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3.11.3. How is the Lisa Power Supply Adjusted?

The Lisa switching power supply has a single voltage adjustment, which is accessed from the side of the PSU case closest to the card cage. Since this adjustment is not accessible while the PSU is installed, the usual technique is to make small adjustments iteratively until the output voltage is satisfactory. This control is moved counter-clockwise to increase the output voltage.

The adjustment controls the +5 output primarily, with the other outputs tracking accordingly. In a situation where there is an unusual imbalance among the loading of the voltages (typically an internal hard disk that draws an unusual amount of current from the +12 supply), you may not be able to achieve satisfactory levels on both the +5 and +12 outputs. The usual solution is to upgrade to the larger capacity "DataPower" power supply (commonly referred to as the 1.8 Amp PSU due to its rating at 120V).

For optimal results, configure the Lisa as it will be used before adjusting the PSU, as adding memory, expansion cards, etc. will change the load and affect the voltage.

Increasing the voltage too much will trigger the over-voltage protection crowbar, causing the power supply to shut-down repeatedly (usually accompanied by a ticking sound). This symptom can also indicate a load that is too small or too large eg. a short-circuit.

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