Amateur Radio Report Card

Herbert Cross KIAMN
59 Fla gler Ave .
Cheshire, Conn.
Improving the Paco GDO
I purchased the Paco G-14 grid dip meter
in kit form about four months ago. I assembled
it and found that it worked quite well on all
but the highest frequency range ( 110-250
mc). There was very little indication of grid
current over much of the tuning range. This
was traced to a defective sensitivity control
which left a small bias on the meter. When
I shorted out the control I was able to get a
usahle reading, but now, I found so many
spurious responses that the instrument was
still of little use over this frequency range.
At this point it was back to the drawing
board for a long look at the circuit diagram
where a new problem became apparent. Even
with the sensitivity control working properly
the meter would be shunted with the 270
ohm resistor, R4, which would lower its sensitivity
anyway. The problem was a twofold
one. I had to kill the residual bias from the
sensitivity control and also block the shunting
action of the resistor, R4. I decided to
insert a small silicon diode between the sensitivity
control and R4 with its cathode towards
R4. In this application I hoped to use
the forward voltage drop of the diode to
solve both my problems. The result far exceeded
my hopes. I now obtained grid current
indication varying from a low of 70 uA. to a
high of 250 uA. and the sensitivity control
functioned nicely. To make sure that the
diode wouldn't misbehave I bypassed both
sides of it to ground with .02 ceramic discs
(probably unnecessary) and established a di-
AUCUST 1965
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to B+
S£P&TIVITT
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Partial Schematic of Poco G- 1S g rid dip
meter. Added ports are enclosed by dotted
line.
rect ground to the cover of the sensitivity
control.
Now there were excellent meter readings
but the many spurious dips throughout the
tuning range left much to be desired. I examined
the meter for possible clues. The most
promising lay in the fact that the front end
of the tuning capacitor rotor was grounded
only by the ball bearings. The shaft extended
through the chassis and connected to the large
aluminum tunin g dial. The top edge of the
dial in tum was exposed to the plug in coil.
I made a small wiping contact from spring
hrass and fastened one end under one of the
condenser mounting screws. The other end
was arranged to rub against the back side
of the tuning dial. This proved to be the answer.
There remained only a small dip at 190
mc and another small one at 250 me. Both
dips are smooth and so slight that they don't
interfere with operation. Although one spring
worked nicely, I placed two springs against
the back of the dial and one against the capacitor
shaft where it extends through the
chassis.
With the electrical details corrected, I
found that there remained a mechanical one.
The plastic hairline indicator was threatening
to wear away the graduations printed on
the tuning dial. I solved this by cutting out
a circular piece of thin, transparent plastic
sligh tly smaller in diameter than the tuning
dial. I clamped it between the hub and the
tuning dial so that it rotates with the dial and
takes the wear from the indicator.
I would also like to offer a suggestion to
the manufacturer. The various coils supplied
with the G-15 are very nicely color coded so
that it would be much more logical and convenient
to designate the corresponding bands
numerically rather than alphabetically.
After making the changes outlined here, I
have been very pleased with the performance
of my G-15 and I hope others may do likewise.
. . . K1AMN
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