Amstrad Mega PC Capacitor Replacement Guide

Mega PC main board VSC386SXD V.2.01. The electrolytics are the blue cans around the power header, the SIMMs and the video section.

The Amstrad Mega PC has three sets of electrolytic capacitors: on the main board, which is the PC7386SX board; on the Mega Drive games card; and in the PC14DSM monitor sold with it. Amstrad's PC7386SX parts list does not list capacitors, so the main-board values below are read from the PC7386SX circuit diagrams. The monitor values are from the PC14DSM parts list in the Mega PC service manual.

Safety

Warning: the power supply module and the monitor carry mains voltage, and the monitor's CRT and its 400 V reservoir capacitor hold a charge after switch-off. Unplug both and read CRT Discharge Procedure before working in the monitor. Amstrad's safety tests after any work are on the Amstrad Mega PC Maintenance Guide.

Main board

The main board runs from +5 V, +12 V, −5 V and −12 V supplied by the power module.

Main board electrolytic capacitors (PC7386SX schematics)

Ref	Value as printed	Sheet	Position on the schematic
C1	33 µF 25 V	1/13	At the power header
C2	33 µF 15 V	1/13	+5 V at the power header
C119, C120, C121	10 µF 25 V	1/13	+5 V rail (drawn at the power header)
C4	10 µF 25 V	1/13	+12 V
C5	10 µF 25 V	1/13	−12 V (positive lead to ground)
C6	10 µF 25 V	1/13	−5 V (positive lead to ground)
C3	10 µF 16 V	1/13	Front panel reset line, with R5 (300 Ω)
C20, C21	10 µF 25 V	2/13	SIMM supply
C60	1 µF 16 V	6/13	WD76C20 battery supply (VBATT)
C131	10 µF 16 V	12/13	ICS1394 clock synthesiser supply
C132	10 µF 16 V	12/13	ICS1394 analogue supply, fed from +12 V through R51 and the 5.1 V zener D14; fitted with the ICS1394 only (the table on sheet 12/13 leaves it out, with R51, R52, R54, R55, D12 and C135, when an ICS1494 is used)
C141	1 µF 16 V	12/13	Beside R57 on the ICS1394 filter pins
C137	10 µF 25 V	13/13	RAMDAC analogue supply (VAA)
C110, C111	10 µF 25 V	13/13	Bulk capacitors on the video section's decoupling row (U51–U54, U56, U58)

C2 is printed as 15 V on the schematic; fit a 16 V or higher part. C5 and C6 sit on the negative rails, so their positive leads go to ground; check the board marking before fitting. The other capacitors on the sheets are ceramic decoupling parts with no polarity mark. Count the electrolytics on your own board against this list: the PC7386SX manual draws board VSC386SXD V.2.01, the marking on the Mega PC board photographed for Commons.

The board has one more electrolytic that is not on the circuit diagrams: C140, beside R16 and R17 near the coprocessor socket U14. It is drawn on the PC7386SX top view and fitted on the photographed Mega PC board, but no value is published for it, so read the value and voltage printed on the part before removing it. The three +5 V capacitors that sheet 1/13 draws at the power header are spread over the board: on the photographed board C120 is beside the 74LS245 at U44 by the ISA riser connector, C119 beside SIMM bank 0 and C121 beside U22.

Games card

The games card's circuit diagrams and PCB views are in the Mega PC service manual, but the only available scan is about 90 dpi and the component values on the circuit diagrams cannot be read. Record the value and voltage printed on each capacitor before removing it. A photograph of the fitted card shows a number of small radial electrolytics around the sound and video sections.

Power supply module

Neither manual includes a circuit or parts list for the supply module (part 273888); Amstrad treated it as a replaceable unit. Its capacitors must be read from the parts themselves. The primary side carries rectified mains.

PC14DSM monitor

The monitor's parts list (Mega PC service manual, PDF p. 32) gives these electrolytics. The scan is low resolution: the values and voltages are clear, but a few reference numbers are hard to read, so check each against the board.

PC14DSM electrolytic capacitors (parts list)

Value	References as printed
150 µF 400 V	C010
100 µF 160 V	C032, C522
47 µF 200 V	C524
33 µF 160 V	C041
100 µF 100 V	C022, C062, C063
10 µF 100 V	C842, C850
4.7 µF 100 V	C804, C814, C824
2.2 µF 100 V	C009, C011
1000 µF 25 V	C071
470 µF 35 V	C052
220 µF 35 V	C053, C514
2200 µF 16 V	C409 (last digit unclear)
220 µF 16 V	C843
100 µF 35 V	C408 or C409 (last digit unclear)
100 µF 16 V	C106, C117
47 µF 50 V	C529
47 µF 35 V	C104, C212
47 µF 25 V	C413
47 µF 16 V	C101, C111, C201, C601, C602, C625, C627
33 µF 35 V	C513
22 µF 16 V	C515
10 µF 50 V	C103, C422, C530, C626
4.7 µF 50 V	C014
2.2 µF 50 V	C102, C202, C302, C415
1 µF 50 V	C503, C510

C010, 150 µF 400 V, is the only 400 V electrolytic in the list; replace it with a 400 V part. The list also gives a 0.47 µF 250 V AC capacitor across the line (C001) and 2200 pF 250 V AC capacitors C003 to C006, which are safety parts. See Capacitor Failure Symptoms.

Replacement procedure

1. Unplug the system unit and the monitor. Open the case as described in the Amstrad Mega PC Maintenance Guide, and discharge the monitor's CRT before working in it.
2. Photograph each board and note the polarity of every electrolytic, including the reversed parts on the negative rails.
3. Remove one capacitor at a time with a desoldering iron or braid, clean the pads and check them for corrosion.
4. Fit parts of the same capacitance and the same or a higher voltage rating.
5. After reassembly, measure the rails at the main board's power header and check the monitor in both PC and game modes.

Related pages

• Amstrad Mega PC · Amstrad Mega PC Maintenance Guide · Amstrad Mega PC Troubleshooting Guide
• Capacitor Failure Symptoms · CRT Discharge Procedure · Recommended Tools