diode, rectifier dead?

probably

What's the forward voltage spec of the diode? What's the highest voltage your meter can supply? if second is lower than the first it will look open.

If the diode tests shorted in both directions it's bad. The one that tests open is probably good, those have a high breakdown voltage, the ohm meter can't exceed that.

The diode that reads short in both directions is definitely bad. The other, if it's a high voltage rectifier, might have too high Vf to be properly tested by a multimeter. Those are best tested by a Huntron Tracker or by replacement.

if

Or test them by applying mains voltage through one to a low wattage incandescent bulb. If the diode is working, the bulb will be dim and flicker slightly running on half wave power.

Testing such high voltage diodes ( really a multi-diode stack ) requires a source of voltage more than what a DMM or analogue ohm meter has available.

Try connecting a 9 volt radio battery in both directions, while monitoring current flow with your meter.

..... Phil

With some sort of current limiter, like a light bulb, of course.

Paul

"Paul E. Schoen Smug Pig "

A standard 9 volt radio battery will only deliver about 200mA into a diode stack that conducts at 6 to 10 volts - plus a test takes only a few seconds.

Best avoid alkaline types for this, unless it is near flat.

..... Phil

Some DMM user manuals suggest that you briefly short the diode while the test leads are attached. This forces the meter to downrange and give a less-inaccurate reading of the forward resistance when the short is removed.

"William Sommerwanker"

Most DMMs have " diode test " ranges fitted, which will supply up to a couple of volts with a fixed current of about 1 mA. The reading is then in volts or millivolts.

...... Phil

If the Fluke 87 is prehistoric, you lose.

The Fluke 87 does have a separate diode test. However, I'm almost certain I read what I read in the 87's manual -- but I'm not sure, and I don't have the time to look for it.

Bzzzt. If the Fluke 87 is prehistoric, he wins.