Commodore 64 Power-Supply Protector (C64 Saver) Installation

The power input corner of a 250407 Rev. B board: the 7-pin power connector CN7, the small coil L5 beside C97, and the power switch SW1 below. An internal Saver goes into the +5 V line between CN7 and SW1.

The Commodore 64 Power-Supply Protector, Ray Carlsen's "Computer Saver", is an over-voltage cut-out that opens the +5 V line from the power supply when the voltage rises above about 5.4 V. It can be fitted in a separate box between the supply and the computer, or inside the computer, where it protects the machine from any supply plugged into it. Carlsen gives the connection points for fitting one inside a Commodore 64 long board. He counts at least five C64 board versions, each with its own connection points; on all of them the Saver goes "electrically between the power connector and the power switch".

Symptoms

• The fault the Saver guards against: a Commodore "brick" supply whose internal 5 V regulator shorts puts excessive voltage into the computer, which damages chips such as the RAM "very quickly... and silently". See Commodore 64 Power Supply Testing and Repair.
• After fitting, a Saver with Carlsen's dual LED shows green while the +5 V is normal and red when it has cut the power because of over-voltage.
• If the computer's red power LED goes out, the supply's +5 V is probably too low or too high.
• A continuous buzz from the Saver's relay means an overload: a short in the computer, or a supply whose output is too low. Switch off; a continued buzz damages the relay contacts.
• A Saver that trips when the computer is switched off and stays tripped until unplugged: some supplies jump above the trip point when the load is removed. This is why Carlsen sets the trip point as high as 5.4 V.

Safety

• Switch off, unplug the supply and remove all cables and anything in the cartridge, cassette and user ports before opening the case.
• Set and test the trip point on a bench supply before connecting the Saver to a computer.
• Under a metal shield there is very little room. Mount the parts as low as possible so the shield cannot short to anything when it goes back, or cut part of the shield away over the new parts.
• Mount the parts away from anything that runs hot: the zener diode is temperature sensitive, and heat moves the trip point.
• A transistor in a tab or metal case usually has its collector connected to the case, so isolate the case from surrounding metal and wiring.
• Small relays often have polarity-sensitive coils; one wired backwards will not energise.

Tools and parts

• A Saver: Carlsen's assembled and tested module, or one built to his circuit (described on the main page). For a home-built unit Carlsen specifies:
  • a relay with a 5 V coil, preferably of 150 Ω or more, and contacts rated at 2 A;
  • low-power NPN transistors rated at 500 mA or more, such as the 2N2222, 2N3565 or 2N3904;
  • a 4.7 V zener diode of 1/2 or 1 W, and a small-signal diode such as the 1N914 across the relay coil;
  • 1/4 W carbon or carbon-film resistors.
• An optional 2 A fuse in the Saver's input, which protects the relay contacts and the supply from an external short.
• A variable bench DC supply and an accurate voltmeter.
• For a 250407 board, a Dremel-type tool with a thin cutting blade, or a small file or knife, to cut a copper trace.
• A Phillips screwdriver (some machines need a T10 Torx driver), a soldering iron and hook-up wire.

Procedure

Set the trip point

1. Connect the Saver's input to the bench supply and the voltmeter to its output.
2. Raise the voltage slowly until the relay cuts in and voltage appears on the output side.
3. Keep raising it until the relay cuts out again. The voltage at that moment is the trip point; it should be about 5.4 V. Set too high, the Saver will not protect the computer; too low, it trips with a good supply.
4. If the trip point is wrong, Carlsen trims it by adding a higher-value resistor in parallel with the 1 kΩ resistor at the zener, and sometimes the 820 Ω one. The added value is usually between 2.2 kΩ and 10 kΩ with his parts; the zener and the transistors both affect the result.
5. Let the parts cool after soldering before measuring: the zener reads low when hot and high when very cold.

Open the computer

1. Turn the computer over and remove the three screws along the front edge of the underside.
2. Lift the front of the top case. Unplug the keyboard connector, rocking it gently and pulling straight up, and the power LED connector, then lift the top case away.
3. Remove the cardboard-and-foil or metal shield over the board if one is fitted. It is held by self-tapping screws.
4. Read the assembly number printed on the board.

Commodore's power input drawing for the 326298 board: +5 V enters on pin 5 of CN7 and passes through L5 to the power switch SW1.

On Commodore's drawing of the 326298 power circuit, the +5 V from pin 5 of the power connector CN7 passes through L5, a 1.2 µH coil, to the power switch SW1. The 326298 and 250425 parts lists both give L5 as a 1.2 µH coil, part 901152-01; the 250407 list gives L5 as a choke coil, part 325559-02.

326298, 250425 and 250466 boards

On these boards the work is done on the top of the board, without removing it from the case or taking off the bottom shield, and no trace is cut.

1. Find the small coil L5. On some boards there may be only a jumper wire in its place.
2. Disconnect one end of L5.
3. Wire the Saver's input and output across the open circuit: the input to the end that leads to the power connector, the output to the end that leads to the switch.
4. Connect the Saver's ground wire to a convenient ground point.

250407 boards

Carlsen finds the 250407 version B the most common board and the hardest to modify, because a trace must be cut on the underside.

1. Remove the seven screws holding the board to the lower case and lift the board out.
2. Unsolder just one of the bottom shield's tabs, at the right rear of the board, and bend the shield up to reach that corner; the rest of the shield can stay in place.
3. Cut the copper trace between the power connector and the power switch.
4. Put the shield back.
5. On the top of the board, connect the Saver's input to one pin at the rear of the power connector and its output to one leg of the power switch, so that the Saver bridges the two ends of the cut. On Commodore's drawing the +5 V is pin 5 of CN7.
6. Connect the third wire to a convenient ground point.

Mount the Saver and reassemble

1. Fix the module where it cannot touch the shield or anything that runs hot. Carlsen offers a pre-wired module that mounts over the cartridge port.
2. Refit the board with its seven screws if it was removed, then the shield, the keyboard and LED plugs, the top case and the three case screws.

Testing

1. Power up with a supply known to be good. The computer should start normally, and a dual LED should show green.
2. Listen for the relay. A continuous buzz means an overload or a low supply; switch off at once.
3. If the computer does not start on a European supply, check its +5 V pin. Carlsen has a European supply that delivers +5 V on pin 4 instead of pin 5; most C64s tie the two pins together inside, but he found one that did not.
4. To check the trip point after fitting, use the bench procedure above on the Saver's input.

Related pages

• Commodore 64 Power-Supply Protector (C64 Saver)
• Commodore 64
• Commodore 64 Power Supply Testing and Repair
• Commodore 64 Voltage Regulator Replacement
• Commodore 64 Troubleshooting Guide