IBM PC (5150) Maintenance Guide

IBM 5150 Personal Computer (1981)

The IBM 5150 Personal Computer needs little routine maintenance. IBM's service manual names only one part of the 5150 system as needing preventive maintenance, the IBM 80 CPS Matrix Printer, whose carriage shafts are to be oiled at every service call. The system unit has no service schedule. The checks below are for a machine that has been stored, has developed a fault, or is being opened for an upgrade.

Identify the system board

IBM made two versions of the 5150 system board, named for the RAM that fits in their sockets. The name is printed on the board. The later board also has a "B" stamped on the rear panel.

IBM 5150 system board versions

Printed on board	Rear panel	On-board RAM	BIOS as supplied
16KB-64KB CPU	No mark	4 banks of 16 KB	04/24/81 or 10/19/81; often upgraded to 10/27/82 with IBM's BIOS upgrade kit
64KB-256KB CPU	"B" stamp	4 banks of 64 KB	10/27/82

All four on-board banks are permanently enabled, so all four must be filled before a memory card is added. The 10/27/82 BIOS has two bugs that make the power-on self test (POST) report far less memory than is fitted unless all four banks are populated and SW1 and SW2 are set to match. The machine still starts. No IBM document states this requirement.

The 5150 has no CMOS setup and no real-time clock, so it has no battery. Everything it knows about the fitted memory, drives, coprocessor and display comes from the SW1 and SW2 switch blocks. IBM's switch charts for both boards are in the Guide to Operations.

IBM 5150 motherboard layout, showing the socket positions referred to on this page

The BIOS ROM in socket U33 also identifies the build; see IBM PC (5150)#Memory and ROM.

Opening and cleaning

Case and keyboard

Switch off and unplug the machine before opening it. Turning off the power supply's switch does not remove mains voltage from every component inside the supply.

IBM's 1981 service manual removes the cover by loosening the cover retaining screws at the rear and sliding the cover forward until it clears the system unit. That manual says two screws. The miscellaneous parts kit for the 5150 in IBM's 1986 parts catalogue holds five system cover screws. Remove whatever screws your cover has.

Wipe the painted steel case and the plastic bezel with a damp cloth and mild detergent.

The keyboard supplied with the 5150 is what IBM's later documents call the 83-key keyboard. "Model F" is a family name that covers several IBM keyboards, some of which do not work on a 5150. IBM documents two types of 83-key keyboard: the motherboard resets the type 1 at power-on, and the type 2 resets itself. Both work on a 5150. Brush dust out from between the keys. Minuszerodegrees warns that after complete disassembly, fitting the curved circuit board back to the curved metal plate is difficult. For the foam sheet inside, see IBM Model F (83-key)#Foam sheet repair.

Inside the system unit

• Lift each expansion card straight out and note which slot it came from.
• Reseat the socketed chips with an IC extractor: the 8088 processor, the BIOS ROM in U33, the Cassette BASIC ROMs in U29 to U32, and the RAM in banks 1 to 3. The bank 0 RAM is soldered in. Minuszerodegrees suggests reseating the BASIC ROMs as the first step when the machine boots from floppy but not to Cassette BASIC.
• Clean card edge fingers with isopropyl alcohol and a foam swab.
• Look for burnt or split tantalum capacitors on the system board and cards, and for cracked solder joints at the power connectors P8 and P9. Faulty tantalums often look undamaged, so a clean board is not proof of a good one.
• Do not clean the floppy drive heads by hand; see #Floppy drives.

Power supply checks

The 5150 supply is designed for continuous operation at 63.5 W and comes in 120 V and 220/240 V versions. Minuszerodegrees has found 110 V units usually labelled 63.5 W and 220 V units 63 W. The IBM FRU part numbers are 8529155 (120 V) and 8654269 (220/240 V). The supply was too small for hard drives of the period. IBM's hard disk option for the 5150 was the separately powered 5161 Expansion Unit, and the 5160 was given a 130 W supply.

The supply is fused at 2 A on 120 V units and 1 A on 220/240 V units. IBM specifies an input range of 104 to 127 V at 60 Hz for the 120 V supply and 180 to 259 V at 50 Hz for the 220/240 V supply.

Output ratings

Each output has its own limit. The 63.5 W total does not mean that 63 W can be drawn from +5 V alone.

