IBM PS/2 Model 25 Troubleshooting Guide

IBM PS/2 Model 25. Photo: Museo de Informática, Wikimedia Commons, CC BY-SA 2.0.

IBM's service procedure for the IBM PS/2 Model 25 (machine type 8525) is in its 1994 Personal System/2 Hardware Maintenance Manual, which covers the Model 25, 25 286 and 25 SX with the other AT-bus PS/2s (Models 25, 30, 33, 35, 40 and 55). It works down to field-replaceable units (FRUs). On the Model 25 the power supply is inside the display assembly, and IBM's FRU for it is the whole display assembly. "Display Assembly (8525)" appears in IBM's lists for power and drive faults.

Which machine you have

The three Model 25 system boards

Machine	Models	Clock and battery	Password jumper
Model 25 (8086)	001, G01, 004, G04, L01, L04	No real-time clock and no battery	None
Model 25 286	006, G06, 036, G36	Dallas DS1287 clock module at ZM35, FRU 8509237	J13
Model 25 SX	K00, K01, L02	CR2032 cell at B1, FRU 33F8354	J2

Sources: ardent-tool's board and battery pages and IBM's manual. The 8086 machine's BIOS reports "No Real-time clock", so it cannot give battery or clock errors.

General checkout

IBM's starting point for all AT-bus PS/2s:

1. Switch off the computer and all external devices.
2. Check all cables and power cords.
3. Make sure there are no diskettes in the drives.
4. Switch on the external devices, then the computer.
5. If a POST error code appears, go to the symptom-to-FRU index below. If several appear, deal with the first one first; it can cause the others.

Configuration and Advanced Diagnostics run from the model's Reference or Starter Diskette (Ctrl-A at the main menu for Advanced Diagnostics). Power on with the diskette already in the drive: soft-booting it can cause false errors and a false report that a power-on password is set. If the diagnostics do not fail, replacing parts will probably not help.

IBM symptom-to-FRU index

Selected entries from IBM's AT-bus index, in IBM's order of FRUs to try:

No-beep and beep symptoms (HMM pp. 12 to 13)

Symptom	FRUs, in order
No beep, power indicator off, fan not running (check the voltage switch)	Power supply
No beep during POST (check the voltage switch)	System board; power supply; display assembly (8525); memory module (8525); any adapter; bus adapter
No beep, fan runs, indicator on, hangs in POST with a message	System board; power supply
No beep, otherwise working	Speaker assembly
One long and two short beeps	System board; display adapter
More than one beep	System board; any adapter
Continuous beep	System board
Repeated short beeps	Keyboard (stuck key?); keyboard cable; system board

Other symptoms and error codes (HMM pp. 13 to 17)

Symptom or code	FRUs, in order
Blank or unreadable display	System board; power supply; display adapter; display assembly; system board video cable
Insert-diskette icon, or the system starts BASIC	Diskette drive; system board; diskette cable; network adapter
Incorrect memory size, or PARITY CHECK 1	System board (see memory isolation below)
Real-time clock loses time	Bus adapter; clock module; system board (often software; see below)
161, 163, 165	Set time and date; bus adapter; clock module; battery; system board
162	Run setup or Automatic Configuration; clock module; system board
1xx	System board
2xx	System board (see memory isolation below)
302	Keylock locked?; keylock assembly; system board
3xx (other)	Keyboard; system board; keyboard cable
604, 662	Wrong diskette drive type
663	Wrong media type
6xx (other)	Diskette drive; external drive adapter; system board; power supply; display assembly (8525); diskette drive cable
17xx	Hard disk drive; system board; hard disk cable; power supply; display assembly (8525)
2401, 2402, if screen colours change	Display
2401, 2402, if screen colours are correct	System board; display

For 305, a keyboard voltage error, IBM's order is the keyboard port fuse, the keyboard cable, then the system board, and if there is no fuse the system board. The keyboard and mouse fuse is F1 on the Type 2, 286 and SX boards; the Type 1 8086 board has F1 and F2. IBM's diskette-error note is that most 6xx codes are media errors: try a known-good diskette, and replace the drive if the error comes back.

Dead machine

IBM's check for a machine that shows no sign of power: confirm the power cord is in place and has continuity, and that the voltage switch is set correctly. If the cord is good, either the supply is defective or a device is making it shut down; measure the supply voltages, replace the supply if they are wrong, and go to IBM's undetermined-problem procedure if they are right.

