Jump to content

Amstrad Mega PC Maintenance Guide

From RetroTechCollection
Mega PC main board VSC386SXD V.2.01. The backup battery is the blue cylinder at top left, beside the power header.

The Amstrad Mega PC is two machines in one case: a PC7386SX main board with its power supply, drives and SIMMs, and a Mega Drive games card in one of the ISA slots. Amstrad's Mega PC service manual covers the games card and the PC14DSM monitor and sends engineers to the PC7386SX service manual for everything else.[1][2] Faults are in the Amstrad Mega PC Troubleshooting Guide and capacitor values in the Amstrad Mega PC Capacitor Replacement Guide.

Safety

[edit | edit source]

Warning: the system unit's power supply module and the PC14DSM monitor carry mains voltage, and the monitor's CRT holds a high-voltage charge after switch-off. See CRT Discharge Procedure.

The rating label on the rear of the system unit says to disconnect it from the mains before removing the top cover or any screws.[3] Amstrad's tests after any work are a flash test at 1.5 kV RMS for 3 s between the live and neutral of the mains lead and all accessible metal, an insulation test at the same voltage showing more than 4 MΩ, and an earth continuity of no more than 0.5 Ω on the mains lead. Parts with the warning-triangle prefix are safety items to be replaced like for like.[2]

Opening the case

[edit | edit source]

Amstrad's exploded views show the cover (A-18) held by screws along its rear edge and sides, the floppy drive (DE-2) and hard disc (DE-1) on a bracket at the front, the power supply module (DE-3) at the rear right, and the expansion card held in the ISA riser.[1][2]

  1. Switch off and unplug the mains lead, the monitor lead and the monitor's power lead from the rear outlet.
  2. Remove the cover screws and slide the cover off.
  3. Note the riser, the games card and the cable routing before removing anything.
  4. Wear an earthed wrist strap when handling the boards and SIMMs.

Backup battery

[edit | edit source]

Schematic 6/13 shows a 3.6 V battery, BATT, feeding the WD76C20's battery input (VBATT) through a 1N4148 and a 1 kΩ resistor (D8, R26). A four-pin header, J9 (marked EXT. BATT on the board), takes an external battery through D7 and R27, and jumper JW1 sits beside it.[2] On the board photographed for Commons the battery is a blue cylindrical cell soldered beside the power header.[4]

  • Inspect the battery and the board around it for leaked electrolyte and green or white corrosion on the tracks, the power header and nearby parts. See Battery Explosion, Capacitor or Corrosion Damage for cleaning.
  • If the battery has leaked or the machine loses its setup, remove it and either fit a replacement of the same voltage or connect an external battery to J9.
  • Record the hard disc type before removing the battery. The rear of the photographed Mega PC carries a "Hard disk drive type 17" label.[3]

Memory and video RAM

[edit | edit source]

Amstrad's specification gives 1 MB of RAM on four SIMM sockets, expandable to 16 MB, and 256 KB of video memory, expandable to 512 KB.[1] The Mega PC parts list gives the fitted modules as 256K SIMMs (272201).[1] Wikipedia gives the standard fit as four 256 KB 30-pin SIMMs and the maximum as four 4 MB modules.[5] The PC7386SX schematic 12/13 draws two 44256 video RAMs (U51, U52) and two more (U53, U54) inside a box marked "512K OPTION".[2] Sega Retro describes the same upgrade as two extra 1-Mbit chips.[6]

  • Reseat the SIMMs and clean their contacts if the machine reports memory errors.
  • Fit the video RAM option at positions U53 and U54 with chips of the type drawn (44256).

Drives

[edit | edit source]

The Mega PC parts list gives a 40 MB Seagate hard disc (DE-1, 273908) and a Sony 3.5-inch floppy drive (DE-2, 273723).[1] See Hard Drive Maintenance and Repair and Floppy Drive Repair.

