IBM PS/2 Model 50 Maintenance Guide

This guide covers preventive maintenance and routine service on the IBM PS/2 Model 50 (machine type 8550) — the original 8550-021 of April 1987 and the Model 50 Z, 8550-031 and 8550-061, of June 1988. All three are low-profile desktops: the planar lies on the chassis floor, the drives sit in a structure over the front of it, and the power supply slides on to an edge connector at the rear left.

A Model 50 Z with the cover removed, showing the planar, the drive structure and the drive riser card.

Safety Warning

The 94 W supply is mains-rectified and its primary-side reservoir capacitor holds a charge after the machine is unplugged. If you open the supply, discharge that capacitor through a 1 kΩ / 5 W resistor and confirm with a meter before touching the board. The supply is universal and voltage-sensing, so there is no selector switch to set wrongly.

There is no CRT in the system unit. The matching IBM 8503, 8512, 8513 and 8514 displays contain their own tube and supply and are serviced separately — see CRT Discharge Procedure.

Identifying Model 50 and Model 50 Z

Model 50 and Model 50 Z compared

	Model 50 (8550-021)	Model 50 Z (8550-031 / -061)
Announced	2 April 1987	2 June 1988
Wait states at 10 MHz	One	Zero
Planar memory	1 MB, as two 30-pin 512 KB parity modules at about 150 ns (FRU 72X8517)	1 MB or 2 MB, as a single 72-pin 85 ns parity module (FRU 90X8624 for 1 MB, 92F0104 for 2 MB); 120 ns modules are also listed
SIMM sockets on the planar	Two 30-pin	One 72-pin
Planar	P/N 90X9161 or 15F6969; system board FRU 72X8516	FRU 90X9533; system board FRU 10G4005
BIOS ROMs	Four 27C256	Two 27C512
Fixed disk as shipped	20 MB MFM / ST-506, 80 ms	30 MB (39 ms) on the -031; 60 MB (27 ms) on the -061
Reference Diskette	The Model 50/60 Reference Diskette covers all of them

The machine type and submodel are on the rear-panel label, for example "Type 8550-061".

Both boards use a Motorola MC146818A real-time clock with the configuration held in CMOS RAM inside it, and both rely on an external battery — see below.

Opening the System Unit

Tools: a Philips screwdriver and an anti-static strap. IBM stowed a small plastic drive-structure removal tool under the battery/speaker assembly; it is the right tool for the white plastic snaps inside.

1. Power off and unplug everything.
2. Loosen the two captive cover screws at the rear. They are captive, so when they spin free they are undone.
3. Slide the cover forward about two inches, then lift it off.

If the cover lock is locked and the key is lost, unscrew the two rear thumbscrews, push the cover forward as far as it will go, then lever a 4–5 mm gap between the cover and the rear wall with a large flat screwdriver — enough to slide the cover over the locking tab. Once open, pull out the lock and remove the locking bracket. IBM's own answer is to order a replacement cover-lock assembly, which comes with two keys, and force the old lock with one of them and a pair of pliers.

Closing it is fussier than opening it: set the cover about an inch back from the front, let it engage the guide at the bottom rear of the case, then slide it forward. If the diskette drive will not line up with the bezel, check that the drive is fully seated on the drive shelf rather than forcing the cover.

Taking the planar out is a defined sequence and skipping a step will crack plastic: remove the adapter cards, the diskette drive, the fixed disk and the drive riser card; on the Model 50 also remove the fixed-disk adapter and the exhaust fan assembly; use the removal tool to lift every white plastic snap, including the one hidden under the speaker assembly, and pull the drive structure straight up; then pull the power supply. Three screws hold the planar. A 1/8-inch centring pin in the base mates with a hole in the planar next to the large inductor, so lift the planar straight up rather than tilting it or you will lever against that pin.

