Amstrad PPC640 Troubleshooting Guide
The Amstrad PPC640 reports most hardware faults through its power-on self test, and most of the rest are power, display, disc or keyboard faults. Amstrad's own fault-finding relied on its R.P.5-2 diagnostic tool and on exchanging boards, so the service manual has no fault-finding chart for the computer itself. What it does have is the schematics, the parts lists and the rail tolerances, and the Technical Reference Manual documents every power-on self-test message.[1][2] Routine care is in the Amstrad PPC640 Maintenance Guide.
Start-up sequence
[edit | edit source]At power-on or reset the firmware shows "Please wait" on the LCD, whatever the switch settings, and adds a dot as it works through the self tests in this order:[2]
- ROS (system ROM) checksum
- DMA controller (8237)
- Interval timer (8253)
- System status ports (8255 emulation)
- Real-time clock (146818)
- Serial port UART (8250)
- Parallel printer port
- System RAM
- Interrupt controller (8259)
- Disc drives (each head is stepped to track 10)
- Keyboard interface
After the tests it shows the sign-on line with the RAM size and ROS version, for example "Amstrad Portable PC 640K (V1.8)", the time and date, and the time it was last used, then asks for a system disc in drive A.[3][2] A soft reset (Ctrl+Alt+Del) skips the system RAM and video RAM tests.[2]
Power faults
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Nothing at all | No working source; adaptor or its plug; internal fuses | Try the batteries and another approved source. Check the wall socket and the adaptor's plug and fuse.[3] Then check F501 and F502 on the fuse board and the ICP protectors (below). |
| Works on one source but not another | Input jack, battery contacts or the input diode for that source | Check the jack J501, the battery tubes and contacts, and the Schottky diodes D510, D511 and D512 that join the battery, monitor and adaptor inputs.[1] |
| Resets when the power selector is moved | Normal behaviour | Moving between EXT ON and EXT OFF resets the machine.[3] |
| Alarm sounds with the keyboard closed | Keyboard switch SW502 | Normal if the power is on; switch off before closing.[3][1] |
| Alarm sounds during disc access | Batteries nearly exhausted | The extra load of the drives sets off the low-battery alarm early. Fit new alkaline cells.[3] |
| LCD fades while saving to disc on batteries | Low batteries under disc load | Temporary; the display returns when the write ends.[3] |
| "Please fit new batteries" and no time or date at start-up | Clock lost power | The clock runs from the main battery pack. Fit cells and set the time and date.[2][3] |
Fuses and protectors
[edit | edit source]The fuse board (PCB502) carries F501 and F502, both marked on the schematic as T3.15A 250 V time-delay fuses (printed "T 3 15A 250V"). The supply also has five ICP fusible protectors: ICP501 (ICP-N10), ICP502 (ICP-N5), ICP503 (ICP-N50), and ICP504 and ICP505 (ICP-N20).[1] Amstrad marks the fuses, the protectors and the regulators as safety-critical parts, to be replaced only with the types in the parts list. A protector opens on overcurrent, so find the cause before fitting a new one.
Supply rails
[edit | edit source].png)
| Rail | Tolerance | Load | Source | Feeds |
|---|---|---|---|---|
| +5 V | ±5% (4.75–5.25 V) | 0.7–2.5 A | Switching regulator, set by VR501 | Main board |
| +12 V | +10% / −20% | 0–50 mA | IC502 L78M12T | RS-232 drivers |
| −5 V | ±5% | 0–50 mA | IC503 L79M05T | Modem board |
| −12 V | ±20% | 0–50 mA | IC504 L79M12T | RS-232 drivers |
| −20 V | ±5% | 0–50 mA | IC505 TA79L020P, with Q508 on PCB504 | LCD |
Measure +5 V at the junction of L501 and C514 with the PPC running from the adaptor; Amstrad's adjustment is VR501.[1] The switching circuits use one half of an NE556 (IC501), the switching transistors Q502 to Q505 and Q507, and the 200 µH choke L501.[1] If +5 V is low and will not adjust, check the electrolytics on the supply first (Amstrad PPC640 Capacitor Replacement Guide).
