IBM PS/2 Model 70 Maintenance Guide

A Model 70 386 with the cover off. The battery/speaker assembly with its yellow 6 V pack is at the front left, the drive riser (bus adapter) and fixed disk behind the plastic support structure, and the power supply on the right. (Image: Peter Ellis, CC BY-SA 3.0, via Wikimedia Commons)

Routine maintenance on the IBM PS/2 Model 70 (machine type 8570) covers the 16 MHz and 20 MHz 386 machines of June 1988 and the later 25 MHz 386 and 486 models. The Model 70 uses the same low desktop case as the IBM PS/2 Model 50: the planar lies on the floor of the chassis, the drives sit in a plastic support structure over it, and the power supply slides onto an edge connector on the planar.

Safety

The supply rectifies the mains, and its primary capacitors can hold a charge after the machine is unplugged. Unplug the machine before taking the cover off, and treat the primary side of an opened supply as live until you have measured it. The supply switches between its 100-125 V and 200-240 V ranges by itself; there is no voltage selector.

IBM's caution for the lithium battery: do not recharge it, disassemble it, heat it above 100 °C, burn it or expose the cell contents to water, and replace it only with part 72X8498 or an equivalent.

There is no CRT in the system unit. IBM's displays have their own tube and supply; see CRT Discharge Procedure.

Specifications

Model 70 physical and electrical specifications

Item	Value
Width × depth × height	360 × 420 × 140 mm
Weight	9.55 kg
Heat output	215 W (735 BTU per hour)
Mains input	90-137 V AC or 180-265 V AC, range selected automatically; the announcement rates it at 4 A maximum on 100-125 V and 2 A on 200-240 V
Operating temperature and humidity	15.6 to 32.2 °C, 8 to 80 % RH
Power supply	132 W, three DC outputs (+5 V, +12 V, −12 V)
Micro Channel slots	Three: two 32-bit and one 16-bit with the auxiliary video extension
Drive positions	One 1.44 MB 3.5-inch diskette drive, a position for a second, and one fixed disk with integrated controller
Keyboard port, maximum current	275 mA
Auxiliary (mouse) port, maximum current	300 mA

Opening the system unit

The cover is held by two captive screws at the rear, which stay in the cover when loosened.

1. Switch off and unplug the mains lead and all cables.
2. Loosen both rear screws until they turn freely.
3. Slide the cover forward about 50 mm, then lift it off.
4. To refit, set the cover on the base with about 25 mm between the rear of the case and the rear of the cover, then slide it back so it drops into the guide at the bottom rear. If it will not go, check that the diskette drive is fully seated on the drive shelf; a drive that sits proud fouls the front bezel.

If the cover is locked and the key is lost, IBM's answer is to order a new cover-lock assembly, which comes with two keys, and force the old lock with one of the new keys and a pair of pliers. Peter Wendt's alternative: remove the two rear thumbscrews, push the cover forward as far as it goes, then lever a 4-5 mm gap between the cover and the rear wall of the chassis with a large flat blade so the cover slides over the locking tab.

The snap tool (IBM part 72X8547) lifts the white plastic snaps that hold the support structure. It is kept inside the system unit in front of the battery and speaker assembly. Some 25 MHz machines have the modified tool 57F2859, which also separates the processor board from the system board.

Removing the planar

IBM's removal order for the system board is: cover, battery and speaker assembly, adapters, drives and bus adapter, support structure, power supply, math coprocessor, memory kits, and on 25 MHz machines the processor board. Ardent Tool's notes on the same job:

1. Take out all adapter cards, the diskette drive, the fixed disk and the drive riser.
2. Lift the white plastic snaps with the snap tool, including the one under the speaker assembly, and pull the support structure straight up.
3. Check that the case thumbscrew nearest the Micro Channel slots is not screwed in far enough to block the support structure.
4. Unscrew and withdraw the power supply.
5. Remove the three planar screws: one in front of the 16-bit video slot, one to the front left of the video port, and one near the power-supply edge connector.
6. Remove the three screws at the rear of the chassis, above the I/O ports, that hold the planar's EMC shield.
7. Lift the planar straight up without tilting it. A 1/8-inch centring pin in the chassis base fits a hole in the planar beside inductor LN2, and levering the board off the pin damages it. You may need to push up on the EMC bezel through the port opening.
8. On refitting, a planar that is not fully down on the pin leaves the rear screws too low to reach their threads.

