Amstrad PC1512 Capacitor Replacement Guide

PC-MM main board, which carries the computer's supply (service manual p. 42)

The power supply of the Amstrad PC1512 is on the main board of its monitor, the PC-MM (monochrome) or PC-CM (colour), so the electrolytic capacitors that matter most for the computer are in the monitor. Amstrad's service manual gives part numbers for the larger electrolytics of each monitor, but its parts lists are not complete: the schematics show further small electrolytics that have no entry in them. The CPU unit's parts list names only one electrolytic, C801 (47 µF 16 V). Read the markings on any part not listed below before removing it. Before ordering parts, check the monitor's 3.15 A time-lag mains fuse, look for bulging or leaking capacitors and measure the computer's rails against Amstrad's targets (see After the recap); the Amstrad PC1512 Troubleshooting Guide covers a dead supply.

Safety

Warning: both monitors run from the mains and contain a CRT. The 150 µF 400 V reservoir capacitors hold a dangerous charge after the plug is pulled.

• Unplug the monitor and discharge the tube; see CRT Discharge Procedure.
• Measure across each 400 V reservoir capacitor (C505 on the PC-MM; C506 and C601 on the PC-CM) and discharge it through a suitable resistor before touching the board.
• On the PC-CM, Amstrad tells engineers to measure the supply circuit with reference to the top of C505, "in order to keep the chopper circuit oscillating", and marks the supply area as isolated.
• After the work, Amstrad's safety test is a 3 kV flash test and an insulation resistance of at least 4 MΩ between the joined mains poles and all accessible metal.

PC-MM electrolytic capacitors

PC-MM electrolytic capacitors (service manual p. 18)

Ref	Value	Voltage	Amstrad part no.
C427	10 µF	25 V	171601
C429	10 µF	160 V	171602
C505	150 µF	400 V	171603
C435	220 µF	35 V	20055
C517	330 µF	35 V	171604
C522	470 µF	16 V	1400248
C425	1000 µF	10 V	171605
C530	1000 µF	16 V	1400158
C521	1000 µF	25 V	171606
C529	3300 µF	16 V	171607
C525	3300 µF	35 V	171608

The PC-MM has a single switching transformer, T501. The parts list gives its switching regulator, IC501, as an STK7356, but the PC-MM schematic labels IC501 STK7358, so read the part fitted before ordering one. The control ICs are a BA6993, LA6324 and BA707 (IC502–IC504), with two TLP580 optocouplers (Q504, Q505), and the IC protectors ICP501 and ICP502 are ICP-N50 parts. The parts list leaves out the small electrolytics of the supply's control circuit; the schematic shows C506 (1 µF 50 V) and C509 (0.22 µF 50 V) beside IC501.

PC-CM electrolytic capacitors

PC-CM electrolytic capacitors (service manual p. 20)

Ref	Value	Voltage	Amstrad part no.
C440	1 µF	250 V	171647
C509	10 µF	100 V	171648
C436	22 µF	250 V	171649
C506, C601	150 µF	400 V	171650
C515	220 µF	160 V	171651
C613	330 µF	25 V	171652
C516	470 µF	35 V	171653
C623	1000 µF	16 V	1422158
C437	1000 µF	35 V	1522159
C408	2200 µF	35 V	1422160
C621, C622	3300 µF	16 V	171654
C618	3300 µF	35 V	171655

The part number for C437 is printed 1522159; the same value carries 1422159 in the PC1640 monitor lists. The PC-CM has two switching transformers, T501 and T601, and two 150 µF 400 V capacitors, C506 and C601. Its parts list adds IC601 (STK7358), IC602 (BA6993), IC603 (LA6324) and IC604 (BA707), with TLP580 optocouplers at Q604 and Q605, alongside the STK7356 at IC501; the presets for the computer's rails are VR601–VR606. The PC-CM power supply schematic shows electrolytics that the parts list does not: C507 (47 µF 50 V), C508 (22 µF 50 V), C602 (1 µF 50 V), C606 (4.7 µF 100 V), C608 (100 µF 50 V) and C609 (100 µF 25 V) round the two switching regulators, and C517, C519 (100 µF 16 V) and C518 (47 µF 50 V) at the L78M12 and L78M05 regulators IC502 and IC503. On the same schematic the C505 named in Amstrad's measuring note is a 0.0022 µF 4 kV capacitor.

Procedure

1. Disconnect the monitor from the mains and from the computer, remove the back and discharge the tube and the reservoir capacitors.
2. Photograph the board and note the value, voltage and polarity printed on every electrolytic in the supply area. Repairers have found PC-CM boards that differ from the layout drawn in the manual, with parts moved and renumbered.
3. Remove each capacitor and clean the pads. Check under the large filter capacitors for electrolyte and corroded tracks.
4. Fit new parts of the same capacitance and the same or a higher voltage rating, observing polarity.
5. Inspect the solder joints of the transformers, the STK regulator and the heat-sinked parts and reflow any that are cracked.
6. Reassemble, connect the computer and check the rails before refitting the back.

After the recap

Set the computer's rails with the monitor's presets, measuring at Amstrad's test points A to D on the monitor board (figures 1 and 3 of the alignment instructions). With the computer connected, the same rails can be read at the 14-way DIN plug, on the pins given in the Technical Reference Manual.

Rail settings (service manual pp. 26–27)

Rail	Test point	DIN pin	Target	PC-MM preset	PC-CM preset
+5 V	A	3, 5	5.10 V ±0.02 V	VR505	VR604
+12 V	B	11	12.15 V ±0.02 V	VR504	VR603
−5 V	C	13	−5 V ±0.05 V	VR501	VR601
−12 V	D	9	−12.0 V ±0.05 V	VR502	VR602
+12 V protection	Dummy load and ammeter on DIN pin 11	11	Trips at 4.5 A	VR507	VR606

On the PC-MM, VR503 sets the monitor's own +12 V to 12.0 V ±0.05 V at test point B. Then run the picture adjustments in the same tables and repeat Amstrad's safety tests. The Amstrad PC1512 Maintenance Guide gives the full adjustment table.

Related pages

• Amstrad PC1512 · Amstrad PC1512 Maintenance Guide · Amstrad PC1512 Troubleshooting Guide
• Amstrad PC1640 Capacitor Replacement Guide
• Capacitor Failure Symptoms · CRT Discharge Procedure · Recommended Tools