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IBM PC XT Maintenance Guide

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IBM Personal Computer XT (5160)

The IBM PC XT (5160) was introduced in March 1983. The parts that need attention on a surviving machine are the tantalum capacitors on the system board and cards, the power supply, the fixed disk, the full-height floppy drives and the keyboard. The XT shares its processor, bus and switch-based configuration with the 5150, and the IBM PC (5150) Maintenance Guide covers much of the same ground.

Identify your system board

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IBM made two versions of the 5160 system board, which differ in how much RAM fits on board.[1]

IBM 5160 system board versions
Marking RAM banks BIOS as supplied Notes
64-256KB SYSTEM BOARD 4 banks of 64 KB 11/08/82 Printed on the board. Also used in the IBM 5155 Portable PC. IBM documents a modification to 256-640KB.
256-640KB SYSTEM BOARD 2 banks of 256 KB + 2 banks of 64 KB 01/10/86 or 05/09/86 Usually on a label rather than printed on the board.
Marking of the 5160 64-256KB system board (minuszerodegrees.net)
Marking of the 5160 256-640KB system board (minuszerodegrees.net)

The marking is not always right. IBM notes that some boards in models 088, 089, 267, 277 and 278 are labelled 64Kb/256Kb although they are 256Kb/640Kb boards. A module fitted at U84 identifies a 256/640 KB board; the 64/256 KB board has nothing there.[2] On the 256/640 KB board, jumper E-2 must be fitted for the 256 KB chips in banks 0 and 1 to be counted; without it the board shows 256 KB.[2]

The 640 KB modification of the older board grounds pin 1 of U44, either by joining pins 1 and 2 of jumper pad E2 or by wiring pins 1 and 8 of U44 together.[1] Mark a modified board so the next owner knows.

The BIOS ROMs in U18 and U19 also date the board; see BIOS revisions.

Regular cleaning

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Case and keyboard

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  • Unplug the machine before opening it. The XT has no battery and no real-time clock.[3]
  • The system cover is held by five screws; IBM's parts catalogue lists "Screw, System Cover (Qty 5)".[4] Some drive securing screws are reached through a well in the underside of the case.[3]
  • Early 5160s shipped with the 83-key keyboard used on the 5150. There are two types: type 1 is reset by the system board at power-on, type 2 resets itself, and both work.[5] Taking the keyboard apart is easy; mating the curved circuit board back to the curved metal plate is not.[5] For dead or twisting keys see foam sheet repair.

Inside the chassis

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  • Lift each expansion card straight out and note its slot.
  • Reseat the 8088 and both BIOS ROMs (U18 and U19) with an IC extractor.
  • Do not clean the floppy heads by hand (see Floppy drive).
  • Inspect the tantalum capacitors, C56 on the +12 V line first. It is the known short-circuit position on the XT board.[6]

Power supply checks

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The XT supply is a 130 W switching regulator with four outputs. IBM made separate 120 V and 220/240 V units, both fused.[7] IBM's parts catalogue lists them as FRU 8529247 (120 V) and 8654269 (220/240 V).[4] minuszerodegrees.net publishes a procedure for converting a 120 V unit to 240 V.[8]

Turning off the red switch does not remove mains voltage from everything inside the supply.[8]

Per-line ratings

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The 130 W figure is the total. Each output has its own maximum:[7]

IBM 5160 power supply load range (Technical Reference p. 3-4)
Output Minimum load Maximum load
+5 V 2.3 A 15.0 A
−5 V 0 A 0.3 A
+12 V 0.4 A 4.2 A
−12 V 0 A 0.25 A

minuszerodegrees.net's per-line table gives 0.3 A for −12 V; IBM's Technical Reference and service manual both give 0.25 A.[7][9] The system board takes about 2 to 4 A of the +5 V, leaving about 11 A for the expansion slots.[7]

Voltage check

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Measure with the supply connected to the system board and the machine switched on. IBM's service manual gives these limits at the system board power connectors:[2]

IBM 5150/5160 system board voltage limits (service manual p. 4-3)
Rail Meter − lead Meter + lead Minimum Maximum
+5 V 5 10 +4.8 V +5.25 V
−5 V 9 6 4.6 V 5.5 V
+12 V 7 3 +11.5 V +12.6 V
−12 V 4 8 10.9 V 13.2 V

IBM numbers the pins of the two connectors 1 to 12 in one row (Figure 4-6): P8 is pins 1–6 and P9 is pins 7–12. With the leads reversed as shown, the negative rails read positive. Power good (P8 pin 1) should read 2.4 V to 5.2 V.[2] On P8, pin 3 is +12 V and pin 4 is −12 V; on P9, pin 3 is −5 V and pins 4 to 6 are +5 V.[10]

