This machine is an Assy No. 250407 Rev. B long board.
The machine would boot normally but there was no Serial Port access – it was not possible to load a game from diskette! My 64 Doctor diagnostic cartridge indicated a faulty CIA chip (MOS 6526) at U1.
As most chips on the board already had sockets, it was pretty fast to test U1 with a working one.
As that did not cure the problem I looked closer at the 7406 logic chip at U8. On this board a MOS 7707 chip was installed as a drop in replacement. This chip is known to have a high failure rate and I had previously identified it as being involved in issues with the Serial Port and non-responding diskette drives (link, link). So I replaced it…
…and that cured the machine cured…
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