS257 Quad 2-Data Selectors/Multiplexers, easy to get. What can be surprising is that this chip can pass tests in an (cheap) IC tester but fail to work properly at the desired frequencies. Example of the malformed signals produced by the faulty chip are shown below.
- MOS 906114 PLA – responsible for driving address chip select lines to various components. On pins 15-18 there must always be at most one pin in the low state at the same time. More than one pin in the low state means that more than one chip is selected for accessing the data lines and causes these lines states to be malformed.
- Modulator module – in the early C64 modulator is not only used for generating the TV signal, but it also participates in the forming of LUMA/CHROMA/COMPOSITE video signals. In later C64C and C64G these signals omitted the modulator. On my C64 LUMA signal was missing on the connector, it occurred re-soldering the modulator fixed the issue.
ROM chips can be replaced with more modern EEPROMs, consult this page for details on replacing and testing C64 ROMs.