Ardent-tool's transcription of IBM's dead-system procedure adds that a short from a device, a badly fitted memory module or a faulty adapter stops the system board presenting Power Good to the supply within 150 ms, and the supply then shuts down. Its listed causes include a SIMM fitted backwards, an adapter not fully seated, an unsupported adapter, and too many drives starting at once. Loose line cords cause intermittent faults and unexpected resets; check both ends.

1. Remove all adapters, options and extra memory, and all external connections.
2. Switch on and listen for a beep.
3. No beep: check the speaker; if it is good, replace the system-board memory and try again; if there is still no beep, measure the supply. Correct voltages mean the system board; wrong voltages mean the supply.
4. A beep: refit the removed items one at a time until the fault returns.

Supply voltages

Measure at J7 on the system board with the machine switched on. Pin 1 is towards the rear.

Models 25, 25 SX and 25-286, connector J7

Rail	− lead	+ lead	Minimum	Maximum
+5 V	J7-5	J7-10	+4.8 V	+5.2 V
−5 V (leads reversed)	J7-9	J7-5	+4.5 V	+5.4 V
+12 V	J7-5	J7-3	+11.5 V	+12.6 V
−12 V (leads reversed)	J7-4	J7-8	+10.8 V	+12.9 V

Power Good (J7-1) should read +3.5 to +5 V on a working machine. If it does not, remove all adapters, cables and extra memory and try again; if Power Good is then correct, refit the devices one at a time to find the one causing the short. The supply's voltage switch must be set to the 100–125 V or 200–240 V range before the cord is plugged in.

For the 8086 machine, IBM's under-voltage sense levels are +3.2 V, −2.4 V, +10.5 V and −8.6 V on the +5, −5, +12 and −12 V outputs, and Power Good is active at 2.4 to 5.25 V. Ardent-tool's shared 8525/8530 power page gives +4.5, −4.3, +10.8 and −10.2 V, which are the figures in IBM's Model 30 Technical Reference.

Memory isolation (8086 machine)

IBM's procedure for the 8086 Model 25:

• PARITY CHECK 1 means system-board memory has failed. With 512 KB fitted, replace the memory module packages. With 640 KB fitted and an address below 20000 shown under the message, replace the optional memory modules; otherwise replace the memory module packages.
• PARITY CHECK 2 means adapter memory has failed.
• Memory errors appear as AAAAAA BB CCCC 20X. For a 205 error, each non-zero digit of CCCC points at a module: from left to right U35, U34, U22 and U16. If CCCC is 0000, the sixth digit of AAAAAA decides: odd (1, 3, 5, 7, 9, B, D, F) means ZM1, otherwise ZM2.

Display

The 2401 and 2402 entries above use the screen colours to split display from system board. On the 8086 machine, intermittent wrong characters or unexpected underlining in applications that load more than one font are covered by IBM's ECA 023: boards with a gate array marked 72X8300 at U11 were replaced with system board FRU 96F7390.

Clock, battery and password

• The Model 25 286 keeps its clock and configuration in the DS1287 module. Ardent-tool lists the DS12887 and DS12887+ as replacements, and the 'A'-suffix parts as usable; it warns against the DS12C887, which uses the century byte differently and apparently does not work in a PS/2.
• The Model 25 SX uses a CR2032. Measure it out of the holder: it should be 2.5 to 3.7 V, and it should be replaced below 2.5 V. Low voltages can corrupt CMOS bits.
• A weak battery loses all the setup information; if only part is lost, the battery is probably not the cause. IBM notes that the time-of-day clock is interrupt-driven and drifts with interrupt activity; check the time in the Reference Diskette's Set Features menu, and if it is right the problem is software.
• To remove a forgotten power-on password on the 286 (J13) or SX (J2): switch off, unplug, remove the cover, move the jumper to join the centre pin and the pin at the other end, and switch on. The password is erased, and the jumper does not need to be moved back. Reset the date, time and configuration afterwards.

Related pages

• IBM PS/2 Model 25
• IBM PS/2 Model 25 Maintenance Guide
• IBM PS/2 Model 25 Capacitor Replacement Guide
• IBM PS/2 Model 30 Troubleshooting Guide
• Capacitor Failure Symptoms