Computer Nerd Kev
Projects > C= Monitor > Diagnostics
Diagnosing Voltage Warnings
This page describes possible causes and basic diagnostic tests related to warnings presented by the Commodore Power Monitor. Note that these are meerly suggested probable causes, and others may be possible.
If all three warnings are visible - 5VDC and RIPPLE both On, while 9VAC is Off - it is likely that there is excessive load on the power supply due to a short or low resistance in either the power supply or the computer. See the "5VDC Low" section.
Note that references to the "external power supply" also include the internal 5VDC regulator in early VIC-20s.
Illumination of this LED indicates that the voltage is significantly either high or low. With the computer turned on (or a dummy load attached to the external power supply output, if available), a Multimeter can be used to determine which of these is the case and the extent of the deviation.
In most circumstances this will indicate a failure of the Voltage Regulator IC or associated circuitry in the external power supply. The computer should be switched off immediately and the external power supply fixed or replaced as damage can easilly caused by excessive 5VDC voltage levels.
If the 9VAC light is also off, this likely indicates excessive loading of the power supply likely caused by a short or component failure in either the computer or the external power supply. The voltage of the 5VDC line at the power supply connector should be checked without the computer powered to determine where a fault may be present. If this shows a normal voltage, then one may use a Multimeter to check the current going to the computer on the 5VDC and 9VAC lines to determine if it is out of specification, or the computer may be immediately checked internally to prevent possible damage during the time it is powered up.
If the 9VAC light is on, this can indicate either a short as above causing the 5VDC Regulator output to drop, or its internal current limiting to turn off the 5VDC output. Alternatively, it could indicate a failure of the 5VDC regulator or associated circuitry in the power supply. If the 5VDC line is 0V with the computer on, but returns to 5VDC when checked without the computer plugged in, it is likely due to a short or low resistance in the computer triggering the Voltage Regulator's automatic current limiting. Otherwise either the Power Supply circuitry may be faulty, or there is a short or low resistance in the computer.
Voltage ripple is constant fluctuations in the regulated supply voltage and can create instability or damage chips. If the RIPPLE LED is on, this could indicate one, or a combination of, these failures:
In the case of b. the voltage of the 5VDC supply may also be Low, causing the 5VDC LED to light. In the case of c. the 5VDC supply might be High, also causing the 5VDC LED to light. Therefore if the 5VDC LED is illuminated, the voltage of the 5VDC supply should be checked with a multimeter or oscilloscope (preferably the latter, to avoid confusion with the ripple) to determine its level. If the ripple is significant enough, the 5VDC LED may also illuminate dimmly as the voltage limits are exceeded on the peaks of the ripple waveform.
Measurement of ripple usually requires an Oscilloscope, though a multimeter with a particularly accurate AC Volts range could be used with a low value capacitor (eg. 10nF, 100nF) in series with one multimeter probe, however the readings could be inaccurate.
If the 5VDC LED is also on, see the "5VDC Low" section.
Remember to check that the Power Monitor is correctly inserted to get power. If the computer shows no instability despite the low 9VAC warning, it may not be a problem. Some Commodore computers were sold with power supplies only just big enough to power the computer, and the 9VAC voltge can drop even under normal load.
If the power supply model normally supplies the right 9VAC level, the dip is likely caused by excessive loading of the 9VAC line inside the computer. The voltage of the 9VAC line at the power supply connector should be checked without the computer powered to determine where a fault may be present. If this shows greater than 9VAC, then one may check the current going to the computer on both the 9VAC and the 5VDC lines to determine if it is out of specification, or the computer may be immediately checked internally to prevent possible damage during the time it is powered up.