Tested with analog meter open circuit both way. will reinstall venthood tomorrow. At least this is keeping me out of trouble I think. thanks Don
What ohmmeter are you using and on which ohms scale?
You need to have enough voltage across the diode to turn it on.
Many people incorrectly believe a diode only lets juice go one way. This is kinda true, but the way they really work is more like a switch. A normal silicon diode requires above 0.6 volts on the anode (compared to the cathode <the line on the diode>)for it to start conducting. When it is reversed you have negative voltage at the anode, so the diode turns off and no longer conducts.
But a Zener diode is designed for REVERSE bias, the opposite of above. That is what makes them good voltage regulators. They "breakdown" at a certain voltage and can be designed for various breakdown voltages. But your ohmmeter has to have above the Zener voltage to see the reverse bias effect.
However, they USUALLY can be cold tested in the forward bias direction and get that 0.7 VDC drop. Then your ohmmeter has to supply more than than 0.7 (in most cases) to turn on.
The problem is there are exceptions. Take HV didoes for an example. Those are usually many didoes in series in a single package. Let's say there are ten diodes. So instead of needing 0.7 volts, you now need a full seven volts for that diode package to show any indication at all on an ohmmeter.
But I do not know how your Zener diode will read on an ohmmeter. But higher ohms scales are higher voltages on analog meters, so you could get a reading.
But think about this. Let's say that diode is really open. That means you still will have a problem after the diode is replaced (most likely). Because that diode, when good, can only drop a voltage. It does nothing else. So when it opens, the voltage goes too high. So what does the too high of voltage do after that? One thing it won't do is stop whatever it is from working unless something is damaged from when the diode opened.
So do NOT expect replacing that diode to fix anything. It is very unlikely (but not impossible).
The only good way to test it is live and check for the ~5.3 volts. The +5.3 volts should be on the cathode, negative lead of voltmeter on the anode. If you have 5.3 volts there, it means your problem is NOT there.
And TBH, I don't think that is your problem. I would be looking elsewhere.
Now if that diode was shorted, I would say you found the problem as then the voltage there would be way too low to run anything.
Anyway, if you mean you're going to put it back and check the voltage, that is the very best way to check. Be sure to post here what you get. My guess is you will find your 5.3 volts there, if you have voltage going into the card to begin with.
If you have no voltage there and the diode is really open, your problem is NOT the diode, but something before it.
-Don- Auburn, CA