What is this thing (shown in Figure
3)? A friend gave it to me many years
ago because he no longer had room
for it, but it was so unique that he
couldn't bear to throw it away — even
though he never knew what it was.
Now I am in the same situation. I
don't know what it is either, yet I hate
to throw it away, too.
It is very well made and appears
to be some sort of wave guide fixture.
A permanently-affixed vacuum tube
with a glass base sits on the top, and
there are no electrical connections of
any sort coming from the tube. I
placed the device on an overturned
coffee cup to give you an idea of its
size. Thanks for taking a look and
diodes on the armature. There is an
electronic voltage regulator that
changes a field voltage to control output, but it only senses one of the 120
volt legs. I suspect that one of the
diodes is bad, but when I tested them
with an ohmmeter, they passed okay.
Is there a bench test for this type
of diode (1300V/3A)?
Lyle A. Nelson
Devils Lake, ND
reverse voltage exceeds the breakdown voltage of the diode junction,
the diode goes into the avalanche
mode (zener region) and begins conducting. With your high-voltage
diodes, though, this test will never
place the diode in the zener region
unless it's really on its last leg. It will,
however, tell you if there's excessive
leakage because the reverse curve
Don't have an oscilloscope?
Reverse leakage current can be measured using a microammeter in series
with a power supply, as shown in
Figure 5. The HI/LO switch selects
the test voltage. In the LO position,
the voltage across the diode is less
than 50 volts; in the HI position, it's
less than 200 volts. A good diode will
have less than 0.1 µA leakage.
Anything above that is suspect.
A. It's an electron tube reference
cavity, used to tune a microwave
oscillator to 950 MHz. You can find its
specs at www.dscc.dla.mil/Down
Capacitor ESR Tester
Q. I have repaired a number of
high current DC power supplies.
Rather than replace all the main filter
capacitors (expensive), I would like to
Q. I have a 240 volt, single-phase
generator that sporadically
browns-out with one of the 120 volt
legs dropping to 85 volts. My oscilloscope shows some glitches in the output on that leg, and they get larger
with increased load. This is a brushless type generator with rotating
A. Semiconductors are strange
devices in that they don't wear
out like vacuum tubes or mechanical
parts. Their failure is generally catastrophic — either they are dead or alive.
However, semiconductor characteristics can change with age. In diodes, the
aging process usually manifests itself
in the form of increased leakage current. This situation (increased leakage) is exacerbated by higher temperatures, like those found inside heavily-loaded generators. Testing with an
ohmmeter won't reveal this flaw.
A better way to gauge diode performance is with a curve tracer,
shown in Figure 4. This circuit plots
the voltage and current
on the screen of an
oscilloscope as the voltage across the Device
Under Test (DUT)
changes amplitude and
polarity. The diode
should begin forward
conductance at about
one volt. When the
LO < 50V
HI < 200V