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Keymaster
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#453 |

Hi Jim,
I’ll deal with the Diodes first:
Digital multimeters can test diodes using one of two methods:
Diode Test mode: almost always the best approach.
Resistance mode: typically used only if a multimeter is not equipped with a Diode Test mode. In some cases it may be necessary to remove one end of the diode from the circuit in order to test the diode. this is true with the following Diodes in your PSU Module D10, D11, D12, D25, if any one of these is short or leaky all will read as faulty.
A multimeter’s Diode Test mode produces a small voltage between test leads. The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. The Diode Test procedure is conducted as follows:

Make certain a) all power to the circuit is OFF and b) no voltage exists at the diode. Voltage may be present in the circuit due to charged capacitors. If so, the capacitors need to be discharged. Set the multimeter to measure ac or dc voltage as required.
Turn the dial (rotary switch) to Diode Test mode ( ). It may share a space on the dial with another function.
Connect the test leads to the diode. Record the measurement displayed.
Reverse the test leads. Record the measurement displayed.
Diode test analysis

A good forward-based diode displays a voltage drop ranging from 0.5 to 0.8 volts for the most commonly used silicon diodes. Some germanium diodes have a voltage drop ranging from 0.2 to 0.3 V.
The multimeter displays OL when a good diode is reverse-biased. The OL reading indicates the diode is functioning as an open switch.
A bad (opened) diode does not allow current to flow in either direction. A multimeter will display OL in both directions when the diode is opened.
A shorted diode has the same voltage drop reading (approximately 0.4 V) in both directions.
Full Details Here:Diode Testing
Capacitor Testing
Use your digital multimeter (DMM) to ensure all power to the circuit is OFF. If the capacitor is used in an ac circuit, set the multimeter to measure ac voltage. If is used in a dc circuit, set the DMM to measure dc voltage.
Visually inspect the capacitor. If leaks, cracks, bulges or other signs of deterioration are evident, replace the capacitor.
Turn the dial to the Capacitance Measurement mode ( Capacitance symbol ). The symbol often shares a spot on the dial with another function. In addition to the dial adjustment, a function button usually needs to be pressed to activate a measurement. Consult your multimeter’s user manual for instructions.
For a correct measurement, the capacitor will need to be removed from the circuit. Discharge the capacitor as described in the warning above.

Note: Some multimeters offer a Relative (REL) mode. When measuring low capacitance values, the Relative mode can be used to remove the capacitance of the test leads. To place a multimeter in Relative mode for capacitance, leave the test leads open and press the REL button. This removes the residual capacitance value of the test leads.

Connect the test leads to the capacitor terminals. Keep test leads connected for a few seconds to allow the multimeter to automatically select the proper range.
Read the measurement displayed. If the capacitance value is within the measurement range, the multimeter will display the capacitor’s value. It will display OL if a) the capacitance value is higher than the measurement range or b) the capacitor is faulty.
Full Details Here:Testing Capacitance
It sounds more complicated than it is really Jim, always measure devices out of circuit to avoid misleading readings, isolating only one end of either a Diode or Capacitor will achieve this. If you find the readings you are getting misleading or inconclusive post again with specific details of what you are reading for help,

regards

George