Q&A
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* Unstable
minute or two. If all’s well, you should
be back in business.
Unfortunately, this method only
guarantees that the caps aren’t shorted;
it won’t tell you if they are open,
which will lead to excessive ripple, but
that’s a topic for another column.
Table 2. 4027 Logic
A. I’m sorry, but a NiCd battery is
like a phone calling card. You
have so many minutes of use and,
when they’re gone, they’re gone.
That is, you can only recharge the
battery so many times before it
wears out. This can vary between
500 and 1,000 recharges, but, when
a battery’s number is up, it’s up
forever. You can’t recondition it.
(Although you can try zapping them
for a temporary fix. See the
September 2000 “Q & A.”)
The good news is that the 9. 6
volt stick configuration is very popular
and it is even used in some
flashlights. This means you should
be able to buy them at a discount
from several sources. Pay attention,
though. They have changed since
you bought your original tool and
now come in 1.3 AH, 2.2 AH, and
NiMH versions. All will work, but
some work better than others —
depending on how often you use the
tool and for how long at a time.
Not All Websites
Are Forever
Take It Easy
Q. In the March 2004 issue, I saw
your answer about restoring old
electrolytics. I have a couple of
Tektronix oscilloscopes and some
other test equipment that have not
been used for over 20 years. What is
the correct procedure to safely
return them to working condition?
No doubt, the electrolytics all need
reforming.
Q. I tried to connect to the three
555 timer websites listed in the
“Commercial Grade Delay Timer,” in
the May 2004 issue. The first one
directed me to a questionable search
service and the other two led nowhere.
Please test these out in the future.
Paul Frankle
via Internet
Joseph De Luccia
Saddle Brook, NJ
Stable Flip-Flop
Power-Up
+5V +5V
C1
0.1
4027
Q
Q Start
LOW
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R
4027
Q Start
LOW
Q
C1
0.1
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A. These websites are tested both
by me and the editors at press
time. In fact, I tested those you
mention just now and discovered
that only the first one is, indeed, no
longer available. Unfortunately, that’s
the way it works because the site will
be dropped unless the person pays
the monthly server bill. The other
two are still up and running.
However, I have noticed that the
fonts that Nuts & Volts and most
other magazines use appear in such
a way that, sometimes, a reader will
use an I (letter) when, in fact,
the website character is a 1
(numeral), so you may want to try
both combinations before giving up.
R1
10k
R1
10k
+5V
F
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NUTS & VOLTS
E
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74HCT73
Q
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Start
LOW
R1
10k
C1
0.1
Figure 5
A. If it’s tube operated,
remove all the tubes,
except for the rectifier tube
(typically something with a
4 as the last digit, like a 5U4
or 6X4). Next, find the B+
output capacitor and monitor
the voltage across it with a
DMM. With the scope
plugged into the Variac, turn
on the power switch and
slowly advance the Variac
voltage to about 60 VAC —
half the AC line voltage. If
the fuse doesn’t pop, the B+
voltage will be somewhere
around 120 to 200 volts.
Let the unit sit for an
hour or two and monitor
the chassis for hot spots —
especially the electrolytics.
Next, power down, replace
the tubes, and — this time —
slowly increase the line voltage
to 100% over a period of a
94
Guaranteed Flip-
Flop State On
Cold Boot
Q. I have to start a motor with one
button and keep the motor
circuit active high while other
buttons are depressed. The problem
is that, during a cold power-up of a
typical flip-flop, the outputs are
unpredictable. This causes the motor
to run when it is not desired. I need a
circuit that will always have one
output low at power-up (motor
normally off). The design I’m thinking
of uses a 4027 dual JK flip-flop,
JULY 2004