A crowbar sits across the +5 V rail: the thyristor SCR501 (SF5B42), triggered through the 6.2 V zener D529 and R526 (33 Ω).[1] If the rail rises far enough for D529 to conduct, the thyristor shorts the rail. A +5 V rail held near zero with an open protector can therefore follow a regulator fault that pushed the rail high, so check the regulator before restoring power.
Display faults
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| LCD blank after "Please wait", CRT indicator lit | Switch 1 selects the external monitor and none is connected | Press Scroll Lock and the right-hand Ctrl key to switch to the LCD, or set switch 1 for the LCD and reset.[3] |
| Screen goes blank after "Please wait" | Switches 4 and 5 disagree with switch 2 (for example, monochrome default with the adapter set to CGA) | Make the switches consistent and reset.[2] |
| Display faint or hard to read | Contrast setting; ambient light (the LCD is not backlit) | Adjust the contrast wheel to the right of the screen. One owner found his PPC640's LCD usable mainly in direct light.[3][4] |
| Colour programs unreadable on the LCD | Rendering mode | With Scroll Lock on, press the right-hand Alt (Gr) key to switch between Amstrad's two colour-to-mono methods.[3] |
| External monitor works, LCD dead | LCD supply or cable; LCD module | Check the −20 V rail and the LCD cable. MCbx notes the cable is held by connector screws and can come out when the case is opened.[5] Amstrad supplied the LCD only as a complete exchange unit (part 177245).[1] |
| Amstrad monitor connected to the DIN socket shows nothing | The DIN socket carries power only | Connect the monitor's video lead to the 9-pin video socket; the PPC then runs from the monitor's supply.[5][3] |
Self-test messages
[edit | edit source]A failed self test stops the machine with one of these messages, after switching the display to 80-column text. Switch off and on again to retry.[2][3]
| Message | Test described in the Technical Reference Manual |
|---|---|
| Error: Faulty ROS ROM checksum | Sum of every byte in the 16 KB system ROM |
| Error: Faulty direct memory access controller | Addressing of the DMA controller's registers |
| Error: Faulty interval timer | Read/write path to counter 2 and the counting rate of counter 1 |
| Error: Faulty real time clock | Seconds counter rate and the NVR checksum byte. The firmware also shows this message when the system status port (8255 emulation) test fails. |
| Error: Faulty system serial port | The 8250 in loop-back mode at 9600 baud |
| Error: Faulty system printer port | Read/write path to the printer data latch |
| Error: Faulty SYSTEM RAM | Data path and address test on all system RAM |
| Error: Faulty interrupt controller | Mask register and an interrupt acknowledge |
| Error: Faulty floppy disk controller or disk drive | Seek to track 10 on each drive (the firmware does not check the track reached) |
| Error: Faulty VDU RAM | No test description given |
| Error: Faulty VDU controller | No test description given |
| Error: Faulty system status register | No test description given |
| Error: Faulty memory (parity error) | A RAM parity error (NMI) at any time; the machine hangs until switched off |
Non-fatal messages:[2]
- "Check keyboard" (the user manual prints "Check Keyboard"; the TRM gives "Check keyboard and mouse"): the keyboard did not return its self-test pass code, 0AAh. The firmware repeats the test, beeping every five seconds, until it passes. Clear anything resting on the keys and look for a stuck key.[3]
- "Error: External ROM checksum incorrect: ROM address = nnnnnh": an add-on ROM in the expansion bus failed its checksum.
- "Insert a SYSTEM disk into drive A / Then press any key": the boot sector could not be read after ten tries.
The system RAM is eighteen KM41256-15 and six KM4164B-15 DRAMs, IC135–158, on the memory and power supply board.[1]
Diagnostic mode
[edit | edit source]The three option links LK1–LK3 on the lower board normally select the language of the messages. With all three fitted, the firmware runs only the keyboard and disc tests and ignores their failures, so that the machine can be brought up for diagnostic testing.[2] The Technical Reference Manual reserves the ROM area E0000–E7FFF for the PPC test board, and MCbx reports that diagnostic mode looks for a diagnostic ROM there.[2][5] Amstrad's dealer tool for in-depth fault-finding was the R.P.5-2, with an upgrade kit for the PPC.[1]
Disc faults
[edit | edit source]- "Insert a SYSTEM disk" with a system disc in drive A: try another system disc and check it with CHKDSK. Amstrad's manual says the drive may simply need cleaning.[3]
- Discs written on another drive will not read: Amstrad lists head misalignment, low head output, a motor speed difference and a format difference as the causes of this interchangeability error.[1] The drives are 720 KB double-density units.
