Amiga 500 Power Supply Repair

An Amiga 500 external power supply with its mains lead, its on/off switch and the five-pin plug that goes to the computer.

The Amiga 500 runs from an external switch-mode supply, Commodore part 312503-01, which feeds the computer through a five-pin plug: +5 V at 4.3 A on pin 1, +12 V at 1.0 A on pin 3 and −12 V at 0.1 A on pin 5, with shield ground on pin 2 and signal ground on pin 4. Commodore's service manual treats the supply as a single replaceable part and gives no circuit for it. Howard W. Sams' Computerfacts CSCS26 does: it documents a 120 V supply with a primary switching stage, a +5 V output regulated through an optocoupler and shunt regulator, separate 12 V regulators, and an overvoltage shutdown. Supplies with the same Commodore label differ inside: World of Jani found four different sets of capacitors in four supplies labelled 312503-03. Check the circuit and designators against the unit on the bench before relying on Sams' numbers; Nightfall's repair of another A500 supply, for example, refers to a zener ZD2 and a 1 µF capacitor C9 that do not match Sams' unit.

Symptoms

• No power LED and nothing on any rail.
• One rail missing. Perry's Amiga Troubleshooter chart notes that the computer works without −12 V but the serial port does not.
• Voltages wrong or unstable. Nightfall's supply had all its outputs unstable.
• The supply shuts down. Sams' unit has a shutdown transistor (Q3A) and an overvoltage SCR (IC1) that switch it off on overload or overvoltage.

A fault can also be in the computer. Commodore's own checks measure the rails inside the A500 at the floppy power header CN12 and the line filter LF1, and separate a bad supply from a shorted component or a bad drive; see the Amiga 500 Troubleshooting Guide.

Safety

Warning: the primary side of the supply runs at mains voltage, and the reservoir capacitor after the bridge rectifier stays charged after the supply is unplugged. On Sams' 120 V unit that capacitor, C5, is 220 µF 200 V and charges to about 156 V DC; on a supply built for 220-240 V mains the voltage is higher. Discharge it before touching the board.

Sams' service conditions:

• Disconnect the supply from the computer, so that high voltages that may occur during servicing cannot damage the system board.
• Never run the supply without a load. A #1129 lamp loads the 5 V output.
• Power it through an isolation transformer with a voltage step-down control.
• If the shutdown circuit has to be defeated, use an isolation transformer with step-down and current limiting, and start at a low voltage and current; defeating the shutdown with full mains applied can cause further damage.

Tools and parts

• A multimeter and an oscilloscope; the scope traces on Sams' schematic are referenced to the isolated ground.
• A #1129 lamp as a dummy load, and a jumper for the plug.
• An isolation transformer with step-down (and current limiting if the shutdown is to be defeated).
• A Phillips screwdriver; the case is held by four screws in the base.
• Parts identified on Sams' schematic: fuse F1 2 A (mains), fuse F2 5 A slow-blow (5 V output), bridge rectifier BD1 (RS204), switching transistor Q1 (2SC3679), the 7912 and 7812 regulators for −12 V and +12 V, the overvoltage SCR IC1 (TYN410), the optocoupler PC1 and the shunt regulator IC2 (µA431). The electrolytics found in several A500 supplies are listed in the maintenance guide.

Procedure

Test the supply on a load

Sams' preliminary check, with the supply unplugged from the computer:

1. Measure the voltages at the supply's plug (Sams calls it CN8): 5 V at pin 1, 12 V at pin 3 and −12 V at pin 5.
2. If they are missing, connect a jumper from pin 2 to pin 4 of the plug and a #1129 lamp from pin 1 to pin 2, switch on and measure again.
3. If they are still missing, check fuses F1 and F2 on the supply board.
4. If the fuses are good, check the setting of the shutdown level control VR1, and if the voltages are present but not regulated, the 5 V adjust control (see adjustments).

Open the supply

1. Unplug the supply from the mains and the computer.
2. Remove the four Phillips screws from the bottom of the supply and lift the supply out of its case.
3. Discharge the reservoir capacitor (C5 on Sams' unit) and confirm with a meter that it is discharged.