Regular Cleaning

• Soft brush and low-pressure air over the planar, the adapter cards, the drive structure and the supply. Hold fan blades still if you are using compressed air.
• Clean the area under the power-supply fan path particularly well. On the tower models IBM's own service experience is that dust collecting over the keyboard controller, the real-time clock, the diskette controller, the serial controller and the 32.768 kHz crystal is conductive enough to cause lost configuration, wrong time and diskette read/write errors that a new battery does not fix. The same chips sit in the same airflow on the desktops.
• The diskette controller chip in particular accumulates dirt that leaks current between pins and eventually looks like a dead drive — 6xx at POST.
• Clean Micro Channel card fingers with a soft eraser or a contact cleaner before reseating. Oxidised contacts produce intermittent card-not-detected faults.
• PS/2 3.5-inch drives have no dust shutter over the slot, so they collect dust continuously. Keep a diskette out of the drive when it is not in use and clean the heads with isopropyl alcohol on a foam swab — never cotton buds, whose fibres lodge in the head gap.

Power Supply Checks

The supply is an ASTEC AA13621, IBM P/N 90X9527, FRU 90X9366, rated 94 W maximum continuous output. It delivers +5 V, +12 V and −12 V only — there is no −5 V rail on this machine.

Measure at the 50-pin planar edge connector, with the black lead on any even pin (all ground):

Model 50 power supply — pinout and IBM service limits

Pin(s)	Signal	IBM minimum	IBM maximum
1	−12 V	−9.0 V	−15.0 V
3, 5, 7, 9, 11, 13	+12 V	+9.0 V	+15.0 V
15–47 (odd)	+5 V	+3.7 V	+6.2 V
2–48 (even)	Ground
49	System Status
50	Power Good

These are IBM's service limits — the window inside which the supply is not to be replaced — and they are deliberately wide. They are not a statement that a rail sitting at +4.0 V is healthy. For that, use IBM's under-voltage sense levels, which are the points at which the Model 50's own supervisory circuit drops Power Good:

Model 50 minimum under-voltage sense levels

Output	Minimum under-voltage sense level
+5.0 V	+4.5 V
+12.0 V	+10.8 V
−12.0 V	−10.2 V

A rail that has sagged towards its sense level is a supply that is on its way out even though it still passes the service check.

Two further points from IBM's own documentation:

• Power Good drives the green front-panel light. When the machine has been off for ten seconds or more and is then switched on, the supply asserts Power Good to reset system logic; its presence means all the minimum under-voltage sense levels have been met. If the green light is not on, the supply is not working properly.
• Protection latches the supply off. PS/2 supplies shut down on over-current, on a short between an output and ground or between two outputs, and before any output exceeds 130 % of nominal — and stay down until the on/off switch is recycled. So a machine that clicks and dies is far more likely to have a short somewhere in the system than a dead supply.

If the supply produces nothing at all, suspect its two internal fuses (F1 and F2) before condemning the unit.

The 94 W rating is modest, and the community consensus is that it is less generous than it looks once a processor upgrade, a stack of adapters and a hard disk are fitted. The Model 60's supply is not a substitute: the tower uses a 15-pin connector plus separate 4-pin drive leads, while the Model 50 uses a 50-pin edge connector.

The case fan is a separate assembly that plugs into the planar (J9 on the Model 50, J8 on the 50 Z) rather than living inside the supply; reported parts are a Panaflo FBH-09A12L (12 V, 0.14 A) or a Nidec Beta SL D09B-12PLL (12 V, 0.09 A).

Battery and Real-Time Clock

The Model 50 and 50 Z do not use a Dallas RTC module. The real-time clock is a Motorola MC146818A soldered to the planar (ZM47 on the Model 50, ZM8 on the 50 Z), and it is kept alive by a separate 6 V lithium battery that clips into the battery/speaker assembly at the front of the chassis.

• Battery/speaker assembly: FRU 33F5950.
• Battery: FRU 72X8498.

The clock chip holds 64 bytes of CMOS RAM. The first 50 bytes carry system configuration and security information; the clock itself uses the remaining 14 for date, time and battery level. The Model 50's assembly connects through a 6-pin card-edge connector on the planar; the 50 Z uses a 6-pin header.

