24 June 2014
schools throughout the US for 'fire
insulation.' Now, it's a big problem.
I'm with you — err on the side of
caution until someone figures out
what's going on. Certainly, the
manufacturers aren't publicizing this
I'm writing about "The Decade
Box Revisited," by Frank Muratore in
the April 2014 issue. This circuit is
similar to one published in Nuts &
Volts by Fred Blechman about 10 or
12 years ago.
However, I believe Mr.
Muratore's circuit arrangement is a
little better than Fred's was.
Frank mentions the possibility of
burnout if a 10-turn potentiometer is
used. This is also possible, though
not as likely in the circuit printed.
The specified pots — according to
the 2013 Parts Express catalog — are
rated at 1/4 watt.
The power rating is much lower
when the pot is set to a low
resistance, and can result in pot
burnout (I know from experience). I
have two suggestions to reduce the
chance of burnout. First, connect a
1/2 watt resistor — 10% of the
resistance of the pot — in series with
Second, consider the use of
higher power pots, especially for
100 ohms and 1,000 ohms. Pots
with a two watt rating are available
at a higher cost.
Definitely a 'retro' article. An
experiment really. I had a similar box
30 years ago.
In any event, your catches are
right on target. Great mods.
Points to Consider
There are some discrepancies in
the April 2014 Ba TESLA article:
1. C1 is listed in the Parts List as
a high voltage cap, but C4 is
referenced in the write-up (p. 26).
2. A five foot CFL is referred to
on p. 27. Where could I purchase
3. The use of the term "winding"
to describe one turn around the
form is not common in electronics
(see p. 22, et al.).
4. The Parts List would be much
more useful if manufacturer's part
numbers were listed.
5. The use of the term "metric"
on p. 23 to mean parameter is out
of place in DIY electronics.
6. It would be a great help if the
schematic had parts values on it.
7. Use of the power rectifier
BR1 at RF frequencies might better
be served by four lower capacitance
high-speed switching diodes.
Likewise, use of an electrolytic
capacitor at RF (C3) is not
necessarily going to produce good
results unless a low inductance
capacitor is paralleled with it.
8. The external DC input
referenced on p. 24 doesn't seem to
be on the schematic as such. There
is a TP there, but that's not usually
considered to be an auxiliary input
connection — especially since there's
no polarity indicated nor another
connection for the other power
9. What is a "rectified input jack"
referenced on p. 24?
10. How is the "programming
I have addressed each of Phillip’s
1. The C4 reference on p. 26 is a
typo and should read "C1." C1 Is
listed as a "1 µF metalized
polypropylene capacitor" in the parts
and does not have a voltage
designation. The article points out
that it must withstand the high
voltages developed. A good source
for this type of capacitor is from the
circuit boards of obsolete CRT
monitors. It is possible to use
multiple 250V or 800V caps.
2. This is obviously a typo and
should read "five foot FL
3. I'm not sure how the term
"WINDING" does not properly
describe something that is
"WOUND" on some type of form.
4. I agree.
5. I am not sure how this is a
6. I am not sure how this is a
7. Any type of diode rectifier
circuit that can be quickly
charged/discharged will work. Feel
free to use any type of
diode/capacitor combination you
have on hand for this circuit.
8. This is a typo and should have
read "external AC source." It should
have been deleted from the article
since I did not include an external
power jack in the schematic.
9. See previous answer.
10. The programming port is the
three-terminal audio jack that
connects Serial In, Serial Out, and
GND to the PIC for programming it
in place (audio jacks are the most
common type interface for PICAXE).
This connection to the programmer
is not necessary if you program the
PIC outside the circuit. Most micros
use an FTDI converter for USB to RS-
232. The DATA(+) and DATA(-)
become Rx/Tx which connect to the
serial pins on the MCU. The Tx
connects to the SeriaI In and the Rx
connects to the Serial Out pin.
Smiling About Arduinos
I’m reading Smiley's Arduino
series and plodding along. Getting
better daily. Well done, Mr. Pardue!
Bill Pointon WA2CG
Love That Pi!
I started with Heathkits,
READER FEEDBACK Continued from page 7