Sams' GridTrace photograph of the supply board, with an index giving the grid square of each part.

Dead supply

Sams' sequence:

1. Check F1 and F2. If F1 is open, check the bridge rectifier BD1 and the power transistor Q1 for shorts. If F2 is open, check the output diode D10 for a short.
2. If the fuses are good, apply mains through the isolation transformer and check for mains voltage (120 V AC on Sams' unit) at the AC inputs of BD1. If it is missing, check the switch SW1 and the coil in the mains filter.
3. If AC is present, check for about 156 V DC at the positive output of BD1. If it is missing, BD1 is faulty.
4. If 156 V is present, check the waveforms, voltages and components round Q1 to Q2, and check the windings of transformer T1 for continuity.

The primary side on Sams' schematic: mains input through F1 (2 A) and SW1, the line filter, BD1 (RS204), the 220 µF 200 V reservoir C5 at 156 V, the switching transistor Q1 (2SC3679), Q2, Q3 and the shutdown level control VR1.

Supply in shutdown

If the supply starts and then shuts down, check the adjustment of the shutdown level control VR1. If adjusting it has no effect, Sams defeats the shutdown by removing the shutdown transistor Q3A and the SCR IC1, under the isolation-transformer conditions in Safety.

One output missing or wrong

The secondary side on Sams' schematic: the −12 V regulator fed with −18.3 V, the 12 V regulator IC3 fed with 19.8 V, the 5 V rectifier D10, F2 (5 A slow-blow), the 3300 µF filters C13-C15, the overvoltage SCR IC1 (TYN410), the optocoupler PC1, the shunt regulator IC2 (µA431) and the 5 V adjust VR2, with the output plug CN8 at the right.

Sams' output checks

Fault	Measure	If missing	If present
Source voltages not correct	Adjust the 5 V adjust control	If it has no effect: voltages and parts round the regulator IC2 and the error-amplifier optocoupler PC1
−12 V missing	About −18.5 V at pin 2 of the −12 V regulator (IC2A)	Continuity of the T1 winding from pin 7 to pin 11; D12, C23, R14	IC2A and C24
+12 V missing	About 19.8 V at pin 1 of the 12 V regulator IC3	Continuity of the T1 winding from pin 8 to pin 12; D9, C11, C26A, R18	IC3 and C19
5 V missing	Fuse F2	If F2 is open: D10 for a short to ground	If F2 is good: continuity of the T1 5 V winding; D10, L1, C13, C15

The schematic prints −18.3 V at the input of the −12 V regulator where the text gives −18.5 V.

Adjustments

Sams' photograph of the regulator board, with Q3A, Q2, Q3, IC2, PC1, VR2 and ZD1 identified.

Sams' two adjustments:

1. Shutdown level: connect the voltmeter's positive lead to the base of the switching transistor Q3A on the regulator board and the negative lead to its emitter. Adjust the shutdown level control for 2.1 V.
2. 5 V output: connect the voltmeter between the 5 V source (CircuiTrace 3 on the schematic) and ground. Adjust the 5 V adjust control for 5.0 V.

Reassemble

Refit the board in the case with the four Phillips screws.

Testing

1. Test on the lamp load first, then on the computer.
2. On the computer, measure the rails at the floppy power header CN12 inside the A500: +5 V on pin 1 (the right pin) and +12 V on pin 4 (the left pin), and −12 V at pin 1 of U38. Commodore's tolerance for its readings is ±10 %.
3. Expect the rated currents to be available: 4.3 A on +5 V, 1.0 A on +12 V and 0.1 A on −12 V for the 312503-01. Other supplies with the same plug are rated lower; Ian Stedman lists the 312503-02 at 2.5 A on +5 V.

Related pages

• Amiga 500
• Amiga 500 Maintenance Guide
• Amiga 500 Troubleshooting Guide
• Amiga 500 Capacitor Replacement Guide
• Sams Computerfacts CSCS26 Amiga 500
• Capacitor Failure Symptoms