60 February 2018
READER - TO - READER TECHFORUM
Legacy Printer Port (LPT1)
I’m doing some work with legacy
computer equipment and have
written Basic software to control
time and temperature in a high
temperature kiln. I’ve used an old
Compaq laptop (LTE Elite 4/40CX).
I use the COM 1 port to control
temperature and the LPT1 port to
cycle the kiln on/off via an SCR.
Everything works great. I’m
able to toggle the printer port pins
via OUT commands. When trying
other legacy computers, everything
works except the printer ports don't
respond to the OUT commands and
remain in a high state. However, they
DO operate the dot matrix printer
indicating they are functional. My
question is how do I gain control of
some of the data pins and be able to
toggle them high and low?
#2181 Jerry Sobel, R.Ph.
Las Vegas, NV
Amplifying Plant Signals
I’m trying to duplicate an ESP
experiment described in the book
“What A Plant Knows,“ by Chamovitz.
Basically, I need an amplifier and
sensor to read signals from stems and
roots. What frequency response does
the amp need? My guess is from DC
to maybe 20 Hz? If this is the case,
then what sort of amp configuration
do I need?
#2182 Carlos Dixon
Measuring Breathing Rate
I’m building a lie detector based
on heart rate, breath rate, and
galvanic skin response. I have it all
figured out except for breath rate.
What sort of sensor should I use to
#2183 Jeanne Villeneuve
Neutralizing Battery Corrosion
What’s the best way to neutralize
battery corrosion? I inherited an old,
but expensive quartz watch from
my grandfather and found a heavily
corroded coin battery inside the
watch case. I need to neutralize the
corrosion without damaging the
#2184 Clarence Dugan
[#11171 - November 2017]
Variable DC Motor Control
I’d like to have continuous
variable control of the motor in my
bench vibrator/polisher. It has a DC
(not AC!) motor that is rated at 90
VDC with a power supply that delivers
10 amps. I prefer to build something
myself. Anyone have some design tips,
or better yet, a schematic?
#1 Here is the “wrong” answer to
the problem: Harbor Freight sells a
router speed control for $20. Plug it
in between the wall outlet and your
tool. See if it does what you want. If
not, take it back and they will give
you your money back.
#2 What about using a variac with
a standard full wave rectifier and
large filter capacitors? The schematic
(Figure 1) lists several commercial
Several things to mention before
any attempt at construction:
• Be aware that most variacs are
not fully isolated, meaning the neutral
on the mains supply is connected to
the neutral on the output.
• Mechanical stops will need to be
implemented on the control dial as
not to exceed a 90 volt DC output
and most likely on the low end to
prevent the motor from stalling.
Atlantic Beach, FL
[#11175 - November 2017]
Troubleshooting Amp Problems
I picked up a CrownDC150a
power amp at a garage sale. When
I tested it on my bench, one side is
rather distorted but then clears up
once the amp warms up, making
it hard to trace. Is this a common
problem with these amps? Any
troubleshooting tips are appreciated.
#1 One can often use a controlled
burst of freeze spray to hit individual
components once it has warmed up.
Hitting resistors or capacitors or even
solder connections may reveal the
#2 You mention the model is
DC150A. Crown does not list that
model but does have a D-150 and
Service info is here: https://www.
The symptom you mention could
be crossover distortion in the output
stage due to an out-of-spec bias
voltage (very low or non-existent). As
the amp warms up, the bias voltage
generator begins to function to some
degree and the distortion becomes
less. The bias voltage is designed to
vary as the output stage temperature
changes, but should never fall to zero.
Connect an audio generator (1
kHz, variable 0 to about 2V RMS sine
wave) to the amp's input and connect
a suitable sized dummy load ( 8
ohm) resistor and oscilloscope to the
output terminals. Power-up the amp
and observe the output sine wave