siliconchip.com.au Australia’s electronics magazine July 2019 75
ity mode to identify the resistors con-
nected to pins 3, 4 and 12 of the IC and
then remove them.
You can do this by heating the ends
of the resistors alternately with a sol-
dering iron while holding the body
of the resistor with tweezers. Once
enough heat has been applied, you
can lift it right off the board.
If you have a hot air rework station,
that makes it even easier.
It’s then just a matter of soldering a
100mm length of light-duty hookup
wire, or Kynar (wire wrap wire) to the
now-empty pad which connects to pin
12 of the IC, as identified in the pho-
to. This will be soldered to the main
board later.
Winding coils L1-L3
The three inductors, L1-L3, are
wound with 0.8mm diameter (26
gauge) enamelled copper wire. These
are air-cored, meaning the coils are
first wound around a suitably sized
former, then the former is removed.
The coil diameters should all be
3mm, so a 3mm drill bit shaft or 3mm
diameter metal tube would be suitable.
The coil is then self-supporting when
mounted on the PCB.
L1 and L3 need to be 160nH while
L2 is 150nH. To achieve this, wind 11
turns for each coil, but then stretch
L2 so that it is around one millimetre
longer than the other two. That reduces
its inductance to the required value.
(You could, of course, use an in-
ductance meter to verify the coils if
you have one).
If you want to achieve the alterna-
tive inductor values mentioned last
month, reduce the number of turns to
six, then stretch L2 by around half a
millimetre.
Now remove the enamel at each end
of the remaining wire on each coil.
Some enamel coatings vaporise while
being tinned, but most must be scraped
off with a sharp knife.
Take care if you use the latter ap-
proach, especially to avoid cutting
yourself. You can verify that you’ve
scraped off the insulation properly by
tinning the wire ends and then check-
ing that the solder has adhered.
But note that you don’t want a lot of
excess lead length on these coils; just
enough to make it through the mount-
ing holes on the PCB and be soldered
on the underside.
So cut the wire ends to length before
stripping the enamel.
Don’t stretch or compress the coils
to fit the pads on the PCB as this will
affect their inductance; just use a short
length of extra wire at one or both ends
to reach the mounting pads.Winding the transformer
T1 is wound on a 14mm-long fer-
rite balun core. Begin with 400mm of
0.315mm diameter (28 gauge) enam-
elled copper wire.
Fold the wire in half so the two cut
ends meet, then twist the two wires to-
gether to produce a twisted wire simi-
lar to that shown in Fig.7.
It can have anywhere from one to five
twists every 20mm; this isn’t critical.
Twisting the wire simply makes wind-
ing the wire onto the core a little easier.
Wind four turns of the twisted wire
onto the core and trim the ends of the
‘bifilar’ wires, so you have four short
lengths of wire each about 20mm long
appearing at one pig-nose end of the
core.
Tin these four ends. Use a multime-
ter to identify the start and end of the
two coils.
The start of one coil and the end
of the other (shown as ‘AS’ and ‘BF’
in the diagram) go to the two central
mounting pads for T1 (either together
into one pad, or separately into each),
while the other two wires go to the
mounting pads at either end.
It doesn’t matter which goes to
which, as the coil is symmetrical.
Again, cut the leads to leave just a
minimal amount and then strip the
enamel off and tin them before solder-Fig.5: use this
overlay diagram
as a guide to
building the
Signal Generator.
We’ve shown
both LCD screens
in place here,
(Jaycar QP5516
and Altronics
Z7013; one on
top of the other)
but you would
only fit one
or the other.
Edge connector
CON2’s middle
pin is soldered
on the underside
of the board. VR2
can be a standard
16mm pot
mounted through
the board, with
the body on the
underside, or
a 9mm vertical
PCB-mounting
type.