Cartridge slot and front controls

[edit | edit source]

The cartridge goes into a 98-pin card-edge connector (DE-18, 274035) behind a door with its own spring and switch guide in the front panel.[1] Amstrad's daughter board circuit shows a PC/Sega mode switch (SW2), a reset switch (SW1) and a cartridge detector switch (SW3).[1] Sega Retro says the front slider is moved one way to open the cartridge slot and control pad sockets and the other way to use the floppy drive.[6]

  • Clean the cartridge's edge contacts with isopropyl alcohol and let them dry.
  • With the case open, inspect the 98-pin connector for bent or dirty contacts.
  • Check that the slider and door move freely and operate their switches.

Monitor (PC14DSM)

[edit | edit source]

The PC14DSM is a 14-inch dual-sync colour monitor with built-in speakers.[1] Its parts list gives the mains fuse F001 as a TL 2.5 A 250 V type and lists a posistor (PR001, PTH451C26) and a degaussing coil (V001).[1]

Amstrad's alignment procedure uses a Mega PC running the KENVGA.EXE test program. It switches the monitor between game mode and VGA mode with the mode select signal on pin 15, high for game mode and low for VGA. The main settings are:[1]

PC14DSM adjustments (alignment procedure)
Adjustment Control Method
H centring S002 (H raster centre switch) With no signal, set the screen control so the raster is just visible, then centre it with S002
H hold, game mode VR508 Short R502 with a 1 µF 50 V capacitor to break H sync, then set VR508 where the picture stops drifting
H hold, VGA mode VR509 As above in VGA mode, then remove the capacitor
H phase V904 (as printed) Centre the data area horizontally
Focus FBT focus control Best focus on an "H" pattern
Side pincushion VR434 Straight sides on a 480-line cross-hatch
V linearity VR422 Equal vertical cross-hatch spacing
V centre VR903 Centre the picture vertically

Set the controls before adjusting: screen and brightness (VR902) at centre, contrast (VR901) at maximum, sub-contrast (VR103) and the R, G and B bias controls (R817, R827, R807) a quarter from minimum, others at centre. The monitor should face east, away from magnetic fields.[1] Discharge the CRT before working near the anode or the flyback transformer.

Tools

[edit | edit source]
  • Cross-head screwdrivers and an earthed wrist strap
  • Digital multimeter
  • Isopropyl alcohol and lint-free swabs
  • For the monitor: insulated trimming tools and a signal source (a Mega PC with a test program)
  • See Recommended Tools.
[edit | edit source]

References

[edit | edit source]
  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 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); MEGAPC3H40 parts lists (p. 4); exploded view (p. 5); PC14DSM alignment procedure (p. 25); PC14DSM parts lists (pp. 31–32).
  2. ↑ 2.0 2.1 2.2 2.3 2.4 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. Technical specification and safety tests p. 2; main PCB top view p. 3; schematic diagrams 1/13 (power header, F1), 2/13 (SIMMs), 6/13 (battery), 12/13 (video RAM) on pp. 5, 6, 10 and 16; cabinet exploded view p. 18; parts lists p. 19.
  3. ↑ 3.0 3.1 Bungle, "MegaPC Rear.jpg", Wikimedia Commons, 2007 (public domain). Retrieved 2026-10-02. Rear panel and rating label of a Mega PC (model MEGAPC 386SX).
  4. ↑ Mister rf, "Amstrad VSC386SXD V2.01.jpg", Wikimedia Commons, 2022 (CC BY-SA 4.0). Retrieved 2026-10-02. Mega PC main board marked VSC386SXD V2.01, with the blue cylindrical battery at top left.
  5. ↑ Wikipedia, "Amstrad Mega PC". Retrieved 2026-10-02. Four 256 KB SIMMs as standard and four 4 MB SIMMs as the maximum.
  6. ↑ 6.0 6.1 Sega Retro, "MegaPC". Retrieved 2026-10-02. VRAM upgrade, front slider, Western Digital chipset, Mega CD and 32X limits.


Retrieved from "https://wiki.retrotechcollection.com/index.php?title=Amstrad_Mega_PC_Maintenance_Guide&oldid=21790"