Amiga 500 Maintenance Guide

An Amiga 500 with its monitor, an external drive and the mouse

Routine care for the Amiga 500: the external power supply, the battery on the A501 memory expansion, the keyboard, the floppy drive and the sockets. For fault-finding see the Amiga 500 Troubleshooting Guide; for the capacitor lists see the Amiga 500 Capacitor Replacement Guide.

Commodore's figures here come from its Service Manual A500 (PN-314981-04, October 1990), and the power supply figures from Sams' Computerfacts CSCS26 for the A500.

Power supply

Ratings and rails

The supply connects through a five-pin plug. Commodore lists:

A500 power connector

Pin	Function
1	+5 V at 4.3 A
2	Shield ground
3	+12 V at 1.0 A
4	Signal ground
5	−12 V at 0.1 A

Ratings vary between supplies with the same plug. Ian Stedman's survey gives 2.5 A on +5 V (25.7 W) for the 312503-02 and 4.5 A (35.7 W) for the 312503-03, and measured a stock A500 with an A501 and one floppy drive at 1.55 A idle and 2 A peak on +5 V.

Checking the rails

Measure with the machine running. Commodore's tolerance for all its readings is ±10 %.

Where Commodore measures the rails

Rail	Test point	If wrong
+5 V	CN12 pin 1 (the right pin of the floppy power header)	Unplug the drive and measure again; then measure pin 7 (VCC) and pin 2 of the line filter LF1. Good at pin 2 but bad at pin 7 means a faulty LF1.
+12 V	CN12 pin 4 (left pin)	Unplug the drive; then measure LF1 pins 5 and 4.
−12 V	Pin 1 of U38 (MC1488)	Measure LF1 pins 6 and 3.

If the rail is wrong at the line filter's input pins, the fault is in the supply or a component shorting the rail. A machine with no −12 V still runs, but the serial port will not work.

Inside the supply

Warning: the supply's primary side runs at mains voltage, and the reservoir capacitor after the bridge rectifier holds a high-voltage charge after unplugging. Discharge it before working inside.

Sams documented a switch-mode A500 supply in CSCS26. Its checks for a dead supply start with fuses F1 and F2: if F1 is open, check the bridge rectifier BD1 and the power transistor Q1 for shorts; if F2 is open, check diode D10. With the fuses good, expect mains AC at the inputs of BD1 and about 156 V DC at its positive output on a 120 V supply. A shutdown transistor (Q3A) and an overvoltage SCR (IC1) shut the supply down on overload or overvoltage. Sams sets the shutdown level control for 2.1 V between the base and emitter of Q3A, and the 5 V adjust control on the regulator board sets the +5 V output. Run the supply on a load when testing it; Sams uses a #1129 lamp on the 5 V output and powers the supply through an isolation transformer with step-down and current limiting.

A500 supplies were built by more than one maker, so check the circuit against the unit on the bench before using Sams' designators. Restorers report these faults:

• Bulged and leaking electrolytics, and RIFA mains filter capacitors, which the World of Jani blog calls "prone to release out the magic smoke".
• Unstable output on all rails, traced on one supply to a corroded 5.6 V zener (ZD2) and a 1 µF 50 V capacitor (C9) that failed under voltage.

Opinion differs on whether a failing A500 supply can over-volt the machine the way a failing Commodore 64 supply can. One AmiBay member says Amiga supplies "don't seem to provide overvoltage if they're failing, unlike the 64's PSU"; the seller of an over-voltage protector in the same thread disagrees. Measure an unknown supply on a load before connecting it.

Supply capacitors

Commodore published no parts list for the supply, and supplies with the same label differ inside. The World of Jani blog recapped four supplies all labelled 312503-03 and found four different capacitor sets:

Electrolytics in four supplies labelled 312503-03 (World of Jani)

Label rating	Capacitors
5 V 4.5 A	47 µF 385 V; two 22 µF 25 V; two 470 µF 16 V; four 4700 µF 16 V
5 V 2.5 A	4700 µF 25 V; 4700 µF 6 V; two 330 µF 25 V; three 22 µF 25 V
5 V 2.5 A	Two 2200 µF 35 V; 330 µF 40 V; 330 µF 25 V; 470 µF 16 V; 10 µF 50 V; two 2.2 µF 16 V tantalum
Not stated	C4 47 µF 400 V; C5 1 µF 450 V; C8 47 µF 50 V; C9 1 µF 50 V; C11, C12 470 µF 16 V; C13 4700 µF 16 V; C14 2200 µF 16 V; C15 4.7 µF 50 V; C18 220 µF 25 V; C19 10 µF 50 V

Count and read the capacitors in your own supply before ordering. The same page notes two 47 Ω resistors in the 4.5 A model that run hot by design.

A501 clock battery

The A501 board. The battery is at the lower left, next to the clock chip.

The A500 main board has no clock battery. The clock is on the A501 512 KB expansion in the trapdoor, and Commodore's parts list gives its battery as a Varta 3/60DK NiCd, 3.6 V 60 mAh, part 380393-01, at BT9, with the OKI MSM6242B clock chip at U9. The A501 charges the cell while the machine is on.

These cells are "prone to leak green acid, damaging the printed circuit boards". Take the A501 out of every machine you acquire and inspect it, even if you do not need the clock.

1. Open the trapdoor and pull the A501 straight out of its edge connector.
2. Remove the shields and look at the battery and the board round it for green or white deposits.
3. Desolder the battery. It is held by three solder points. Do not twist it off; the pads lift with it.
4. Clean any residue. Amiga Alive notes that most owners use vinegar and then alcohol. Follow Battery Explosion, Capacitor or Corrosion Damage for corroded tracks, and check the traces under the battery for continuity.
5. If you want the clock back, fit a rechargeable 3.6 V cell of the same type, or a lithium coin cell. A non-rechargeable coin cell needs a diode and a series resistor of at least 200 Ω so that the A501's charging circuit cannot charge it.

See also Battery Refurbishment.

Keyboard

A500s were fitted with keyboards from more than one maker. Hackup.net describes the Mitsumi type as a single membrane with two graphite contacts per key, and the Samsung type as "almost impossible to repair". Identify the type before buying parts.

Floppy drive

Commodore fitted Chinon (part 312554-01) and Panasonic (327142-01) drives. The Chinon FB-354 has three 22 µF and one 1 µF electrolytic, which one restorer replaced. For head cleaning and mechanism care see Floppy Drive Repair.

Ports

Do not plug or unplug joysticks, mice or anything else with the machine on. A connector's metal shell can short a pin as it goes in, "blowing a resistor (or worse an 8520 CIA chip!)". The game ports are wired to CIA U7 and to Denise through U15 (74LS157), so a damaged port often means one of those chips.

Sockets

Agnus sits in an 84-pin PLCC socket (U2), and the 68000, Paula, Denise, Gary and the CIAs are in DIP sockets on the Rev. 5 board. Tom's reseating notes suggest reseating Agnus for chip RAM faults, using a PLCC chip puller.

Related pages

• Amiga 500
• Amiga 500 Troubleshooting Guide
• Amiga 500 Capacitor Replacement Guide
• Amiga 500 Service Manual (314981-04)
• Sams Computerfacts CSCS26 Amiga 500
• Recommended Tools