Great Hierophant
Veteran Member
I would nominate an IBM PC/XT as the most repairable PC I have come across. Other close seconds are the IBM PC 5150 and the Apple II/II+ series.
The IBM PC/XT's motherboard consists solely of off-the-shelf parts. The RAM is standard 256Kx1 and 64Kx1 modules, the glue logic has no custom chips or PALs, almost entirely 74LS series based. The important chips, Intel 8088, 8087, 8284, 8288, 8255, 8237 and 8259 are not custom and were manufactured by a variety of vendors. The ROM sockets are compatible with standard 27256 EPROMs, and those are pretty common. The basic IBM expansion boards, including the floppy, serial, parallel, game, CGA & MDA also contain completely off-the-shelf parts and the important chips, NEC 765/8272, MC6845, 8250B were all available from multiple manufacturers. The same cannot be said for IBM's hard drive controllers, its AT and XT/286s (use PALs), its Jr. (has a custom video gate array), or its later devices. The XT beats the PC and the Apple II in that it uses a standard ROM/EPROM socket while the latter machines use an earlier-style ROM socket.
Additionally, the XT is easier to use with keyboards. The IBM Model M's (true IBM manufacture and probably made in the millions, not hundreds of thousands) will work reliably with a late BIOS XT while they are questionable in a PC. Proper XT or XT/AT switchable keyboards will work in both PC & XT, its just that the XT has more replacement options available. The Apple II/II+s use really old ASCII keyboards that are hard to find these days in all-key working condition.
Additionally, you could put an XT motherboard into any AT case without difficulty. The PC motherboard has non-standard slot spacing, so some of its slots may be obstructed in a 3rd party case.
On the other hand, all the chips in an Apple II/II+ are socketed. And except for the CPU, they are all standard 74 series chips with the exception of a very few oddballs. The chips on Apple's most basic and common cards are almost always socketed (Language, Super Serial, Parallel, Disk II) as are many 3rd party cards. Good luck trying to get an Apple II motherboard to fit inside another chassis unless you designed it.
As far as other systems go, each is more proprietary. Atari's 8-bits used proprietary GTIA, ANTIC and POKEY chips, and later models even more custom logic. The Commodore 64 uses a custom PLA and VIC-II and SID chips that are not quite proprietary, but not especially common either.
The IBM PC/XT's motherboard consists solely of off-the-shelf parts. The RAM is standard 256Kx1 and 64Kx1 modules, the glue logic has no custom chips or PALs, almost entirely 74LS series based. The important chips, Intel 8088, 8087, 8284, 8288, 8255, 8237 and 8259 are not custom and were manufactured by a variety of vendors. The ROM sockets are compatible with standard 27256 EPROMs, and those are pretty common. The basic IBM expansion boards, including the floppy, serial, parallel, game, CGA & MDA also contain completely off-the-shelf parts and the important chips, NEC 765/8272, MC6845, 8250B were all available from multiple manufacturers. The same cannot be said for IBM's hard drive controllers, its AT and XT/286s (use PALs), its Jr. (has a custom video gate array), or its later devices. The XT beats the PC and the Apple II in that it uses a standard ROM/EPROM socket while the latter machines use an earlier-style ROM socket.
Additionally, the XT is easier to use with keyboards. The IBM Model M's (true IBM manufacture and probably made in the millions, not hundreds of thousands) will work reliably with a late BIOS XT while they are questionable in a PC. Proper XT or XT/AT switchable keyboards will work in both PC & XT, its just that the XT has more replacement options available. The Apple II/II+s use really old ASCII keyboards that are hard to find these days in all-key working condition.
Additionally, you could put an XT motherboard into any AT case without difficulty. The PC motherboard has non-standard slot spacing, so some of its slots may be obstructed in a 3rd party case.
On the other hand, all the chips in an Apple II/II+ are socketed. And except for the CPU, they are all standard 74 series chips with the exception of a very few oddballs. The chips on Apple's most basic and common cards are almost always socketed (Language, Super Serial, Parallel, Disk II) as are many 3rd party cards. Good luck trying to get an Apple II motherboard to fit inside another chassis unless you designed it.
As far as other systems go, each is more proprietary. Atari's 8-bits used proprietary GTIA, ANTIC and POKEY chips, and later models even more custom logic. The Commodore 64 uses a custom PLA and VIC-II and SID chips that are not quite proprietary, but not especially common either.