Editing IBM PC XT Maintenance Guide (section)

== Common Failure Points ==

=== Tantalum capacitor short-circuit ===
The single most documented XT fault. The PSU may latch off the moment the short rail comes up; the fan may still spin. Both rails (+5 V and +12 V) have known tantalum positions on the motherboard. C56 on +12 V is the textbook example.

=== Bank 0 RAM dead ===
A failed RAM chip in bank 0 produces a silent "dead" motherboard — no beep, no video — easily mistaken for a CPU or PSU fault. Reseat all bank-0 chips first; piggyback a known-good chip onto each position in turn.

=== MFM hard drive ===
The 10 MB Seagate ST-412 and the 20 MB Seagate ST-225 are MFM drives with their own age-related faults: stiction (the heads stick to the platter on cold start; tap firmly to free), stepper grease drag, head alignment drift, and capacitor failure on the drive's analog board.

If the HDD will not spin up:
* Listen for the spindle motor. A faint hum followed by silence is stiction; a stuck-hum-then-click pattern is a failed spindle bearing.
* The drive uses '''+12 V''' for the spindle motor and '''+5 V''' for the controller logic. A weak +12 V from the PSU is a common cause of HDD failure on a freshly-restored XT.

If the HDD spins but won't be detected:
* '''1701''' at POST = the [[IBM Fixed Disk Adapter|Fixed Disk Adapter]] cannot talk to the drive. Reseat the 34-pin control and 20-pin data ribbons.
* '''C800 ROM''' message at POST = the HDD controller's BIOS expansion ROM at C8000 is corrupt. The 11/08/82 5160 BIOS displays this correctly. On the 1986 BIOSes the same condition may produce a "1 long + 2 short" beep instead.

=== Floppy drive ===
Same Tandon TM100-2 / TM100-2A or equivalent as on the [[IBM PC (5150)|5150]]. Belt failure, hardened stepper grease, and dirty heads are the standard issues. See [[Tandon TM100-2]].

=== Keyboard ===
The Model F (83-key) and the later Enhanced (101-key) keyboards both share age failures: degraded foam-and-foil pads (Model F), broken cable conductors at the strain relief, and corroded DIN-5 contacts.