IBM 5150 power supply outputs (Technical Reference p. 3-4)

Output	Minimum load	Maximum load	Maximum power (calculated)	Regulation
+5 V	2.3 A	7.0 A	35 W	+5 % / −4 %
−5 V	0 A	0.3 A	1.5 W	+10 % / −8 %
+12 V	0.4 A	2.0 A	24 W	+5 % / −4 %
−12 V	0 A	0.25 A	3 W	+10 % / −9 %

The minimum loads on +5 V and +12 V are why the supply will not run unloaded. A working system board alone is normally enough load, but not always. Minuszerodegrees' per-line ratings page gives 0.3 A for −12 V; IBM's Technical Reference gives 0.25 A. The system board and diskette drives take most of the +5 V output, leaving about 4 A for adapters in the expansion slots.

Voltage checks

Measure at the system board power connectors with the board connected and running. IBM's service limits are:

IBM 5150 supply checks at the system board connectors (HMS pp. 020-5, 020-6)

Signal	Meter − lead	Meter + lead	Reading
Power Good	Pin 5	Pin 1	+2.4 to +5.25 V
+5 V	Pin 5	Pin 10	+4.8 to +5.25 V
−5 V (leads reversed)	Pin 9	Pin 6	+4.5 to +5.40 V
+12 V	Pin 7	Pin 3	+11.52 to +12.60 V
−12 V (leads reversed)	Pin 4	Pin 8	+10.80 to +12.96 V

Pins 1 to 6 are P8 and pins 7 to 12 are P9. P8 carries Power Good, a key, +12 V, −12 V and two grounds; P9 carries two grounds, −5 V and three +5 V pins. At each diskette drive power connector IBM's limits are +4.8 to +5.25 V between pins 2 and 4, and +11.52 to +12.60 V between pins 3 and 1.

The Technical Reference gives the supply's own sense levels. Power Good goes high (2.4 to 5.5 V) 100 ms after all four outputs pass their under-voltage sense levels, and drops (0 to 0.4 V) when any output falls below its under-voltage level or rises above its over-voltage level:

IBM 5150 supply sense levels (Technical Reference p. 3-6)

Output	Under-voltage sense	Over-voltage sense
+5 V	+4.0 V	+5.9 V
−5 V	−4.0 V	−5.9 V
+12 V	+9.6 V	+14.2 V
−12 V	−9.6 V	−14.2 V

If any output is overloaded or over voltage, the supply shuts down every dc output for as long as the condition lasts. The fan may still turn, because on some supplies it is powered from the mains.

System board resistance check

With the power off and P8 and P9 unplugged from the system board, measure resistance on the system board's own pins. IBM's charts treat a reading below the minimum as a system board fault and readings above it, with the voltages wrong, as a supply fault. On IBM 51xx boards the short is usually a tantalum capacitor.

Minimum resistance at the system board power pins

Rail	Common lead	Ohms lead	1981 HMS (p. 020-9)	1986 HMS (p. 0020-9)
+12 V	Pin 5	Pin 3	6 Ω	17 Ω
−12 V	Pin 6	Pin 4	48 Ω	17 Ω
−5 V	Pin 7	Pin 9	17 Ω	17 Ω
+5 V	Pin 8	Pins 10, 11, 12	0.8 Ω	0.8 Ω

The 1986 manual, which covers the 5150, 5160 and 5155 together, also rounds the voltage limits to +4.8 to +5.2 V, +11.5 to +12.6 V and +10.8 to +12.9 V (−12 V, leads reversed).

Minimum off time

IBM states that the supply generates Power Good "when the power supply is turned on after it has been off for a minimum of 5 seconds". Wait at least five seconds before switching back on, or the system board may not start.

The −5 V rail

IBM describes −5 V as the dynamic memory bias voltage. It follows +5 V and +12 V quickly at power-on and decays more slowly at power-off. Minuszerodegrees lists these users of −5 V:

• the 4116-class RAM on the 16KB-64KB system board
• chip U1 on both system boards
• the 4116-class RAM on the IBM 32 KB Memory Expansion Option and the 4132-class RAM on the IBM 64 KB Memory Expansion Option
• the early version of the IBM 5.25-inch Diskette Drive Adapter

An ATX supply with a P8/P9 adapter often has no −5 V output. On a 16KB-64KB board this leaves the RAM without its bias supply, and minuszerodegrees lists it as the first thing to rule out when that board appears completely dead. On a 64KB-256KB board it causes a 131 error at POST, because the cassette port wrap test fails when U1 has no −5 V.

