Amstrad PC1640 Troubleshooting Guide

The Amstrad PC1640 is diagnosed the same way as the Amstrad PC1512: the supply is in the monitor, the BIOS runs a self-test that prints a dot for each stage, and Amstrad's own tools were the RP5-2 diagnostics card and its display test disc. The Amstrad PC1512 Troubleshooting Guide has the full self-test table and the supply checks; the faults below are the ones reported on the PC1640 itself. Procedures are in the Amstrad PC1640 Maintenance Guide.

PC1640 CPU board, top view (service manual p. 28)

Self-test

After "Please wait" the BIOS prints a dot as each test starts: ROM checksum, DMA controller, interval timer, status ports, real-time clock, serial port, parallel port, mouse counters, system RAM, interrupt controller, floppy drives and keyboard. A failure stops the machine with a message, except the keyboard test, which repeats. Fitting all three links LK1–LK3, next to IC113, makes the BIOS run only the keyboard and floppy tests and carry on past a failure; with the links fitted a PC1640 reports 512K instead of 640K, which owners have confirmed is normal in this mode.

Floppy controller error

"Error: Faulty Floppy Disk Controller or Disk Drive" appears when the BIOS cannot make drive A: seek. The disc test is treated as fatal, so a PC1640 with no working drive A: will not reach the boot prompt.

1. Check that drive A: is powered from its Molex plug and jumpered to the first drive select (DS0), and that the card-edge connector is the right way round; a drive LED that lights as soon as the power is applied means the cable is reversed.
2. Test the drive in another PC if possible.
3. If the drive is good, watch drive select 0 during the self-test. On a working board, pin 2 of IC117 (74HC240) goes high, and pin 18 low, a few seconds after switch-on, and the drive LED comes on.
4. If drive select never changes, the floppy controller's interrupt may not be reaching the interrupt controller. The interrupt passes through IC110, a 74LS125, whose +5 V supply comes through R181, a 5.6 Ω fusible resistor.

On the board in this report, R181 had failed open, leaving IC110 without power. A new resistor smoked within seconds because C124, a 100 µF capacitor on the same feed, had failed short. With C124 removed and R181 replaced the machine passed its self-test and booted. Replace C124 as well as R181; the owner used a 5.1 Ω ¼ W fusible resistor, and other owners measured about 5.8 Ω across a good R181 in circuit. Amstrad's parts list gives R181 as a 5.6 Ω fuse resistor, part 175089. See the Amstrad PC1640 Capacitor Replacement Guide.

Power

The computer's rails come from the monitor through the 14-way DIN cable; check them at the plug as in the PC1512 guide. Owners who lack a working monitor power the PC1640 from a standard supply through the same pinout and use a separate display card with switch 10 on. A PC1640 can also be powered through a prototyping card in an expansion slot, since nothing sits between the supply input and the slots on the main board.

Monitor supply faults reported by owners

Monitor	Symptom	What was found
PC-MD	Supply dead	STK7358 hybrid regulator shorted between pins 12, 13 and 14; both optocouplers and a transistor also failed
PC-ECD	Computer runs, screen stays black	The 5 V regulator IC503, feeding the TTL decoder board, gave 0.92 V from a 15 V input. A new regulator brought the picture back; the picture then rolled until the vertical hold was set
PC-ECD	Thought dead	Worked once the brightness and contrast controls were turned up

The PC-MD's supply regulator is IC501, an STK7358, with BA6993, LA6324 and BA707 control ICs. On the PC-ECD the decoder board is soldered to the main board and has to be desoldered to work on it.

Display

• No picture, computer otherwise working: check the DIP switches against the monitor before anything else. An owner who knocked them during a repair lost the picture until they were set back for the PC-MD. The chart is in the maintenance guide.
• Third-party monitor on the internal adaptor shows nothing: owners report that the IGA works with Amstrad's monitors but not with ordinary MDA or CGA monitors, and run other monitors from an add-in card with the IGA disabled. John Elliott explains that the IGA behaves as an EGA unless the BIOS or a DOS utility sets bit 6 of port 3DBh to switch on CGA or MDA emulation.
• Do not select a Hercules 720 × 348 mode with the PC-ECD connected. DOS Days warns that its higher horizontal frequency gives a scrambled picture and can damage the monitor. The ECD runs at 21.8 kHz for 350 lines and 15.85 kHz for 200 lines.

Memory

On the Schneider board in one repair report the eighteen main memory chips are soldered to the board with no sockets, which makes a RAM fault slow to find.

• Memory parity errors and random hangs: DOS Days had both on its PC1640, and in the end the machine stopped starting at all; a replacement main board cured it.
• Corruption that follows a pattern, such as missing characters in BIOS messages: on the leak-damaged Schneider board, an address line was stuck high after the corroded CPU socket was replaced. Ruud's Diagnostic ROM, split into even and odd halves for the PC1640's two ROM sockets and used with a separate CGA card, located it.
• Owners build the ROM pair from 27C256 EPROMs, using a 64 KB file of eight copies of the 8 KB image split into even and odd bytes.

Backup cells

"Please fit new batteries" means the AA cells are flat or missing, and the configuration has been reset. Leaking cells have run down onto the CPU and 8087 sockets on at least one board; on that machine the CPU had died and the replaced socket left an address line fault. Look at the board under the tray on any machine that has had cells left in it.

Related pages

• Amstrad PC1640 · Amstrad PC1640 Maintenance Guide · Amstrad PC1640 Capacitor Replacement Guide
• Amstrad PC1512 Troubleshooting Guide · Amstrad PC1640 VDU Tests Manual
• Capacitor Failure Symptoms · CRT Discharge Procedure · Battery Explosion, Capacitor or Corrosion Damage · Floppy Drive Repair