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Low capacity or an open capacitor affect ignition because there is insufficient surge current to
reliably light the bulb. If disconnecting a capacitor does change either flicker or ignition, then the
capacitor is functioning. Reconnect it. High ripple current caused by a diode failure will result in
overheating of R3. You may use a clamp-on A.C. ammeter on the common D.C. line to the
capacitor bank as an indirect method of checking ripple. The 8502 capacitor bank will have a
nominal 25 ampere (AC) current flow. Higher current indicates a high ripple condition. Lower
currents, may indicate open or deteriorated capacitors.
Current into the capacitor bank varies with adjustment of the power output. As the output power is
decreased, current flow of the capacitor bank will decrease.
Diode testing and replacement
To test a diode it must be disconnected from the circuit in which it is used. In many failure modes,
the diode failure can be tentatively identified by discoloration of the bright surface because of
excessive heat. You will still have to disconnect the diode and make further checks to verify failure.
After disconnecting the diode, sue your VOM on the R x 1 scale. With the meter leads connected in
one direction, the reading should be zero or close to it; reversing the meter leads, the indication
should be a very high resistance. If the diodes does not exhibit these characteristics, replace it. If
you have removed a diode from its heatsink or you are installing a new diode, observe the
following instructions: Clean the area of the heaksink in which the diode is to be installed.
Warning
You must use heatsink compound. It is caustic in nature. Do not use for fingers; keep it
away from your eyes; and do not ingest. Follow instructions on the container in which the
compound is packaged.
You may obtain heatsink compound at your local electronics dealer. Apply the compound with a
wood or plastic spatula; a popsicle stick will work well. A thin layer of compound is adequate. Use
internal star washers as opposed to split washers. When tightening the diodes in the model 8502
power supply, the torque should be 90 in.lbs., 125 in.lbs. In replacing diodes, observe polarity.
Model 8502 – diodes 1-3 are 1N3291R types, diodes 2-4 are 1N3291S types.