IBM PC AT Maintenance Guide

IBM 5170 type 1 system board, with the 01/10/84 BIOS ROMs

The IBM PC AT (5170) has more to look after than the 5150 or 5160: a battery-backed clock and configuration memory that has to be set up again when the battery goes, a supply that needs a minimum load, and a diskette controller that depends on pin 34 of the drive cable. The three system board types and three BIOS revisions differ, so do not assume that what applies to one 5170 applies to another. IBM's AT service manuals of 1984 and 1986 are organised into diagnostics, removal and replacement, setup and parts sections; neither has a preventive maintenance section.

Identifying the system board

IBM numbered the three boards type 1, type 2 and type 3.

IBM 5170 system board types

Type	Speed	Size	Main crystal	System board RAM	BIOS supplied	FRU
1	6 MHz	13.8 × 12 in	12 MHz	2 banks of 256 KB, stacked 41128 128K-bit chips	01/10/84	6480170
2	6 MHz	13.8 × 9.4 in	12 MHz	1 bank of 512 KB, 41256 256K-bit chips	06/10/85	6480072
3	8 MHz	13.8 × 9.4 in	16 MHz	1 bank of 512 KB, as type 2	11/15/85	6489922

The table's first six columns are from minuszerodegrees; the FRU numbers are from IBM's 1986 parts catalogue, which also notes that the type 3 board is identified by the FRU number printed on it. The RAM module FRUs are 8286139 (128 KB, type 1) and 6480008 (256 KB, types 2 and 3). RAM must be rated at 150 ns or faster. On a type 1 board, changing between one and two banks needs SETUP run again and may need jumper J18 moved.

ROM sockets

Very early boards carry four 16 KB 27128 BIOS ROMs in U17, U27, U37 and U47. Later boards have two 32 KB 27256 ROMs in U27 and U47, and type 1 boards have a shunt block, U131, set for the ROM type fitted. On most 5170s U17 and U37 are therefore empty; they occupy E0000 to EFFFF. Code placed in them must fill 64 KB with an 8-bit checksum of 00, or the POST ignores it.

Opening and cleaning

• Switch off and unplug. Turning off the supply's switch does not remove mains voltage from every component inside the supply.
• The cover is held by five screws at the rear. With the keylock locked, the cover can still be removed by IBM's procedure; see IBM PC AT (5170)#Storage.
• Clear dust from the system board, cards, drive bays and supply vents with a soft brush and low-pressure air. Hold the fan still while blowing air through it.
• Reseat the expansion cards and the keyboard cable.
• Wear eye protection the first time an unknown board is powered up. Aged tantalum capacitors can explode at power-on.

Power supply

Minuszerodegrees gives the rating as about 192 W. The IBM FRU number is 8286122. The input voltage is selected with a switch above the power cord socket: 115 V for 100 to 125 V ac, 230 V for 200 to 240 V ac. IBM's service procedures check that switch first.

Output ratings

Each output has its own limit; the 192 W total cannot all be drawn from one rail.

IBM 5170 supply outputs (Technical Reference p. 3-4)

Output	Minimum load	Maximum load	Maximum power (calculated)	Regulation
+5 V	7.0 A	19.8 A	99 W	+5 % / −4 %
−5 V	0 A	0.3 A	1.5 W	+10 % / −8 %
+12 V	2.5 A	7.3 A	87.6 W	+5 % / −4 %
−12 V	0 A	0.3 A	3.6 W	+10 % / −9 %

Sources: the currents and tolerances from IBM's Technical Reference; the wattages as calculated by minuszerodegrees.

Voltage checks

With the system running, check Power Good between P8 pin 1 and pin 5 (ground): IBM's limit is 2.4 to 5.2 V. Then check the system board connectors. IBM's 1986 service manual and its 1989 service information manual give slightly different limits:

IBM 5170 supply checks at the system board connectors

Signal	Meter − lead	Meter + lead	HMS 1986 (p. 0020-3)	FSIM 1989 (Figure 9-17)
+5 V	P8-5	P9-4	+4.8 to +5.2 V	+4.8 to +5.25 V
−5 V (leads reversed)	P9-3	P8-6	+4.5 to +5.4 V	+4.6 to +5.5 V
+12 V	P9-1	P8-3	+11.5 to +12.6 V	+11.5 to +12.6 V
−12 V (leads reversed)	P8-4	P9-2	+10.8 to +12.9 V	+10.9 to +13.2 V

At the diskette and fixed disk power connectors the 1986 manual's limits are +4.8 to +5.2 V between pins 2 and 4, and +11.5 to +12.6 V between pins 3 and 1.