Power supply checks

The supply is an ASTEC AA15530, IBM part 90X9409, FRU 90X8626, rated at 132 W. It plugs into a 50-pin edge connector on the planar.

8570 power-supply edge connector

Pins	Signal	Current per pin
1	−12 V DC	300 mA
Odd pins 3-13	+12 V DC	535 mA
Odd pins 15-47	+5 V DC	1130 mA
Even pins 2-48	DC return (ground)	915 mA
49	System Status	–
50	Power Good	–

The supply has three DC outputs: +5 V, +12 V and −12 V to the system board, and +5 V and +12 V to the drives. There is no −5 V rail. IBM's service check is made with the supply out of the machine, connector facing up, black lead on pin 2:

Models 50 and 70: IBM service test voltages

Leads	Rail	Minimum	Maximum
Pin 2 to pin 1	−12 V	−9.0 V	−15.0 V
Pin 2 to pin 3	+12 V	+9.0 V	+15.0 V
Pin 2 to pin 15	+5 V	+3.7 V	+6.2 V

These limits only decide whether IBM replaces the supply. The tighter figures are the under-voltage sense levels in the Model 70 HMR: the supply raises Power Good, which lights the green power-good lamp on the front, only once +5 V is above 4.5 V, +12 V above 10.8 V and −12 V beyond −10.2 V. If the green lamp stays off, the supply is not working properly. If the voltages are wrong, IBM checks the mains cord for continuity before replacing the supply. Ardent Tool adds that a cord whose plug contacts have taken a set can make intermittent contact at the supply's inlet.

The supply protects itself. A short between any two outputs, or between an output and DC return, latches every output off without damage, and an internal over-voltage fault latches every output off before any output passes 130 % of nominal. It restarts only after the fault is removed and the power switch has been off for at least ten seconds. At switch-on the outputs track within 50 ms of each other at their 50 % points. A machine that clicks off and will not restart at once may simply be latched; wait ten seconds before switching on again.

A supply with no output at all often has a blown fuse. There are two, F1 and F2, one in each mains line, at the lower left of the supply board near the AC inlet. Each is an ASTEC 084-00300030-0217, a BEL 3AG 5 A 250 V fast-blow cartridge (6 × 32 mm).

To remove the supply, take out the standard screw at the front (IBM left an access hole through the drive shelf for it) and the two at the rear, then slide the supply out horizontally off the edge connector. The shell is closed with security Torx screws: T15 on the metal case and T10 on the black plastic around the edge connector.

Each Micro Channel connector may draw up to 1.6 A at +5 V (16-bit slot) or 2.0 A (32-bit slots), plus 0.175 A at +12 V and 0.040 A at −12 V.

Cleaning the supply

The 8570 supply has its fan built in. Ardent Tool's author found a dust ball the size of a finger jammed under a cable and dust on about half the components. With the supply out and the Torx screws removed, prise the two halves apart at the retaining rib at the top centre; the inner cover carrying the fan pivots on tabs at the lower inner edge, and its cable pulls straight off the power tap. One Torx screw at the upper rear has a toothed lock washer; refit it.

Battery and real-time clock

The clock is a Motorola MC146818A on the planar, with a 32.768 kHz crystal (Y1) beside it: U150 on the Type 1 planar, U81 on the Type 2 short planar and U143 on the 25 MHz planar. It holds 64 bytes of CMOS RAM: IBM uses the first 50 for configuration and security data and the remaining 14 for the clock. A separate 2 KB CMOS RAM extension holds further configuration data, and the battery keeps both alive when the machine is off. On the Type 1 planar that extension is the µPD446G-20L at U145; on the Type 2 it is the HM6264LFP-15T at U79.

The battery is a 6 V two-cell lithium pack, FRU 72X8498, held in the battery/speaker assembly FRU 33F5950. The Model 70 has no Dallas clock module; the Model 65 SX is the machine in this family that uses Dallas parts with internal cells. The pack is sold as a camera battery: CR-P2, Duracell DL223A, Ever Ready EL223AP, Kodak K223LA and Ray-O-Vac PC223 are equivalents. Fit it with its contacts towards the rear of the assembly, against the spring clips.