Replacing the battery is a plug-and-clip job, not a desoldering job. Remove the assembly (press the retaining clip at the bottom of the assembly inwards — it sits right above the orange crystal holder — then lift the assembly away from the base), swap the cell, refit, then boot the Reference Diskette and restore the configuration. Be careful not to pull the battery header out of the planar while working elsewhere in the machine; doing so loses the configuration.

IBM's service check, from retain tip H024809: remove the battery from the assembly, set the meter to the 12 V DC range, and replace it if it reads below 5.5 V DC. Measuring it in circuit gives a misleading reading. Low battery voltage causes intermittent 161, 162 and 163 errors.

A useful early warning: when the cell weakens, the 32 kHz oscillator in the clock tends to stop before the CMOS contents are lost, so a machine whose time is frozen at the moment it was switched off — with no 161 or 163 yet — has a battery about to fail.

IBM's FRU order for 161, 163 and 165 is: set the time and date, then bus adapter, clock module, battery, system board. For 162 it is: run Setup and Automatic Configuration, then clock module, then system board — unless the machine also cannot run advanced diagnostics, in which case IBM points first at the diskette drive, then the system board, then the drive cable.

If you need to clear an unknown power-on password: power off, then short pins 2 and 3 on the battery/speaker assembly (or, if the assembly has no pins, short connector 1 to frame ground with a meter lead), power on, and remove the short once POST has finished.

The cover lock exists specifically to stop the battery being pulled and the password and configuration with it.

Connector Care

• Keyboard and pointing device. Two identical 6-pin mini-DIN connectors at the rear. The keyboard connector is marked with a small keyboard symbol moulded into the back panel; the unmarked one is the pointing device. The interface logic is the same for both, so swapping them does no damage but leaves the keyboard dead and produces a 3xx error. IBM's maximum allowable current is 275 mA at the keyboard port and 300 mA at the auxiliary device port.
• Keyboard/mouse fuse. F1 on the planar, 3 A / 250 V, FRU 72X8561. A blown fuse gives error 305 — IBM's FRU order for 305 is the fuse first, then the keyboard cable, and it notes that if there is no fuse in the system the board is the replacement.
• Serial and parallel ports. A DB-25 serial port and a parallel port on the rear panel.
• Micro Channel slots. The Model 50 has four channel connectors, one with the auxiliary video extension and one occupied by the fixed-disk adapter, giving three usable slots. The cards plug directly into the planar — there is no Micro Channel riser in this machine.

Drive Riser and Drive Interface

The diskette controller reaches the drives through a single 40-pin connector on the planar, with signals and supply routed via a bus adapter — the drive riser card — that plugs into it. That riser can run two 3.5-inch drives. The drives have no separate power connector: everything arrives on the one interface. The fixed disk plugs into its own dedicated adapter slot, which is separate from the three Micro Channel slots.

On the riser used with these machines, the resistor/capacitor pairs near the connectors are DC buffering and decoupling; earlier risers lacked them and suffered data loss when two drives were fitted.

Diskette Drive Maintenance

The machine ships with a 1.44 MB 3.5-inch drive. IBM's published figures for it: 80 tracks, 135 tracks per inch, two heads, 500 kbps transfer in high-density mode, track-to-track 6 ms, seek settle 15 ms, motor start 500 ms maximum, and 300 rpm ±1.5 %.

Use double-sided high-density media for 1.44 MB. IBM's own warning is that the drive should not be used to format a 1 MB diskette as 1.44 MB, nor a 2 MB diskette as 720 KB.

The spindle is direct drive. The Mitsubishi MF355C specification — the drive family IBM used here — states that the drive uses a DC brushless direct-drive motor coupled straight to the spindle at 300 rpm. There is no belt to replace.

