On Wed, 2007-08-01 at 10:03 +0100, Bob Towers wrote:
> Stuart Rohre wrote:
> > You an check for gross shorts or opens with the diode test function on the
> > finals too. I would imagine they might fail short rather than open. Do
> > they get hot, even with low power output? That would be a short. Of
> > course, if open, they might act as a capacitor and pass some driver power
> > to
> > the antenna connection.
>
> I think the leads are accessible without taking the board out. Maybe a
> bit of poking around, but definitely worth a try.
> >
> > Just be sure any meter you use has low current if acting as a ohm meter for
> > testing. And voltage. Modern DMM's are probably safe for finals and
> > drivers.
>
> Yes, I only use a DMM these days.
But a DMM can lead you astray when checking around semiconductors. Many
have two resistance check voltages one well below 1/2 volt so as to not
forward bias a silicon junction, and one greater so you can forward bias
such a junction. You can check circuits outside the silicon devices with
the low voltage and you can check the devices with the high voltage. The
high voltage is usually not high enough to make a junction break down in
reverse bias. But may get close on a small geometry UHF bipolar
transistor.
> >
> > Might be reluctant to use an old AVO meter VOM to test in today's
> > transistor
> > circuits.
>
> Years ago I had an ancient AVO but it probably went in a clear out.
> They're worth a bit nowadays...
> >
> > A surely safe test for a final, is to check the source or emitter resistor
> > DC voltage drop. If there is a voltage drop, it is handling some current.
> > If no voltage at the resistor high side, the device is open.
> > 73,
> > -Stuart
> > K5KVH
>
> Just a thought.. When diodes blow, I presume they go open circuit. Or do
> they? Is it possible just to destroy the semiconducting properties and
> leave a linear resistor?
Most go short, because the junction has melted. Generally the junction
melts easier than the leads, but a really bad abuse can burn off the
leads inside the case.
>
> 73
>
> Bob
> MM0RKT
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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