The original supply can be recapped; see IBM PC (5150) Capacitor Replacement Guide#Power supply capacitor replacement. Opening it can expose hazardous voltages, and minuszerodegrees warns not to assume they disappear soon after the supply is unplugged.

Connectors and sockets

The socketed parts on the system board are the 8088, the optional 8087, the BIOS ROM (U33), the Cassette BASIC ROMs (U29 to U32) and the RAM in banks 1 to 3. Reseat them when chasing an intermittent fault, and clean the expansion card edge fingers. A bad solder joint on the keyboard DIN socket is one of the causes of a 301 error.

Capacitors

Tantalum capacitors on IBM 51xx system boards and cards fail short-circuit, most often at the first power-on after years in storage and within about 30 seconds of it. Minuszerodegrees reports a high failure rate for boards left unpowered for many years and a very low one for machines used every few months. They can fail with a shower of sparks or explode; minuszerodegrees suggests wearing glasses if the board is exposed to your face at power-on. A shorted tantalum makes the supply shut down, so the machine looks dead. See IBM PC (5150) Capacitor Replacement Guide for the positions on each board revision and the replacement procedure.

Common failure points

Shorted tantalum capacitor

This is the most documented 5150 fault. With the supply disconnected, measure the resistance of each rail on the system board against IBM's minimums in the table above. A near-zero reading on any rail means a shorted part on that rail; all four rails carry tantalums, and the capacitor guide gives the count on each line for both boards. The capacitor guide's procedure for finding it takes the diagnosis from there.

POST indications

The 5150's POST is a confidence test only. The absence of an error does not prove a subsystem good. IBM's own table of error indications is in the service manual (p. 010-6) and on the product page. Minuszerodegrees gives these known causes:

IBM 5150 POST indications

Indication	Meaning
1 short beep	POST completed. Not an error.
1 long, 1 short	The 8259 interrupt controller test or the 8253 timer test failed, including their supporting circuitry.
1 long, 1 short, then 1 short about 15 seconds later	04/24/81 BIOS only. One or more of the Cassette BASIC ROMs (U29 to U32) has failed.
1 long, 2 short	Video. SW1 set for a video card the POST cannot find or that has failed; EGA/VGA set on a BIOS older than 10/27/82; a video card's own ROM reporting a fault; or the "C800 ROM" bug below.
1 long, 3 short	Generated by the BIOS on an IBM EGA card when it finds a fault with itself.
Continuous tone	Not generated by the POST. Faulty system board, a faulty card interfering with the system board, or a faulty power supply.
Repeating long beeps starting 15 to 20 seconds after power-on	Keyboard clock line held low, by a faulty keyboard or faulty keyboard interface circuitry.
131	Cassette port wrap test failed. On a 64KB-256KB board, most often a missing −5 V (U1 needs it).
201	RAM: a failed chip, poor socket contact, wrong chip type, wrong switch settings, the 10/27/82 four-bank requirement, or a fault in the supporting circuitry.
301	Keyboard. A preceding byte identifies a stuck key. Otherwise: no keyboard, keyboard in the cassette socket, an AT keyboard, a broken cable conductor, or a bad joint on the DIN socket.

Because of a bug in the 5150's POST, a checksum failure in an expansion ROM, which should display "C800 ROM", gives the video beep pattern instead.

RAM

Faulty RAM refresh circuitry may not produce a 201 error; minuszerodegrees has seen it leave only a flashing cursor on screen. For a board that shows nothing at all, minuszerodegrees suggests its minimum diagnostic configuration and a diagnostic ROM (Ruud's diagnostic ROM or the Supersoft/Landmark ROM) fitted in U33 in place of the BIOS.

Floppy drives

The full-height 5.25-inch drives in the 5150 are belt driven. IBM's service manual checks that the belt is on its pulleys and undamaged, then that the spindle turns freely with the belt off, before condemning the drive motor. IBM's 1986 parts catalogue lists the belt as a separate FRU for each of the three full-height drive types: 8529154 (type 1), 8654251 (type 2) and 1696625 (type 3).