These agree with the regulation tolerances in the Technical Reference (+5 % / −4 % on +5 V and +12 V, +10 % / −8 % on −5 V, +10 % / −9 % on −12 V). The same page gives the sense levels below which the supply withholds power good: +4.5 V, −4.3 V, +10.8 V and −10.2 V.[7]

A supply that is not connected to the system board or drives cannot be measured properly. IBM's service manual says to load it with 1.5 ohm, 25 W across P9 pins 1 and 6, made from two of IBM's 3 ohm 25 W resistors (P/N 0337213) in parallel.[2] A working system board alone is normally, not always, enough load; so is a working full-height Tandon drive.[8]

The −12 V line feeds nothing on the system board; it goes only to the expansion slots.[11] −5 V is used by the analogue circuits of the diskette adapter's phase-lock loop.[7]

Minimum off time

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Wait at least one second after switching off before switching on again. The supply regenerates power good only after being off for at least 1.0 second.[7][3] Power good rises between 100 ms and 500 ms after the outputs reach their sense levels.[7]

The supply shuts all outputs down on a DC overcurrent or overvoltage and stays down until the fault is removed; its fan may still turn.[7][3] The primary fuses are 5.0 A on 110 V units and 3.5 A on 220/240 V units.[7]

Expansion slots

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Slot 8, the slot nearest the power supply, is different from the others. IBM's Technical Reference says a card in J8 must return a "card selected" signal whenever it is selected, and that J8's timing requirements are stricter than J1 to J7's.[7][12] Slot 8 sits on the external address and data buses, which is why a card there must generate the signal for its data to reach the processor.[12]

  • IBM's service manual names the Asynchronous Communications Adapter, with jumper J13 fitted, as the only IBM card that provides the signal.[2]
  • Other cards that work in slot 8 usually have a "slot 8" jumper: minuszerodegrees.net lists the Video-7 VEGA (P1), Video-7 VEGA VGA (P3) and Microsoft InPort (J2).[12]
  • An accelerator card in slot 8 can cause an 1801 error at power-on.[12]

Connector and socket care

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  • Reseat U18 and U19, the 8088 and the optional 8087 socket, and the RAM in all four banks.
  • Clean card edge fingers with isopropyl alcohol on a foam swab.
  • The 5-pin DIN keyboard socket is soldered to the system board, and the keyboard interface is discrete logic with no controller chip, so a bad joint here looks like a keyboard fault.[3][13]
  • When fitting the fixed disk cable, IBM's service manual gives the orientation: the 10 MB cable (P/N 8529271) goes into the adapter with the trace wires facing up, away from the system board; the 20 MB cable (P/N 6480086) with the trace wire facing down, towards the board.[2]

Capacitor health

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The 5160 system board carries sixteen tantalum capacitors, thirteen of them on the +5 V line. On +12 V there are only C55, a ceramic, and C56, a tantalum. Positions, photographs and procedure are in IBM PC XT Capacitor Replacement Guide.

Common failure points

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Tantalum capacitor short-circuit

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A shorted tantalum makes the supply shut down, so the machine looks dead, sometimes with the fan still turning.[7][14] C56 on +12 V is the usual one, and the board runs with it removed.[6]

POST beep codes

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The XT's POST is a confidence test only. No 201 error does not mean the RAM is good, and the POST outputs no codes a POST card can read.[3][13] It can take minutes before an error appears; IBM gives 13 to 90 seconds for a complete POST, depending on the options fitted.[3][2]

IBM 5160 POST indications
Indication Meaning
1 short beep POST completed. Not an error.
1 long + 2 short Video. SW1 is set for an MDA or CGA the POST cannot find or that has failed; a system board fault makes the POST think an MDA or CGA is fitted; or a video card's own ROM found a fault.
1 long + 3 short Produced by the ROM on an IBM EGA card when it finds a fault in itself.
Continuous tone Not produced by the POST. A faulty system board, a card interfering with it, or a faulty supply. A supply giving power good is not proved good by that alone.
2 short beeps with an on-screen code Accompanies an xx01 error and "ERROR. (RESUME = F1 KEY)".
101 System board: 8259 interrupt controller, 8253 timer or 8237 DMA controller and their support logic.
201 (with an address and byte, e.g. "30000 10 201") RAM: failed or badly seated chip, wrong chip type, wrong switch settings, board fault, or a memory expansion card.
301 Keyboard. A preceding byte means a stuck key, and identifies it. Otherwise no keyboard, an AT keyboard, a broken cable conductor, a bad joint on the DIN socket, or the board's keyboard circuit.
601 Diskette subsystem. The 1982 BIOS tests only drive A:; the 1986 BIOS tests as many drives as SW1 switches 7 and 8 say are fitted, so a wrong setting alone gives 601.
1701 Not a system board error. The fixed disk controller's own ROM produces it, and it covers the controller, cables, drive, configuration and power.

