-- o ICOM

Number 25 o n your Feedb8Ck C8rd
Mike Bryce WBBVGE
222S Mayflower NW
Massillon, OH 44646
Building one's equipment is a
lot of fun-great fun, in fact. On
the olher hand, it sure is no picnic
when you're all done, and then
nothing happens. So, we'll lake a
close look at getting that dead radio
to lIy with some simple troubleshooling
technique.
Before you start to co nstruct
any project, give the schematic a
good look over. You will occasionally
discover errors and, somewhat
more often, important omissions
in product documentation
and magazines. Look over the
schematic for proper Vcc connections,
ground points, input/outputs,
and other important details.
In many cases, lhe Vee pins are
not marked in schematics, assuming
that everyone koows that pin
14 of a 401 1 chip is Vee and pin 7
is ground . Of course if you don't
know that, and you don't connect
the pins to the proper points, the
circuit just won't work! Aside from
that, there are zillions of reasons
for a non-working project. Let's
take a look at several ouncse reasons
this month.
The Approach
Other than looking for errors in
the schematics and wiring, what
do you do? Break the problem
down into small bite size pieces.
You'll need some rather simple
test gear, including a good quality
VOM , either an analog or digital
model. You'll also need a logic
probe lor tfose digital projects,
and a variable power supply. The
supply shOuld sport a current me.
ter.
Now for the troubleshooting.
First things first. Just what is the
project doing? Or not doing? Just
because the transmitter has no
output doesn't necessarily mean
the circuit is totally dead. Let's
connect our small transmitter to
the power supply. With the ammeter
moniloring me current being
drawn, we can get a reasonably
clear picture of what is going on.
After you power up, what kind of
current do you see flowing? Depending
on th e ty pe of ci rcuit
you're working on, you should see
some current being drawn by the
oscmatcns). Key the transmitter.
Does the current go up? If not.
76 73AmateurRadio · June, 1989
Low Power Operation
then either you're not applying a
key voltage to the PA, or the PA
transistor is defective. Remember,
most failures occur because
of heat. The PA transistor and the
power supply pass transistors
generate heat, so you should
watch those parts. If you don't see
any current being drawn by Ihe
circui t, check for Vcc on the oscillator.
Whal do you do if your dead
project has a self-contained power
supply? Well, don't laugh, but
be sure that it' s plugged into a live
o u tlet. Check for the proper
voltage coming out of the power
supply. Look for internal luses.
Some transformers use a fused
primary wire inside th e transformer.
When these go, you can't
fix them, short of replacing the
defective unit.
Another common problem with
power supplies is that there may
be steering diodes used fO( battery/AC
operation. Check these
diodes with your VOM .
Without the oscillator, you'll not
get a microwatl of power out. You
can check for proper oscillator operation
with either an AF probe or
a general cove rage receiver. I use
a frequency counter on my bench.
A small two or three loop pick up
coil will sniff out the RF. I won't get
into PLLs. They can cause a lot of
trouble and are beyond the scope
of this column. In troubl eshooting
most simple two or three transis-
... c.
~
Ph%A. Some of the projects that didn't quite make it . . .
tor QRP transmitters, check the
oscillator first . Next, check for output
of the oscillator. A coupling
capacitor may be at fault. From
the oscillator, check for output at
the buffer transistor (if used). If all
is as it should be, but there's still
00 output shOwing at the antenna,
check lor Veeon the PA transistor.
Now check the outpct coupling
capacitor. Capacitors rarely go
bad, but sometimes you pick out a
dud from the junk box . An cut-ottolerance
capacitor can cause
you a lot of fuss.
Next in line to look at is the
tra nsmitter. Does it u se TRI
switching? If so, check the diodes.
A shorted diode will cause the output
to come up zero. In this case,
monitor the current meter on the
power supply. It should show normal
current, perhaps even draw
more current than it should. If that
Photo B. A little home-brew unit that sports a VMOS power amp and a
direct conversion receiver.
is the case, check tne output seclion
for a short. You can use your
VOM 10 find a DC short, but not an
RF short caused by a capacitor
breaking down under the presence
of RF. In lhis case, use a
WCF. A WCF? Yup, a Well Calibrated
Fingertip.
Happy, heahhycapacitors don't
get hot during operation. With
your fingertip, check the output
capacitors. (With the power off!
No RF burns, please.) If you find a •
warm capacitor, replace it. I also
use this technique when troubles
hoot ing computer mother
boards. All those 0.1 IlF caps on
each chip. It has saved me a lot of
time. Take ca re, however, to
ground yourself when using th is
method when working around
com ponents such as C MOS
chips. Static electricity ca n kill
these components.
After yo u get some troubleshooting
time under your belt, the
WCFwill save you a lot of grief. PA
transistors, audio power amplifiers,
and ot her heat producing
devices all run warm . You can tell
just how well the device is doing
just by the amount of heal it generates.
Reg ulator ICs should be
warm. If not, then the circuit is not
drawing much current, or none at
all. If the regulator is steaming hot,
too muc h current is being drawn.
After exhausting all the easy-tofix
problems, we have to look a bit
deeper into the circuit. I've atways
had a hard time with fixing a radio
thai I did not build. However, I' ve
had some luck by having someone
look over my work. Sometimes,
after spending hours and
hours looking over a circuit, you
just can't see the problem. A
friend can sometimes pick out the
troub l e i n a fe w secon ds!
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