Clean the heads with isopropyl alcohol on a foam swab. If a drive reads nothing at all, or errors get worse as it warms, the cause is usually the surface-mount electrolytics on its logic board rather than the heads — see IBM PS/2 Model 50 Capacitor Replacement Guide and Floppy Drive Repair.

Fixed Disk Maintenance

Model 50 family fixed disks

Machine	Drive	Interface	Access time
8550-021	20 MB, FRU 72X8522	MFM / ST-506, on adapter FRU 90X8070 or 90X8642	80 ms
8550-031	30 MB, FRU 6128287	Integrated drive-plus-controller on passive riser FRU 90X9441	39 ms
8550-061	60 MB, FRU 6128294	Integrated drive-plus-controller on passive riser FRU 90X9441	27 ms

Two things about these drives are commonly got wrong:

• The 20 MB drive on the 8550-021 is the only ST-506 drive that adapter supports.
• The drives IBM's error codes call ESDI on the desktop machines are not ESDI drives on a separate controller. They are a hard disk and a Micro Channel controller in one physical assembly; the riser is a passive interconnect. Genuine ESDI controllers with separate drives belong to the towers, the Model 60 and Model 80.

Stiction. A drive of this age that has stood for years may not spin up: the heads adhere to the platter surfaces and the spindle motor cannot break them free. Peter Wendt's advice is to unplug the unit, shake it vertically, then rotate it sharply clockwise a few times and try again — and explicitly not to drop the front of the case on the desk, which will kill the drive. Repeated power cycling also sometimes works but is hard on the drive and the supply. Whichever works, back the contents up immediately: the drive has been mistreated and will not improve.

Drive configuration is held in CMOS and set through the Reference Diskette. After a drive change, run Set Configuration.

Micro Channel Card Maintenance

Every Micro Channel card carries an adapter ID. The Reference Diskette reads it, looks up the matching Adapter Description File and assigns interrupts, DMA channels and addresses. After any card change:

1. Start from the Reference Diskette with the diskette already in the drive at power-on. Warm-booting it with Ctrl-Alt-Del can produce false errors and a false indication that a power-on password is set.
2. Select Set Configuration, then Automatic Configuration.
3. Supply the option diskette if a needed ADF is missing.
4. Save and restart.

A configuration that does not match what is fitted produces a 16x error; IBM's action for 165 is to set the configuration and features before suspecting the board.

Automatic Configuration is not free. IBM warns that running it can overwrite customised settings. Note the current settings with View Configuration first, or back them up with Backup Configuration, so they can be restored afterwards.

One self-inflicted fault worth knowing: some non-IBM video adapters drive the VGA signals rather than receive them. Fitting two adapters that both drive VGA will damage both.

Capacitor Health

The planars use surface-mount tantalum and ceramic capacitors, not surface-mount aluminium electrolytics, and are not subject to the electrolyte leakage seen on some later PS/2 boards. The parts that do leak on this machine are the surface-mount electrolytics on the diskette drive's logic board and on the integrated fixed-disk logic board. IBM published no capacitor values for any of it. Full procedure and sourcing: IBM PS/2 Model 50 Capacitor Replacement Guide.

Recommended Tools

• Philips screwdriver and an anti-static strap bonded to chassis ground.
• T10 security Torx bit, if you intend to open the power supply.
• Digital multimeter.
• Isopropyl alcohol and foam swabs.
• The plastic drive-structure removal tool stowed under the battery/speaker assembly, or a plastic spudger.
• Reference Diskette, plus the option diskettes for every fitted Micro Channel card.
• A spare 6 V battery, FRU 72X8498 or an equivalent.

See Recommended Tools for the general kit.

Related Pages

• IBM PS/2 Model 50
• IBM PS/2 Model 50 Troubleshooting Guide
• IBM PS/2 Model 50 Capacitor Replacement Guide
• IBM PS/2 Model 60 Maintenance Guide — the tower sibling
• Battery Refurbishment
• Floppy Drive Repair
• Hard Drive Maintenance and Repair
• Recommended Tools