Editing IBM 5120 (section)

== Common Faults ==

* '''Processor card''' — same unobtainable gate arrays as the 5100's G2 and the 5110's J2 cards. Reseat before condemning.
* '''8-inch floppy drives (×2 built-in)''' — drive belt aging is the dominant failure mode; head load solenoid fatigue; spindle motor brush wear; media (IBM Diskette 2D) oxide shedding. Drives are typically Shugart-class 8-inch FDDs.
* '''9-inch CRT and deflection board''' — separate monitor sub-chassis inside the 5120 case; high-voltage flyback and deflection electrolytics are concerns.
* '''Power supply — two supplies in one enclosure, and the difference matters.''' Board '''A2''' is the same '''transistor switching regulator''' as the 5100's and 5110's, producing +5, &minus;5, '''+8.5''', +12 and &minus;12 V for the logic. Board '''A1''' is a '''ferroresonant''' supply for the diskette drives, and it is the part the 5120 adds. A1 is fused on each of its three DC outputs — '''F2''' (+5 V), '''F3''' (&minus;5 V), '''F4''' (+24 V) — and A2 is not fused at all: it shuts down instead.<ref name="mim5120" /> So a blown F2, F3 or F4 means the fault is on the '''diskette''' side and does not implicate the computer's supply; a dead machine with all three fuses intact points at A2. The enclosure also contains transformer '''T1''', the '''resonant capacitor C4''' (an AC-rated part, not an electrolytic and not a general recap candidate) and circuit breaker '''CB1'''.<ref name="mim5120" /> See [[IBM 5120 Capacitor Replacement Guide]].
* '''The 60-second rule.''' Where the 5100 and 5110 manuals say five seconds, the 5120 manual says that after an automatic shutdown the system should be powered off '''for at least 60 seconds''' before being powered up again.<ref name="mim5120" /> Cycling a 5120 faster than that makes a healthy supply look dead. This is the single most common way to misdiagnose one.
* '''Keyboard''' — remove keycaps with the proper tool rather than by hand. The claim that the 5120 uses a Model F buckling-spring keyboard could not be confirmed in IBM's own documentation and has been removed.
* '''Industrial-design "carry handles" on the bottom''' — plastic fatigue reported by some collectors.