IBM 5170 supply connectors and maximum current per pin (Technical Reference p. 3-7)

Connector	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6
P8	Power Good	+5 V, 3.8 A	+12 V, 0.7 A	−12 V, 0.3 A	Ground	Ground
P9	Ground	Ground	−5 V, 0.3 A	+5 V, 3.8 A	+5 V, 3.8 A	+5 V, 3.8 A
P10, P11	+12 V, 2.8 A	Ground	Ground	+5 V, 1.8 A
P12	+12 V, 1.0 A	Ground	Ground	+5 V, 0.6 A

The Technical Reference specifies this behaviour:

• The supply switches off within 20 ms if any output is overloaded, and an overcurrent does not damage it.
• With the minimum loads present, the outputs track within 300 ms of each other at power-on and power-off.
• With no load the supply may switch off and need a power-on reset; no damage results.
• Power Good is generated when the supply is switched on after being off for at least one second, with a turn-on delay of 100 to 500 ms.

Minimum load and the load resistor

The supply needs at least 7.0 A on +5 V and 2.5 A on +12 V. On a machine with no fixed disk, IBM's Technical Reference requires a 5 ohm, 50 W load resistor on P10. Its FRU number is 8286123. At 12 V it draws 2.4 A and dissipates about 29 W. IBM's service manual repeats the requirement for systems with no fixed disk, on P10 or P11. A 5170 with its hard drive removed and no resistor may or may not start. IBM never stated why the resistor is needed, and minuszerodegrees suggests later supplies may not require it.

XT-class supplies

Powering a 5170 system board from an XT-class supply can cause a type 1 board with the 01/10/84 BIOS not to start at all (a POST card shows the "verify shutdown byte" test failing), intermittent start-up, and intermittent loss of the SETUP configuration. An AT-class supply has a wire on P8 pin 2 and an XT-class supply does not, but that is not the only difference.

Supplies that fail when warm

Some 5170 supplies fail after two or more hours powered on, then work again after the machine is switched off and has cooled. IBM's instruction is to replace the supply.

Battery

The configuration and the clock are held in a Motorola MC146818 real-time clock and CMOS RAM chip, powered from a battery on connector J21 when the machine is off. IBM supplied a 6 V lithium battery, FRU 8286121, and used more than one type over the machine's life.

IBM's battery check is:

1. Switch off and remove the cover.
2. Disconnect the battery from the system board. Measured while connected, the reading varies because of the board's drain.
3. Set the meter to 12 V dc, common lead on pin 1 and voltage lead on pin 4 of the battery connector.
4. Look for at least 6.0 V.

Replacements described by minuszerodegrees, all 6 V nominal unless stated:

• a CR-P2 cell in a holder, with the leads and connector moved across from the old battery
• four 1.5 V AA cells in a holder with the right connector
• three AA cells (4.5 V)

The system board has no charging circuit, so a rechargeable battery achieves nothing. Minuszerodegrees advises against more than 6 V nominal: the drop across diodes CR1 and CR2 is about 1 V and the MC146818 datasheet gives 3 to 6 V operation, so a battery measuring over 7 V takes the chip past its rating. It considers a 3.6 V lithium cell borderline; some boards keep their settings on it but lose time. Old third-party stock sold under 8286121 is likely to be well discharged. For leaking batteries, see Battery Explosion, Capacitor or Corrosion Damage.

Battery symptoms

The first sign of a low battery is that the settings survive but the clock loses time while the machine is off, and the loss grows as the battery weakens. The clock's oscillator is more sensitive to battery voltage than the clock chip, and runs only part of the time as the voltage falls.

