Amstrad PPC640 Capacitor Replacement Guide

PPC512/PPC640 power supply schematic (service manual p. 26), where most of the larger electrolytics sit

Amstrad's electrical parts list for the Amstrad PPC640 gives 35 aluminium electrolytic capacitors, with value, voltage and Amstrad part number for each. Nine of them, the C9xx positions, are on the modem board, which only the PPC640 has; the other 26 are common to the PPC512. The table below is Amstrad's list, checked against the power supply and modem schematics where those are legible. General signs of failure are described at Capacitor Failure Symptoms.

Electrolytic capacitor list

PPC640 electrolytic capacitors (electrical parts list p. 29)

Ref	Capacitance	Voltage	Amstrad part	Notes
C114	47 µF	16 V	20027
C139	4700 µF	16 V	170613	Largest in the machine
C140	1000 µF	10 V	171605
C401	1 µF	50 V	20062	Part number as printed; the list gives the same number to the 0.1 µF ceramics
C402	47 µF	16 V	20027
C403	1000 µF	10 V	171605
C404	47 µF	16 V	20027
C405	1 µF	50 V	20062	As C401
C409	22 µF	25 V	177378
C505	10 µF	50 V	177375	Power supply
C506	10 µF	25 V	171601	Power supply
C507	100 µF	25 V	177379	Power supply
C508	100 µF	25 V	177379	Power supply
C509	100 µF	50 V	177380	Power supply
C511	100 µF	63 V	177381	Power supply
C512	10 µF	50 V	177375	Power supply; schematic marks 10 µF 20 V
C513	1000 µF	16 V	1400345	+5 V output; schematic note "FDD+B"
C514	1000 µF	16 V	1400345	+5 V, after L501 (the alignment test point); schematic marks 100 µF 16 V
C515	10 µF	25 V	171601	−20 V LCD output; schematic prints "(7MM)"
C516	1 µF	63 V	177374	Power supply; schematic prints "(7MM)"
C517	100 µF	63 V	177381	Power supply
C518	10 µF	63 V	177376	Power supply; schematic prints "(7MM)"
C519	470 µF	16 V	1422248	Power supply
C521	1 µF	50 V	20062	−20 V drive board PCB504; part number as C401
C522	47 µF	16 V	20027	−20 V drive board PCB504; schematic marks 47 µF 6.3 V
C541	100 µF	16 V	20028	Real-time clock supply line; schematic marks 100 µF 10 V
C902	2.2 µF	50 V	151598	Modem board (PPC640 only)
C903	2.2 µF	50 V	151598	Modem board (PPC640 only)
C904	10 µF	100 V	177377	Modem board (PPC640 only)
C918	2.2 µF	50 V	151598	Modem board (PPC640 only)
C920	100 µF	16 V	20028	Modem board (PPC640 only)
C929	100 µF	16 V	20028	Modem board (PPC640 only)
C930	1000 µF	10 V	171605	Modem board (PPC640 only)
C935	100 µF	16 V	20028	Modem board (PPC640 only)
C936	100 µF	16 V	20028	Modem board (PPC640 only)

Totals by value: 1 µF ×4, 2.2 µF ×3, 10 µF ×6, 22 µF ×1, 47 µF ×4, 100 µF ×10, 470 µF ×1, 1000 µF ×5, 4700 µF ×1.

The parts list does not say which board each C1xx and C4xx position is on. The C5xx positions all appear on the power supply schematic, which belongs to the memory and power supply board PCB501 except for C521 and C522 on the small −20 V drive board PCB504. The C9xx positions are on the modem board PCB901 (MC0053A), whose parts all carry 900-series references.

Where the schematic and the parts list differ

For four supply capacitors the value printed on the schematic differs from the parts list: C512 (schematic 20 V, list 50 V), C514 (schematic 100 µF, list 1000 µF), C522 (schematic 6.3 V, list 16 V) and C541 (schematic 10 V, list 16 V). Read the part actually fitted before ordering. Where only the voltage differs, the higher rating from the parts list is safe to fit. For C514, check the can on the board.

Other capacitors on the telephone line

C901, across the telephone line on the modem board, is a 0.47 µF 250 V metallised polyester part (177385), not an electrolytic. The modem's line transformer T901 is marked on the schematic as a 3000 V RMS isolation barrier. Replace C901 only with a part of the same type and rating.

Replacement procedure

1. Disconnect the adaptor, the car lead and the telephone line, and take out the battery tubes.
2. Open the case as described in the Amstrad PPC640 Maintenance Guide, separating the two stacked boards and the modem board as needed.
3. Photograph each area and note each capacitor's value, voltage and polarity before removing it. Compare with the table above.
4. Fit new capacitors of the same capacitance and at least the listed voltage, observing polarity. Check that the larger parts, C139 (4700 µF) and the 1000 µF positions, will fit under the case before ordering. The schematic prints "(7MM)" beside C515, C516 and C518; measure the originals and match their size.
5. Reassemble, then run the PPC from the adaptor and check the rails below before fitting discs.

After a recap

Measure +5 V at the junction of L501 and C514 and, if needed, set it to 5 V ±5% with VR501, with the PPC running from the power adaptor.

Rails to check (power supply schematic, service manual p. 26)

Rail	Tolerance	Regulator
+5 V	±5%	Switching regulator, VR501
+12 V	+10% / −20%	IC502 L78M12T
−5 V	±5%	IC503 L79M05T
−12 V	±20%	IC504 L79M12T
−20 V	±5%	IC505 TA79L020P

The Amstrad PPC640 Troubleshooting Guide covers a rail that is missing or out of tolerance, including the +5 V crowbar.

Related pages

• Amstrad PPC640 · Amstrad PPC640 Maintenance Guide · Amstrad PPC640 Troubleshooting Guide
• Amstrad PPC 512 / PPC 640 Service Manual
• Capacitor Failure Symptoms · Recommended Tools