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Typical problems and solutions concerning the M20 power supply

The Olivetti M20 power supply unit is a typical Flyback power supply. This component is the first one to test in the case of system failure, as it is one of the most solicited part of the whole system.

BE CAREFUL, anyway: those circuits have BIG capacitors charged up to 350V, which can sore enough energy to seriously hurt you, even when the unit is disconnected from mains!

Symptoms:

The M20 does not start the diagnostic sequence. When powered up, the power on led blinks, the buzzer beeps and then all remains in a quiescent state, with a low low volume continous beeeeeeeeep.
The voltage on the 5V, 12V and -12V lines is correct for several tenths of second after the power up, but then drops to 1/3 or 1/4 of the nominal value.
Other simptoms can be that the computer does not reset correctly. The ON led is on, but the buzzer beeps continuously and the disc drives moves randomly making noise. The voltage are below their nominal value.

Possible solution:

Avoid to keep the computer switched on continously. Try to disconnect the video, the disk drives and every expansion card to see if the voltages remain constant. There can be a problem in the power supply. Verify every soldering in the power supply unit to see if it is in good condition. Pull gently every wire that connects the flyback transformer to the four shottky diodes mounted on the heat dissipator. If only 2 or 3 diodes are connected, the power supply can not give enough current to the M20 motherboard, but the voltages appear to be correct if tested with a small charge.

Replacing the Power Supply

Sometimes, the power supply is dead. You can probably try to repair it, but you must have the skills, the time and at least an oscilloscope to do it. As part of the circuit is connected directly to the mains, you should know VERY well what you are doing.

Do not even think to find now an original Olivetti power supply for your M20, if you are very lucky, you can find a broken M20 and you can reuse its power supply. Anyway, the M20 circuitry requires rather standard voltages: +5V, +12V, -12V, which can be obtained from a standard PC power supply. The problem is that the shape of the Olivetti power supply unit is quite different from the average PC power supplies' one.

You can think of replacing the circuit inside the box and adapting the M20 connectors to the new power supply. In the picutre, you can see the new power supply, with its fan. Be careful, as for disk drives the connector is the same, BUT the wiring is different. Also, heat dissipators can be connected to the mains, fix the circuit in the M20 power supply case and avoid them touch the metallic walls.

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