To fit a belt, IBM's procedure is to remove the drive, lay it with the belt and pulleys facing up, place the belt around the small pulley with the dark shiny side facing the pulleys, then place it around the large pulley.

Where the drive is a Tandon TM100-1 or TM100-2, Tandon's manual gives the spindle speed as 300 RPM ± 1.5 %, the belt as part 914000-001 and the head life as 20,000 media contact hours. Tandon forbids cleaning the heads by hand: "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 to keep them clean, and a cleaning diskette may be used, following its maker's instructions, when contamination is severe. Tandon's manual gives no lubrication procedure.

On the Tandon TM100-2, minuszerodegrees reports that the lubricant on the head carriage rails deteriorates until the carriage moves poorly or not at all. With the power off, the carriage should slide freely along its rails by hand against slight resistance from the stepper motor. Minuszerodegrees cleans off the old lubricant and applies fresh silicone lubricant, preferring silicone grease to spray because spray reaches places it should not. This is the site's own practice, not a Tandon or IBM instruction. See also Tandon TM100-2 and Floppy Drive Repair.

Keyboard

• A keyboard that does not respond may have a broken cable conductor. Minuszerodegrees publishes a diagram for metering the cable out end to end.
• An AT keyboard does not work on a 5150; the 5170 introduced a different keyboard protocol.
• Keys that twist in their barrels point to degraded foam; see IBM Model F (83-key)#Foam sheet repair.

Tools and consumables

• ESD wrist strap and mat
• Digital multimeter
• IC extractor
• Isopropyl alcohol and foam swabs
• Contact cleaner for sockets and edge connectors
• Soldering iron, flux, desoldering pump and braid
• Safety glasses for the first power-up of an unknown board

See also Recommended Tools.

Bringing a stored 5150 back into use

1. Unplug the supply from the system board and check the system board resistances against IBM's minimums.
2. Start with the minimum diagnostic configuration: system board, power supply and speaker only, no cards, no keyboard, nothing else connected to the supply, and SW1 switches 5 and 6 both off. About five seconds after power-on it should give one long and two short beeps. Wear glasses in case a tantalum fails.
3. Measure the supply outputs against IBM's limits.
4. Add the display adapter, disk adapter and drives, then the other cards one at a time, switching off before each change. IBM's power procedure isolates a faulty adapter the same way, removing the display adapter last.
5. Set SW1 and SW2 to match the fitted memory, drives, coprocessor and display, using IBM's charts for your board.
6. Check the floppy drive belt and head carriage (#Floppy drives) before trying to read a disk.

Quick-fix flowcharts

No power, fan does not turn

• Check the mains lead and the fuse: 2 A on 120 V supplies, 1 A on 220/240 V supplies.
• IBM's first step for a power symptom is to disconnect the keyboard and every external device except the display, then reconnect them one at a time.
• Remove the adapters one at a time, then disconnect drive A and drive B power in turn.
• A supply with nothing connected may stop after a second or not start at all. That alone does not prove it faulty.
• Wait five seconds between power cycles.

Fan runs, no beep, nothing on screen

• On a 16KB-64KB board, confirm that the supply provides −5 V.
• Check Power Good between pins 1 and 5: 2.4 to 5.25 V. If it is missing, go to the resistance check.
• Try the minimum diagnostic configuration.
• Reseat U33, U29 to U32, the 8088 and the socketed RAM, or fit a diagnostic ROM in U33. A fault in the soldered bank 0 RAM stops the POST with no beep and no picture.
• Check the speaker for continuity; IBM's chart replaces the speaker before the system board when there is no beep.

Documentation

• IBM PC 5150 Technical Reference: design reference, including the power supply specification.
• IBM PC 5150 Hardware Maintenance and Service (August 1981): IBM's service manual for the first system board.
• IBM PC 5150 Guide to Operations (April 1984): owner's manual with the switch charts.
• IBM Hardware Maintenance Service (March 1986): the later service manual, including the parts catalogue.
• Tandon TM100-1 and Tandon TM100-2: the drive manufacturer's manual.

Related pages

• IBM PC (5150)
• IBM PC (5150) Troubleshooting Guide
• IBM PC (5150) Capacitor Replacement Guide
• Capacitor Failure Symptoms
• Floppy Drive Repair
• Recommended Tools