Checking the battery

IBM retain tip H024809, filed for the Models 60, 65 SX and 80, which use the same battery and assembly: take the battery out of the assembly before measuring it (a reading taken in place is not accurate), set the meter to a 12 V DC range, and replace the pack if it reads below 5.5 V. The tip also names the assembly itself as a cause of intermittent 161, 162 and 163 errors.

A clock that stands still while the configuration survives is an early sign of a failing pack. Alfred Arnold's explanation on Ardent Tool: as the battery voltage falls, the RTC's 32 kHz oscillator stops first, while the voltage is still high enough to hold the CMOS RAM, and 161 and 163 errors follow later. A clock that merely drifts is usually software: IBM notes that the time-of-day clock is interrupt driven and its accuracy varies with interrupt activity. Check the date and time from the Reference Diskette's Set Features menu; if they are right there, the fault is in software.

When a pack has gone flat, restore the configuration from the user's backup Reference Diskette if the machine had custom settings, or run Automatic Configuration if it used the defaults.

Removing the assembly

Take the battery out of the assembly before unplugging the assembly's cable, and plug the cable in before fitting the battery, so the battery is not accidentally discharged. The assembly is held by a clip at its base, directly above the orange crystal holder on the planar; push the clip inwards and lift the assembly up. On the Type 1 and 25 MHz planars the assembly also plugs into a card-edge connector on the planar.

Clearing a power-on password

The battery/speaker assembly carries the Clear POP line, so it is also the password reset.

1. Switch off and unplug the mains lead.
2. If the assembly has pins, short pins 2 and 3 together. If it has none, put a meter lead into connector position 1 and hold the other end on frame ground.
3. With the short in place, switch on. This erases the power-on password.
4. Remove the short once POST has finished. To set a new password, start from the Reference Diskette and use Set Features.

Micro Channel adapters

The configuration stored in CMOS depends on which slot each adapter is in. Refit a replacement adapter in the same slot as the one removed; if it goes in a different slot, run Set Configuration from the Reference Diskette. After adding a card:

1. Switch on with the Reference Diskette in drive A. Ardent Tool notes that starting it with Ctrl-Alt-Del instead can produce false errors.
2. Copy the card's option diskette to the working Reference Diskette so its adapter description file (ADF) is available.
3. Select Set Configuration, then Run Automatic Configuration.
4. Save and restart.

For intermittent faults IBM's advice is to check cables and connectors for damage, reseat all adapters, drives and memory modules, check the system fan and check the supply voltages.

Memory

The Model 70 shipped with a 1 MB or 2 MB memory kit. The 16 MHz and 20 MHz planars hold 6 MB (2 MB in each of three connectors), the 25 MHz planar 8 MB (four connectors), and memory-expansion adapters in the 32-bit slots take the system to 16 MB. Run Set Configuration after adding or removing memory.

Model 70 memory-module kits (HMM parts list)

Kit	Planar	FRU
1 MB	16 MHz and 20 MHz	90X8624
2 MB	16 MHz and 20 MHz	92F0104
2 MB	25 MHz	92F0103

The 16 MHz and 20 MHz machines were announced with 85 ns 1 MB and 2 MB kits. Ardent Tool reports that the 25 MHz BIOS supports only the 2 MB 80 ns parity SIMM, and that 85 ns 1 MB and 2 MB SIMMs should not be used on that planar. One contributor there got a 215 error before the memory count when he started filling the sockets at J17 or J18, and recommends starting at J21 and J20; a 225 error points to unsupported presence-detect coding or SIMMs in the wrong sockets. On 25 MHz machines IBM removes memory kits starting from the rightmost connector.

If POST finds an error in the first 512 KB, it disables the first 1 MB block and moves its addresses to the second block if one is fitted. An error elsewhere is reported but not disabled; the customer diagnostics on the Reference Diskette can then disable the failing 1 MB block, and POST ignores it from then on.

Diskette drive

IBM's parts list gives the Model 70's 1.44 MB diskette drive as FRU 64F0207 (supplied with its bezel), on slide 72X8507, and the hard-disk and diskette-drive bus adapter (the riser) as 90X9287. The drive takes power and signals through the riser; there is no separate power lead. The PS/2's 3.5-inch drives are direct drive, with no belt: Mitsubishi's MF355C specification, for example, describes a DC brushless motor driving the spindle at 300 rpm.

