Amstrad PC7386SX Capacitor Replacement Guide

PC7386SX main board VSC386SXD V.2.01, from the service manual's top view. Electrolytics are drawn as circles with a polarity mark.

The Amstrad PC7386SX main board uses a small number of radial electrolytic capacitors, mostly on the supply rails at the power header and around the video section. Amstrad's electrical parts list covers ICs, transistors, diodes and miscellaneous parts only, so the values below are read from the circuit diagrams and the main PCB top view in the PC7386SX service manual. The same board is in the Amstrad Mega PC, whose capacitor guide adds the games card and the PC14DSM monitor.

Safety

Warning: the power supply module carries mains voltage, and its primary capacitors hold a charge after the machine is unplugged. Unplug the mains lead before removing the cover. After any repair Amstrad specifies a flash test at 1.5 kV RMS for 3 s between the mains lead and the exposed metal, an insulation resistance above 4 MΩ and an earth continuity of no more than 0.5 Ω.

Main board electrolytic capacitors

Electrolytic capacitors on the circuit diagrams

Ref	Value as printed	Sheet	Circuit
C1	33 µF 25 V	1/13	+5 V at the power header CON1
C2	33 µF 15 V	1/13	+5 V at CON1
C119, C120, C121	10 µF 25 V	1/13	+5 V at CON1
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	Battery supply to the WD76C20 (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
C141	1 µF 16 V	12/13	ICS1394 circuit, beside R57
C137	10 µF 25 V	13/13	ADV471 RAMDAC analogue supply
C110, C111	10 µF 25 V	13/13	Video RAM supply (U51–U54)

The schematics draw all of these with a polarity mark. C2 is printed as 15 V; fit a 16 V or higher part. C5 and C6 decouple the negative rails, so their positive leads go to ground; follow the board marking, not habit.

Positions on the top view that are not on the schematics

The main PCB top view also draws electrolytic outlines at C128, beside the MC1489A line receiver U42 and the 74LS245 at U44, and at C140, beside R16 and R17 near the coprocessor position U14. Neither reference appears on the 13 circuit diagrams, so no value is published for them. Read the value and voltage printed on each part before removing it. Check your own board against the whole list: a board revision other than VSC386SXD V.2.01 may differ.

Power supply module

Amstrad supplied the power supply as a complete module (DE-3, part 273888) and printed no circuit or parts list for it in either the PC7386SX or the Mega PC service manual. Its capacitors have to be read from the parts. The primary side runs from rectified mains, so the input reservoir must be replaced with a part of the same or higher voltage rating.

Replacement procedure

1. Unplug the mains lead and the monitor's power lead from the rear of the system unit.
2. Remove the top cover and lift out the expansion card riser, then the drive bracket if it blocks access (see the Amstrad PC7386SX Maintenance Guide).
3. Photograph the board and note the polarity of every electrolytic, including C5 and C6.
4. Remove the main board if needed to reach the solder side. Desolder one capacitor at a time and clean the pads; check the pads and tracks near the backup battery for corrosion while the board is out.
5. Fit parts of the same capacitance and the same or higher voltage rating, observing polarity.
6. Refit the board and measure the rails at CON1 before connecting the drives: +5 V on pins 2 and 10–12, +12 V on pin 3, −12 V on pin 4 and −5 V on pin 9, with pins 5–8 as ground.

See Capacitor Failure Symptoms for what failed parts look like.

Related pages

• Amstrad PC7386SX · Amstrad PC7386SX Maintenance Guide · Amstrad PC7386SX Troubleshooting Guide
• Amstrad Mega PC Capacitor Replacement Guide · Recommended Tools