Source for the table: minuszerodegrees.net.[13]

With the 11/08/82 BIOS, a flashing cursor and nothing else often means an MDA card with SW1 set for CGA; nothing at all often means a CGA card with SW1 set for MDA.[13] The IBM PC XT Troubleshooting Guide has the full list of messages and the minimum diagnostic configuration.

Bank 0 RAM

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A failed chip in bank 0 leaves the board apparently dead. The same symptom has many other causes, so run the minimum diagnostic configuration first.[14]

U19 and the 1986 BIOS

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On the 1982 BIOS, U19 holds part of Cassette BASIC only: a failed U19 gives "F6000 ROM" and the machine carries on booting. On the 1986 BIOS, U19 also holds part of the BIOS, and a failed U19 leaves the board apparently dead.[15] With U19 removed on an 08/16/82 or 11/08/82 board, the RAM count completes, the screen clears to an underline cursor, and the floppy drive light and motor stay on.[13]

1986 BIOS notes

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The 01/10/86 and 05/09/86 revisions can stop some third-party floppy controllers booting from 1.44 MB diskettes (reading is fine), and they may not support a 5161 Expansion Unit.[15] IBM states that the 5161 is not supported on 5160 models 088, 089, 267, 268, 277 and 278.[16]

Fixed disk

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IBM's controller is the IBM Fixed Disk Adapter, made by Xebec. Its first version supports only a 306-cylinder, 4-head drive, the Seagate ST-412 IBM shipped with it; a later version supports the 20 MB drives of later models.[17] IBM's parts catalogue lists the 10 MB drive as FRU 8529275.[4]

IBM's service notes for these drives:[2]

  • Before moving the machine, run the advanced diagnostics option "Prepare Fixed Disk for Relocation", which parks the heads beyond the data area.
  • IBM-made 10 MB drives with serial number F274092 or lower can make a high-pitched noise from the static eliminator. IBM says this causes no operational problem (ECA019).
  • The IBM 20 MB drive in models 088 and 089 is type 13, identified by "WD25" on a label on its black cover if the type label is missing.

A 1701 error points at the drive system as a whole. Reseat both ribbons at both ends, check the controller's seating, and check that the drive is a type the fitted controller supports.[13][17]

Floppy drive

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The XT needs at least one floppy drive.[18] The IBM adapter and drives are double density and cannot read 1.2 MB media.[3] Early 5160s usually have a full-height Tandon TM100-2 or TM100-2A, or a Micropolis equivalent.[14]

The full-height drives are belt driven. Tandon's manual for the TM100-1 and TM100-2 says the spindle is driven by belt from a DC motor with an integral tachometer and held at 300 RPM ± 1.5 %, and lists the belt as part 914000-001.[19] IBM's parts catalogue lists a belt for each full-height drive type: FRU 8529154 (type 1), 8654251 (type 2) and 1696625 (type 3). IBM identifies the type from the serial number: A, B or no prefix for type 1, D for type 2, E for type 3.[4] On type 2 drives whose file control card is stamped 010 on its left edge, IBM's fix for read/write errors is to remove capacitor C39, which generates noise.[2]

Do not clean the heads by hand. Tandon's manual says: "Do not use cotton swabs, solvents, or otherwise clean the head(s) by hand. Damage to the head(s) may result." Good media is normally enough; for heavy contamination Tandon allows a cleaning diskette used to its maker's instructions.[19] Tandon specifies head life as 20,000 media contact hours.[19]

Tandon's manual has no lubrication procedure. On TM100 drives left unused for years, minuszerodegrees.net finds that the lubricant on the head carriage rails often gums up, leaving the carriage sluggish or stuck. With the power off, the carriage should slide freely along its rails by hand against slight resistance from the stepper motor. Their practice is to clean off the old lubricant and apply a silicone-based lubricant, preferring silicone grease to spray because spray tends to reach parts that should stay dry.[20] This applies to the TM100's carriage rails, not to the heads or spindle.