IBM 5170 configuration errors

Code	Message	IBM's description	Causes (minuszerodegrees)
161	161-System Options Not Set-(Run SETUP)	Dead battery	A flat battery, a new battery fitted but SETUP not yet run, or a faulty system board. Fitting a good battery does not clear 161 until SETUP is completed.
162	162-System Options Not Set-(Run SETUP)	CMOS checksum error	A corrupted SETUP, a flat battery on boards that give 162 instead of 161, or a video card that does not match the display type set in SETUP.
163	163-Time & Date Not Set-(Run SETUP)	Clock not updating	Invalid date or time in the clock chip, a SETUP that was started and not finished, or a fault in the clock circuitry.
164	164-Memory Size Error-(Run SETUP)		Memory found by the POST differs from SETUP: RAM added without running SETUP, wrong amounts entered, or faulty RAM.

Source: minuszerodegrees' 5170 POST errors page. IBM's service manual adds that 161 always appears on the first POST after a battery is fitted.

A clock that will not keep time with a good battery may be the MC146818 itself: the failure history kept by minuszerodegrees records three faulty MC146818 chips, all on type 1 boards.

SETUP

The AT has no SETUP program in ROM. Minuszerodegrees describes three ways to run it, all assuming IBM BIOS ROMs: IBM's own "Diagnostics for the IBM Personal Computer AT" 5.25-inch diskette (an image is available for anyone who can write a 360 KB disk); typing a short program into Cassette BASIC, which needs no diskette; and the third-party GSETUP program. Run SETUP after adding or removing RAM, changing the video card type, or replacing the battery.

Connectors and cards

• The AT keyboard uses the same 5-pin DIN plug as the XT keyboard but a different protocol, introduced with the 5170. The 83-key PC and XT keyboard does not work on the AT. The keyboard controller chip, U126, appears more than once in minuszerodegrees' failure history.
• Some cards designed for the 5150 and 5160 do not work in a 5170, partly because the slot timing is different. Some cards designed for the 6 MHz 5170 do not work in the 8 MHz model.
• The Fixed Disk and Diskette Drive Adapter's signal cable is FRU 8286124 and the fixed disk data cable 8286129. The diskette cable must be the type IBM supplied, which manipulates the motor-on and drive-select lines; a hard disk-style twisted cable is different.

Diskette drives

IBM's FRU numbers are 8286130 for the 1.2 MB high-capacity drive and 8286131 for the optional double-sided (360 KB) drive. IBM marked 360 KB drives fitted to a 5170 with an asterisk on the front. The POST does not boot from drive B.

Pin 34

The diskette controllers in the 5150 and 5160 ignore pin 34 of the drive cable; the controller in the 5170 monitors it.

• A 1.2 MB drive in a 5170 must generate a DISK CHANGED signal on pin 34. The BIOS requires it, and without it diskettes can be corrupted.
• A 360 KB drive need not generate DISK CHANGED, but some 360 KB drives put a READY signal on pin 34, which is a problem for the 5170. The simple rule when fitting a 360 KB drive is to disconnect pin 34. Some drives have a jumper for this (the 'RY' jumper on the JU-455-7), and some connector adapters have a jumper that controls it.

Media

IBM states that a 160/180 KB or 320/360 KB diskette written in the high-capacity drive can afterwards be read only in a high-capacity drive. Minuszerodegrees explains that such a disk no longer conforms to the 360 KB standard in track width and recording coercivity, and that this is why IBM offered the 360 KB drive as an option for the AT: owners used it with double-density media to write disks for 360 KB machines.

Head carriage

In 5.25-inch drives unused for many years, the lubricant on the head carriage rails often gums up until the carriage moves poorly or not at all. With the power off, the carriage should slide freely by hand against slight resistance from the stepper motor. The remedy is to clean off the old lubricant and apply new. Check the drive's own service manual before choosing a lubricant; see Floppy Drive Repair.

Fixed disk

The first hard drive IBM supplied in the 5170 was the CMI 6426-S (also seen as CM-6426), a 20 MB drive. Its high failure rate led IBM to move to the Seagate ST-4026; minuszerodegrees collects contemporary Computerworld, InfoWorld and PC Magazine articles on the failures. The Model 239, released in October 1985, has a 30 MB drive made by IBM. IBM's FRU numbers are 6278099 (20 MB) and 8286216 (30 MB); AT fixed disks made after 1 October 1985, other than the 20 MB, carry an identifying label.

