Amstrad Mega PC Troubleshooting Guide
Neither of Amstrad's manuals for the Amstrad Mega PC has a fault-finding chart. The checks below come from the PC7386SX schematics, which Amstrad says apply to the Mega PC's main unit, the Mega PC service manual's games card, front panel and monitor sections, and the rating label on the machine.[1][2] Procedures are in the Amstrad Mega PC Maintenance Guide.
Power
[edit | edit source]The power supply module's rating label gives these outputs, from a 100–240 V AC input:[3]
| Rail | Rated current (label) | CON1 pins (schematic 1/13) |
|---|---|---|
| +5 V | 7.5 A | 2, 10, 11, 12 |
| +12 V | 2 A | 3 |
| −12 V | 0.2 A | 4 |
| −5 V | 0.1 A | 9 |
| Ground | – | 5, 6, 7, 8 |
| Power Good | – | 1 |
The pin numbers are read from the CON1 power header on schematic 1/13.[2] The supply module is replaced as one part (273888); neither manual includes its circuit.[1][2]
| Symptom | Probable cause | Action |
|---|---|---|
| Completely dead, no lamps | Mains lead, rear switch, supply module | Check the mains lead and switch, then measure the rails at CON1 with the board connected |
| Power lamp lit, no start | Low rail, Power Good absent, short on a card | Measure +5 V and Power Good at CON1; remove the games card and any added card and try again |
| Monitor dead, system running | Rear AC outlet, monitor fuse | The outlet is rated 1 A on the label; the PC14DSM fuse F001 is a TL 2.5 A 250 V type |
Keyboard and mouse
[edit | edit source]Schematic 1/13 feeds the +5 V to the keyboard and mouse sockets (CON3, CON4) through F1, a 2 A fuse resistor (BEL 2A MQ, part 273945), and the inductor L2.[2] If both the keyboard and the mouse are dead, measure +5 V on the socket and check F1. The keyboard and mouse are handled by the 8042 controller (U32).[2]
Setup lost at switch-on
[edit | edit source]The real-time clock and setup memory run from a 3.6 V battery on the main board, through D8 and R26 to the WD76C20 (U33); a four-pin header, J9, takes an external battery.[2] A machine that forgets its setup, its date or its hard disc type has a flat or leaking battery. Check the board around the battery for corrosion before fitting a new one; see Battery Explosion, Capacitor or Corrosion Damage. The rear of the photographed Mega PC carries a "Hard disk drive type 17" label.[3]
Picture
[edit | edit source]The PC and Mega Drive pictures leave through the same 15-pin VGA connector. The PC side runs at 31 kHz and the Mega Drive at 15 kHz, so an ordinary VGA monitor shows the PC but not the games.[4][5] Amstrad's PC14DSM alignment procedure switches the monitor between the two with a mode select signal on pin 15: high for game mode, low for VGA mode.[1]
| Symptom | Probable cause | Action |
|---|---|---|
| PC picture good, games scroll or roll | Game-mode H hold on the monitor (VR508), or a monitor without 15 kHz support | Use the PC14DSM or another dual-sync monitor; on the PC14DSM set VR508 by Amstrad's procedure |
| Games good, PC picture rolls | VGA-mode H hold (VR509) | Set VR509 by Amstrad's procedure |
| Picture shifted sideways in one mode | H phase or H hold setting | Follow the H hold and H phase steps of the alignment procedure in order |
| No PC picture on any monitor | VGA controller WD90C11 (U50), RAMDAC ADV471 (U57), clock synthesiser ICS1394 (U55), video RAM | Check +5 V at the video section and reseat any added video RAM in U53 and U54 |
Sources: Amstrad's alignment procedure[1] and the PC7386SX schematics.[2]
Games side
[edit | edit source]| Symptom | Probable cause | Action |
|---|---|---|
| Cartridge does not start | Front slider or mode switch in PC position, cartridge detector, dirty contacts | Move the slider to the games side; clean the cartridge and the 98-pin connector; check the cartridge detector switch (SW3) and the PC/Sega mode switch (SW2) |
| No reset on the games side | Reset switch SW1 | Check SW1 on the front panel board |
| No sound | Volume control VR1, amplifiers NJM2070M (U31, U32) | Check the rear volume control and the headphone socket, then the amplifiers on the games card |
| Mega CD or 32X will not work | Not supported without modification | Sega Retro says the Mega PC cannot interface with either without radical modifications |
Sources: the Mega PC daughter board circuit and parts list,[1] and Sega Retro.[4] The Mega PC's control pads are internally the same as Sega's, so a Sega pad can be used to rule out a faulty pad.[5]
The games card also carries the PC's AdLib-compatible sound, built on a Yamaha YM3812 (U3) and YM3014B DAC (U6), so PC sound also depends on the games card.[1]
Ports
[edit | edit source]The serial ports use MC1488A and MC1489A line drivers and receivers (U38–U42), which run from the ±12 V rails; the parallel port is driven by the WD76C30 (U17).[2] A serial port that fails while the parallel port works points at the ±12 V supply or the line drivers.
Related pages
[edit | edit source]- Amstrad Mega PC · Amstrad Mega PC Maintenance Guide · Amstrad Mega PC Capacitor Replacement Guide
- Sega Mega Drive / Genesis (Model 1)
- CRT Discharge Procedure · Hard Drive Maintenance and Repair · Floppy Drive Repair
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Amstrad plc, MEGAPC3H40 with Games Card / PC14DSM 14" Dual Synch Colour Monitor Service Manual. Hosted as File:Amstrad Mega PC Service Manual.pdf, described at Amstrad Mega PC Service Manual. Specification (PDF p. 2); note to engineers (p. 3); parts lists (p. 4); daughter board circuit (p. 7); PC14DSM alignment procedure (p. 25); PC14DSM parts lists (pp. 31–32).
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Amstrad plc, PC7386SX Personal Colour Computer Service Manual, first published October 1992. Hosted as File:Amstrad PC7386SX Service Manual.pdf, described at Amstrad PC7386SX Service Manual. Schematic diagrams 1/13 (power header CON1, F1, front panel CON2/CON11), 6/13 (battery), 7/13 (serial and parallel), 12/13 (video clock and RAM), 13/13 (RAMDAC and VGA) on pp. 5, 10, 11, 16 and 17; parts lists p. 19.
- ↑ 3.0 3.1 Bungle, "MegaPC Rear.jpg", Wikimedia Commons, 2007 (public domain). Retrieved 2026-10-02. Rating label of a Mega PC (model MEGAPC 386SX).
- ↑ 4.0 4.1 Sega Retro, "MegaPC". Retrieved 2026-10-02. Shared VGA output and timings, the front slider, Mega CD and 32X limits, region locking.
- ↑ 5.0 5.1 Wikipedia, "Amstrad Mega PC". Retrieved 2026-10-02. VGA connector, 15 and 31 kHz outputs, SCART use for the Mega Drive.