If the spindle speed is out, minuszerodegrees.net adjusts R4 on the servo board at the rear of the drive, and checks the speed with the 50 Hz or 60 Hz strobe ring on the flywheel under mains-powered lighting.[20]

See also Floppy Drive Repair and Tandon TM100-2.

Keyboard

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From 1986 IBM supplied the 101-key keyboard in the US and the 102-key keyboard elsewhere.[5] A keyboard made only for the AT will not work on an XT; the choices are a switchable keyboard or a protocol converter. Some IBM 101-key keyboards work directly, but their LEDs do not, and some added keys need a 1986 BIOS.[5]

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  • ESD wrist strap and mat
  • Digital multimeter
  • IC extractor
  • Contact cleaner, isopropyl alcohol and foam swabs
  • Soldering iron with a fine tip, flux, desoldering pump and braid
  • Glasses for the first power-up of a board that has not been used for years[3]

See also Recommended Tools.

Preventive maintenance checklist

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  1. Measure the rails against IBM's limits in the voltage check table.
  2. Inspect the system board and card tantalums, C56 first.
  3. Clean card edge fingers.
  4. On TM100 drives, check that the head carriage moves freely by hand; if not, clean and relubricate the rails as above.[20]
  5. Check SW1 against the fitted RAM, display and drives. The XT has no CMOS setup.[3]
  6. Run the machine. minuszerodegrees.net reports tantalum failures mostly on boards that have not been powered for years, and rarely on machines used every few months.[21]

Quick-fix flowcharts

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No power, fan dead

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  • Check the mains lead and the rear switch, and leave a full second between switching off and on.[7]
  • Disconnect P8. Does the fan now turn continuously? If so, the system board is loading the supply down: suspect C56 first, then the +5 V tantalums.[22][6]
  • If the fan still does not turn with P8 out, suspect the supply, bearing in mind that an unloaded supply may shut down by design.[8]

Dead machine, fan runs, no beep

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  • Check SW1 5 and 6 against the display adapter. An 11/08/82 BIOS with a CGA card and SW1 set to MDA gives exactly this.[13]
  • Reseat U18, U19, the 8088 and the bank 0 RAM. On a 1986 BIOS a failed U19 alone gives a dead board.[15]
  • Try the minimum diagnostic configuration in the troubleshooting guide.

1701 at power-on

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  • Reseat the Fixed Disk Adapter and both ribbons at both ends.
  • Measure +12 V at the drive power connector; IBM's limits there are +11.5 to +12.6 V, and +4.8 to +5.2 V on +5 V.[2]
  • Check that the drive's geometry is one the fitted controller supports.[17]
  • The message comes from the controller's own ROM, not the system board.[13]

