Commodore 64 with a faulty serial port. 7406 logic chip replaced.

Assy 250407 Rev. B #4

This machine is an Assy No. 250407 Rev. B long board.

Commodore 64 Serial port error. Repair job. breadbox64.com

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.

Commodore 64 Serial port error. Repair job. breadbox64.comAs most chips on the board already had sockets, it was pretty fast to test U1 with a working one.

Commodore 64 Serial port error. Repair job. breadbox64.com

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…Commodore 64 Serial port error. Repair job. breadbox64.com

…and that cured the machine cured…

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