The FD-5 section of the service manual has a trouble analysis table for nine failure types:[1]
| Trouble | Causes listed by Amstrad |
|---|---|
| Index detection failure | Direct-drive motor control board or motor (replace the motor assembly), index LED or detector, board motor-on or index circuit |
| Not ready | As above, or the board's ready circuit |
| Track 00 not detected | Track 00 assembly, board detection circuit |
| No seek | Stepper motor, guide shaft contaminated or damaged, stepper driver circuit |
| No write | Index faults, head disconnected or shorted, write circuit |
| No read | Index or head faults, read circuit |
| Read error | Alignment, limiter or track 00 flag (adjustable); azimuth and burst (not adjustable); index period (replace the motor); read circuit |
| In-use LED does not light | LED, board circuit |
| Write protect not detected | Write protect switch or circuit |
Amstrad's policy was to exchange a faulty drive complete; its alignment procedures need alignment discs and a drive exerciser.[1] See Floppy Drive Repair.
Keyboard
[edit | edit source]The keyboard has its own controller, IC801 (AMS40112) with a 6 MHz ceramic resonator, X801, and talks to the main board over a two-wire serial clock and data link.[1][2] The main board resets the keyboard controller by holding the clock line low for 10 ms or more, and the controller answers with 0AAh when its own test passes.[2] A permanent "Check keyboard" with nothing pressing on the keys points to a stuck key, the keyboard cable or the controller.
Modem
[edit | edit source]- Amstrad says not to use the modem on battery power.[3]
- Software must address the modem as COM2 (2F8h, IRQ3).[2]
- The modem board takes its −5 V from the main supply; check that rail if the modem does not respond.[1]
- On British machines the line plugs into the BT 431A connector in the covered compartment to the right of the LCD; US machines have LINE and PHONE sockets at the rear.[2][3]
Related pages
[edit | edit source]- Amstrad PPC640 · Amstrad PPC640 Maintenance Guide · Amstrad PPC640 Capacitor Replacement Guide
- Amstrad PPC 512 / PPC 640 Service Manual · Amstrad PPC 512 / PPC 640 User Instructions
- Capacitor Failure Symptoms · Battery Explosion, Capacitor or Corrosion Damage
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Amstrad plc, PPC512S/512D, PPC640S/640D Personal Portable Computer Service Manual. Hosted as File:Amstrad PPC 512 PPC 640 Service Manual.pdf, described at Amstrad PPC 512 / PPC 640 Service Manual. Notes to service engineers p. 2; FD-5 trouble analysis pp. 3–5; alignment p. 14; power supply schematic p. 26; modem schematics pp. 27–28; electrical parts list pp. 29–31.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 Amstrad Consumer Electronics plc, Amstrad PPC Technical Reference Manual (1987), transcribed by John Elliott at seasip.info. Retrieved 2026-10-02. Section 1.9 (real time clock), 1.10.3 (option links), 1.16 (modem), 1.20 (power connectors), 1.21 (switch settings); section 2.1 (power-up initialisation), 2.2 (self tests), 2.3.1 (parity error), 2.6 (ROS messages).
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 Amstrad plc, The Amstrad Portable PPC 512/640 User Instructions (US edition, first edition 1988). Hosted as File:Amstrad PPC 512 PPC 640 User Instructions.pdf, described at Amstrad PPC 512 / PPC 640 User Instructions. Batteries and alarms p. 9; switching on pp. 13–14; start-up p. 41; switching off p. 43; external monitor and switches pp. 317–318; troubleshooting pp. 327–328; hardware error messages p. 339.
- ↑ GXL750 and others, "My newest old computer, an Amstrad PPC", VOGONS forum, June 2012. Retrieved 2026-10-02. An owner's PPC640 with poor LCD contrast except in strong light.
- ↑ 5.0 5.1 5.2 MCbx, "Amstrad PPC512", oldcomputer.info. Retrieved 2026-10-02. Disassembly, the LCD cable, the DIP switches and the option links.