Peter Wendt's list of the usual reasons a drive is dropped from the configuration with a 162 at POST: POST drives the heads out from track 0 and back within a fixed time, and a sticking head carriage (hardened grease or dirt on the guides), a dirty track-0 photo-sensor, or a head damper pad that has come unstuck (common on older ALPS drives) makes it miss. He recommends Tamiya ball-gear grease for the head guides and the head servo spindle. Drive errors in the 6xx range can come from failed surface-mount electrolytics on the drive's own board; see IBM PS/2 Model 70 Capacitor Replacement Guide.

Fixed disk

The Model 70's fixed disks are Direct Bus Attachment (DBA) ESDI drives with the controller built in. They plug into the drive riser, so there is no disk adapter card. They park their heads automatically at power-off. IBM warns that careless shipping or handling can destroy all the data and the formatting on the drive, so back it up before moving the machine.

Model 70 fixed disks (HMM parts list)

Drive	FRU	Notes
60 MB DBA	6128294	762 cylinders, 6 heads, 26 sectors per track, 27 ms average access
80 MB DBA	56F8894	Terminator 92F0269
120 MB DBA	6128291	920 cylinders, 8 heads, 32 sectors per track, 23 ms average access
160 MB DBA	56F8895
Slide kit	90X8628

Older drives can stick after long storage and fail to spin up ("stiction"). Ardent Tool says the problem is common on the Seagate ST-177I 60 MB drive fitted to some 70-061 machines. Peter Wendt's method: unplug the system unit, shake it up and down several times, then turn it sharply clockwise a few times, and switch on. Never lift the front of the unit and drop it on the desk. Whatever frees the drive, copy the data off at once.

IBM's codes for the drive: 010450XX, 010451XX and 010490XX-010499XX are read/write or controller errors (run Advanced Diagnostics, then replace the riser, the drive and the system board in that order); 010454XX is a sector-buffer test error, 010455XX and 010456XX controller errors, 010462XX a controller seek error and 010464XX a drive read error, all "run Advanced Diagnostics"; 010473XX means corrupt data that may need a low-level format.

Ardent Tool notes that the riser's three resistor/capacitor pairs buffer and decouple the drive supply. Early risers lacked them, and machines with two diskette drives, or a diskette drive and a tape drive, lost data during copies.

Engineering change announcements

8570 ECAs

ECA	Applies to	Symptom and fix
021	8570-A21 system boards, P/N 15F7657	POST error 110 with five question marks, typically after a memory upgrade; board replaced
029	25 MHz -Axx and -Bxx boards with a barcode prefix from B1HC"A" to B1HC"H"	Bus-master adapters such as the IBM SCSI adapter fail to work; board replaced with P/N 15F7657
092	25 MHz machines with an 80 MB or 160 MB fixed disk (Models A81 and A16, and the 486 B81 and B16) running DOS or OS/2	A possible problem with data transfer from the fixed disk; fixed by an ABIOS patch diskette (70bios.dsk)

Cleaning

• Brush and blow dust off the planar, the riser, the support structure and the supply vents. Hold the fan still if you use compressed air.
• Clean the supply as described above; the fan is inside it.
• The keyboard and mouse connectors are identical 6-pin sockets with the same interface logic; the keyboard socket is the one marked with a keyboard symbol. Check the pins are straight before pushing a plug home.

Tools

• A volt-ohm meter. IBM names one "similar to the Triplett Model 310".
• The snap tool from inside the system unit (72X8547, or 57F2859 on some 25 MHz machines).
• The Tri-Connector wrap plug 72X8546 for the serial and parallel port diagnostics.
• Security Torx T15 and T10 bits for the power-supply shell.
• A coin or large flat blade for the cover screws.
• The Reference Diskette for the machine and the option diskettes for every adapter. The 16 MHz and 20 MHz machines use the Model 70 386 Reference Diskette; the 25 MHz and 486 machines use the Model 70 486 Reference Diskette. Ardent Tool hosts both images.
• A CR-P2 type 6 V lithium pack.
• An anti-static wrist strap.

See also Recommended Tools.

Related pages

• IBM PS/2 Model 70
• IBM PS/2 Model 70 Troubleshooting Guide
• IBM PS/2 Model 70 Capacitor Replacement Guide
• IBM PS/2 Model 50 Maintenance Guide (same case)
• IBM PS/2 Model 80 Maintenance Guide
• Battery Refurbishment
• Floppy Drive Repair
• Hard Drive Maintenance and Repair