Amstrad ALT-386SX Capacitor Replacement Guide

ALT386SX main PCB, top view (Amstrad service manual p. 16)

The capacitors that owners have found failing in the Amstrad ALT-386SX are the electrolytics in its power supply and C121, a surface-mount 33 µF capacitor in the speaker circuit on the main board. Amstrad's service manual gives no capacitor values in its parts lists, and lists the power supply only as a complete unit (UN-1, part 270492) with no schematic; the main board values appear only on the schematics. For the supply, oldcrap.org has published the ratings read from the parts removed from one machine's supply; they are given below. The failures below are owners' reports, cited as community experience. The Amstrad ALT-286 uses the same supply.

Safety

Warning: the power supply runs from the mains; its heatsinks are live in operation and its reservoir capacitor can hold a charge after the lead is pulled.

• Remove the battery pack and the mains lead before opening the case.
• The supply board in oldcrap.org's photographs (RPS-07M-1) has two fuses, F1 "250V T2A" and F2 "250V 2A", and a printed warning that the heatsink presents a risk of electric shock and should be tested before touching. Replace fuses with the same type and rating.
• Measure the voltage across the large reservoir capacitor before working on the supply, and discharge it through a suitable resistor if it holds a charge.

Power supply

The supply is a Rubycon RPS-07 on oldcrap.org's machine. Every electrolytic capacitor in it had leaked, and the outputs were much lower than required; replacing the capacitors restored the correct voltages. Another owner reported recapping the same supply and finding "some of the caps def bad", and a third recapped the RPS-07 in an ALT-286.

Before recapping, measure the rails at the main board's power connector CN104 (see After the recap) and check the contacts in the 7-wire cable between the supply and the main board. One owner who lost 12 V traced it to failed contacts in that plug, not to the capacitors.

oldcrap.org lists the ratings printed on the 15 electrolytics removed from the RPS-07 in its ALT-386SX; the board in its photographs is marked RPS-07M-1.

Rubycon RPS-07 electrolytics, read from one ALT-386SX supply (oldcrap.org)

Ref	Capacitance	Voltage
C6	100 µF	400 V
C21	1000 µF	25 V
C22	1000 µF	25 V
C25	680 µF	35 V
C26	680 µF	35 V
C29	1000 µF	25 V
C30	100 µF	25 V
C31	1000 µF	25 V
C32	1000 µF	25 V
C33	470 µF	10 V
C34	47 µF	35 V
C35	33 µF	35 V
C36	47 µF	50 V
C37	33 µF	35 V
C54	10 µF	50 V

Fifteen parts: five 1000 µF 25 V, two 680 µF 35 V, one 470 µF 10 V, two 100 µF (25 V and 400 V), two 47 µF (35 V and 50 V), two 33 µF 35 V and one 10 µF 50 V. The list covers electrolytics only. The photographs show other C numbers on the board, such as the film and ceramic capacitors C1–C5 at the mains input; for the other numbers missing from the list it does not say whether the position holds another type of part or is empty. The author had no schematic, and one reader whose supply had been worked on before asked whether the values were original, so read each part as you remove it and compare it with the list.

Owners' practice when choosing parts:

• Match the capacitance, and fit the same or a higher voltage rating.
• One owner ordered exact matches from Nichicon and Panasonic; the author used parts from stock, preferring Nichicon, and notes that the capacitance of the smoothing capacitors in the supply need not be exact.
• Note the value, voltage and polarity of each part before removing it.

Main board

Amstrad's main board schematics give the value of each polarised capacitor. Plain C numbers on the board also include ceramic capacitors, so the table lists only the parts drawn as polarised. The ALT386SX schematics are on pp. 24–25; the ALT286 main board schematics (pp. 19–20) show the same capacitors at the same references and values.

ALT286 and ALT386SX main boards: polarised capacitors (Amstrad schematics)

Ref	Value	Circuit
C110	47 µF 6.3 V	+5 V from the power connector CN104
C111	10 µF 16 V	+12 V from CN104
C113	10 µF 16 V	−12 V from CN104
C115	10 µF 35 V	−24 V from CN104
C117	100 µF 16 V	+12 V at the hard disk power connector CN105
C121	33 µF 10 V (printed "MX33u/10V")	Speaker: in series between the driver transistor T102 (2SD1664), through R142 (22 Ω), and the speaker connector CN103
C122	10 µF 6.3 V	+5 V decoupling
C128	4.7 µF 10 V	Real-time clock supply, backed by the battery BT101
C130	33 µF 6.3 V	Power-on reset, after the reset IC U137 (PST529D)
C177	1 µF 50 V	Clock supply switching (T104, T105)
C171–C176	100 µF 6.3 V (six)	+5 V decoupling

Sixteen parts. The ALT286's co-processor/memory schematic (p. 21) shows C301, 22 µF 6.3 V. The ALT386SX co-processor/memory schematic is not in the hosted scan, so the capacitors on that board are not listed here.

The part owners have found failing is C121, a surface-mount capacitor beside the memory connector, marked "4W 33 10V". One owner found it had leaked and corroded the tracks under U144, replaced it with a tantalum part and noted that it is in the speaker circuit, so low speaker volume is the warning sign; Amstrad's schematic confirms that C121 feeds the speaker. Another found C121 loose with the board blackened beneath it, and fitted a new 33 µF part. A third owner, replying to the first, found a leaking surface-mount capacitor in a machine whose speaker was barely audible and replaced it with a standard electrolytic, without giving the reference. Check the board around C121 and the backup battery for residue and corrosion; see Capacitor Failure Symptoms.

Procedure

1. Remove the battery pack and unplug the mains lead.
2. Open the case, unplug the supply and lift it out; discharge and check the reservoir capacitor.
3. Photograph each board and record every electrolytic's value, voltage and polarity.
4. Remove each capacitor with a desoldering iron or hot air. Do not twist surface-mount parts off: the pads and tracks under a leaking part are often weakened.
5. Clean away the electrolyte with isopropyl alcohol, then inspect the tracks and vias under and around the part with a magnifier. Repair broken tracks with fine wire, as described in the maintenance guide.
6. Fit new parts of the same capacitance and the same or a higher voltage rating, observing polarity.
7. Refit the supply and its 7-wire cable, checking the contacts in the plug; one owner traced an intermittent 12 V to failed contacts there.

After the recap

Amstrad publishes no voltages or tolerances for the supply. The main board schematic names the seven pins of the power connector CN104: +5 V, +5 V, +12 V, −12 V, −24 V and two grounds (p. 24; p. 19 for the ALT286). One owner measured 5 V, 5 V, 12 V, −12 V and −19 V on the connector of his ALT-386SX supply; another, testing an ALT-286 supply on the bench, found +5 V, −12 V and −24 V but no +12 V. Check that each rail is present and steady with the supply loaded by the main board, that the machine passes its self-test and reaches the setup screen with F2, and that the speaker is at full volume.

Related pages

• Amstrad ALT-386SX · Amstrad ALT-386SX Maintenance Guide · Amstrad ALT-386SX Troubleshooting Guide
• Amstrad ALT-286 Capacitor Replacement Guide
• Capacitor Failure Symptoms · Battery Explosion, Capacitor or Corrosion Damage · Recommended Tools