In SETUP, IBM's 20 MB drives are type 2 and its 30 MB drives type 20. Entering type 20 needs diagnostics at level 2.0 or later and a BIOS that supports it. The 30 MB upgrade kit for type 1 machines included 06/10/85 BIOS ROMs for this reason. Controllers that carry their own BIOS ROM, such as SCSI and XT-IDE cards, usually ignore the SETUP drive type.

See also Hard Drive Maintenance and Repair.

Capacitors

Minuszerodegrees' failure history lists tantalum capacitors against many 5150, 5155 and 5160 boards but none of the 5170 boards it records. The 5170 entries are RAM, the MC146818, the U126 keyboard controller, RAM delay lines, BIOS ROM contents, ROM sockets and individual logic chips. Tantalums on cards and electrolytic capacitors in the supply still age. See IBM PC AT Capacitor Replacement Guide.

Things that are not faults

• A hot 80286. Measured by minuszerodegrees on the chip top 30 minutes after a cold start, idle at a DOS prompt in a 20 °C room, a 6 MHz 80286 reached about 38 °C and an 8 MHz part about 50 °C.
• A hot load resistor on a diskette-only machine; it dissipates about 29 W.
• Empty ROM sockets at U17 and U37.
• A POST memory count that does not match what is fitted. The count excludes the gap between conventional and extended memory, and all expanded memory.
• No boot from drive B.
• A third half-height drive that will not fit the right-hand cage. IBM designed it for two half-height diskette drives, or one half-height diskette drive above a full-height fixed disk.
• The "Bytes free" figure in Cassette BASIC, which is not the amount of RAM fitted.

POST beeps

IBM 5170 beep indications and causes

Indication	Causes (minuszerodegrees)
1 long, 1 short	06/10/85 and 11/15/85 BIOS only. System board speed outside certain bounds, typically after a faster crystal has been fitted.
1 long, 2 short	Video: a faulty card, a poorly seated card, dirty edge contacts, or interference from another card. An IBM EGA card can also produce this.
1 long, 3 short	The ROM on an IBM EGA card reporting a fault with itself.
Memory count completes, then nothing	On a type 3 board, an MDA card with SW1 set for colour. Also seen with a faulty Fixed Disk and Diskette Drive Adapter.

Source: minuszerodegrees' 5170 POST errors page. IBM's own table of POST errors and the procedures they lead to is in the IBM PC AT Troubleshooting Guide.

A failure in the first 64 KB of bank 0 shows as "000000 xxxx 201" with an MDA or CGA card, where xxxx gives the failed bits, but as a dead board with EGA or VGA. A port 80h POST card shows DD in that case. Reseat the bank 0 chips first. On a type 1 board, swapping the bank 0 and bank 1 chips turns the dead board into a 201 error if a bank 0 chip was at fault; types 2 and 3 have only one bank, so swapping proves nothing.

Tools

• Screwdrivers and nut drivers
• Anti-static wrist strap
• Digital multimeter
• Low-pressure air, lint-free wipes, isopropyl alcohol
• A 6 V battery with the J21 connector, or the means to move the connector across
• Eye protection
• Soldering iron, flux and desoldering braid for capacitor work

See also Recommended Tools.

Documentation

• IBM PC AT 5170 Technical Reference (September 1985) and IBM PC AT 5170 Technical Reference (March 1984)
• IBM Hardware Maintenance Service (March 1986): AT parts catalogue, start procedure and power procedure
• IBM Personal Computer Family Service Information Manual: Chapter 9 covers the 5170

Related pages

• IBM PC AT (5170)
• IBM PC AT Troubleshooting Guide
• IBM PC AT Capacitor Replacement Guide
• IBM PC XT Maintenance Guide
• IBM PC (5150) Maintenance Guide
• Battery Refurbishment
• Floppy Drive Repair
• Hard Drive Maintenance and Repair
• Recommended Tools