References

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  1. ↑ 1.0 1.1 IBM 5160 – Motherboard Versions, minuszerodegrees.net. The two versions, their markings, bank layouts, the BIOS supplied with each, the use of the 64-256KB board in the 5155, and the IBM-supported 640 KB modification.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 IBM, Personal Computer Family Service Information Manual, SA38-0037-00, First Edition, January 1989, hosted as File:IBM Personal Computer Family Service Information Manual SA38-0037-00.pdf. Chapter 4, "5150/5160-Based System Diagnostics": POST timing (p. 4-3), dead machine tips and service voltage limits at the system board connectors (p. 4-3), dummy load (p. 4-4), E-2 jumper and mislabelled boards (p. 4-7), Type 2 drive C39 (p. 4-9), drive power connector limits and relocation before moving (p. 4-11), 10 MB drive noise and 20 MB identification (p. 4-12), fixed disk cable orientation and slot 8 (p. 4-13), DSEA and 74LS32 (p. 4-14).
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 IBM 5160 – Basics, for 5160 beginners, minuszerodegrees.net. No RTC or CMOS setup, the underside case well, the one-second minimum off time, slot 8, no keyboard controller chip, the POST as a confidence test only, no POST-card codes, and the advice to wear glasses when powering up equipment that has not been used for years.
  4. ↑ 4.0 4.1 4.2 4.3 IBM, Hardware Maintenance Service parts catalogue, part 6280087 (PC, XT and Portable PC), hosted as File:IBM Hardware Maintenance Service 6280087 Parts Catalog (PC, XT, Portable PC).pdf. 5160 system unit interior, p. 12 (system board 8529254, supplies 8529247 and 8654269, cover screws); diskette drive belts pp. 18, 22 and 26; 10 MB fixed disk 8529275 and cable 8529271, p. 28.
  5. ↑ 5.0 5.1 5.2 5.3 IBM 5160 – Keyboards, minuszerodegrees.net. Type 1 and type 2 83-key keyboards, reassembly of the curved board and plate, the 1986 enhanced keyboards, AT-only keyboards, and the LED and added-key limits of IBM 101-key keyboards on an XT.
  6. ↑ 6.0 6.1 6.2 Short-circuit on the +12 volt line of an IBM 5150/5155/5160 motherboard, minuszerodegrees.net. C55 and C56 are the only components on the 5160's +12 V line; C56 is the known failure and not critical to operation.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 IBM, Personal Computer XT and Portable Personal Computer Technical Reference, part 6139821, March 1986, hosted as File:IBM PC XT 5155 5160 Technical Reference Mar86.pdf. Section 3: description, currents and tolerances (p. 3-3); input, output load range and sense levels (p. 3-4); fuses and power good (p. 3-5).
  8. ↑ 8.0 8.1 8.2 8.3 IBM 5160 – Power Supply Unit, minuszerodegrees.net. Mains still present inside with the switch off, per-line ratings, the one-second minimum off time, start-up loading, and the 120 V to 240 V conversion.
  9. ↑ IBM, Personal Computer Family Service Information Manual, SA38-0037-00 (1989), chapter 6, "5160 PC XT", p. 6-2: 15 A at +5 V, 4.2 A at +12 V, 300 mA at −5 V and 250 mA at −12 V.
  10. ↑ Minimum Diagnostic Configuration: fan turning, minuszerodegrees.net. P8/P9 pin functions and wire colours with the IBM limits.
  11. ↑ Vintage Computers – Filter Capacitors, minuszerodegrees.net. On the 5160 the −12 V line feeds only the expansion slots.
  12. ↑ 12.0 12.1 12.2 12.3 IBM 5155/5160 – Expansion Slot 8, minuszerodegrees.net. IBM's Technical Reference wording, the slot's position, slot-8 jumpers on named cards, the external-bus explanation, and the 1801 error with accelerator cards.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 13.7 IBM 5160 – POST – Some errors (on-screen, or otherwise), minuszerodegrees.net. Meanings and causes of the POST beeps and messages, the continuous tone, 1701 and 1801, the ROM messages, PARITY CHECK 1 and 2, and the SW1 video-setting symptoms.
  14. ↑ 14.0 14.1 14.2 IBM 5160 – Known Problems/Issues, minuszerodegrees.net. Shorted tantalums, the RAM sizing flaw, bank 0 failure, the looping POST with SW1-1 on, slot 8, 16-bit cards, the C8000 ROM conflict, MK38000 ROM sockets, the V20, and the 1986 BIOS notes.
  15. ↑ 15.0 15.1 15.2 IBM 5160 – BIOS Revisions, minuszerodegrees.net. BIOS revisions and part numbers, U19 behaviour, 720 KB and 1.2 MB notes, the 1.44 MB boot problem with some third-party controllers, and the 5161 note.
  16. ↑ IBM, Personal Computer Family Service Information Manual, SA38-0037-00 (1989), chapter 7, "5161 Expansion Unit", p. 7-1.
  17. ↑ 17.0 17.1 17.2 IBM Fixed Disk Adapter, minuszerodegrees.net. The three versions and their part numbers, the 306-cylinder/4-head limit, the ST-412, and 20 MB support.
  18. ↑ IBM 5160 – Significant Differences to the IBM 5150, minuszerodegrees.net.
  19. ↑ 19.0 19.1 19.2 Tandon, TM100-1 / TM100-2 48 TPI Operating and Service Manuals (1983), hosted as File:Tandon TM100-1 TM100-2 48TPI Operating and Service Manuals 1983.pdf. Belt drive and tachometer (p. 1-2), 300 RPM ± 1.5 % and 20,000-hour head life (Table 2-1, p. 2-3), head-cleaning caution (p. 6-1), drive belt 914000-001 (Appendix A, p. A-1).
  20. ↑ 20.0 20.1 20.2 Tandon TM100-2 – Problems, minuszerodegrees.net. Deteriorated rail lubrication and the silicone remedy (the site owner's practice), spindle speed adjustment at R4 on the servo board, and the 50/60 Hz strobe rings on the flywheel.
  21. ↑ Commonly Failing Electronic Components (in vintage computers), minuszerodegrees.net. Tantalum failure modes, failures at or soon after power-on, and the higher rate on boards left unpowered for years.
  22. ↑ Minimum Diagnostic Configuration: fan not turning, minuszerodegrees.net.
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