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CPREC Newsletter 723 April 2020 Page 1
G2LW
Crystal Palace Radio & Electronics Club Affiliated to the Radio Society of Great Britain (www.rsgb.org)
Established 1956
April 2020
Due to the Coronavirus (COVID-19) pandemic,
club meetings have been suspended until further
notice.
G2LW
Web: www.cprec.btck.co.uk
Email: [email protected]
Club Net: Each Wednesday at 20:00 on 145.525MHz (S21) ± QRM
CPREC Newsletter 723 April 2020 Page 2
Welcome
As you are aware Bob (G3OOU) stepped down as
editor of the Palace Pulse at the end of last year and
his last edition of the ‘Pulse was in February. We
advertised the position of editor, but unfortunately
the pay wasn’t sufficiently attractive for anyone
wanting to step into Bob’s boots. The upshot of this
change is that until we get a permanent editor the
frequency of the newsletter will drop from monthly
to bi-monthly and I will act as a sort of compositor
collecting content sent to me by members or
associates and generating the newsletter as best I
can. In the past we sent a few newsletters out by
post but from now on distribution will be by email
only. You may well find that future newsletters
will be light on content but of course this depends
on how willing you are to support me in this
endeavour.
In February we had the club’s AGM and all
members with email addresses should have
received a copy of the minutes (distributed on 3rd
March). I haven’t received any comments on the
minutes so I’m rather pleased that everyone agreed
their contents. In March we had a talk on Chassis
and Box construction by Jim (MOJFL) a summary
of which is included below.
I never imagined that my first attempt at putting
together the club’s newsletter would coincide with
a pandemic. We’ve all been told to keep away from
others and I’m certainly practicing self-isolation
although I still need to go out and buy groceries.
You can keep up to date with breaking news from
CPREC by visiting the club’s web site or our
Facebook page and of course you can always email
us at [email protected].
Alan (G8NKM).
March’s Meeting - Sheet Metal Working by Jim
(M0JFL)
Jim explained his talk was aimed at those home
constructors who required small boxes or chassis’
for their projects. He suggested it’s best to buy
ready fabricated items rather than trying to make
them yourselves although this does force
constructors to standardise on what’s available.
Unfortunately these days it’s not easy to obtain
chassis’ or small metal boxes suitable for home
projects and sometime constructors are forced to
make their own.
There are a number of materials that can be used,
however constructors need to be aware of how
these various materials behave while being cut or
bent. Aluminium sheet is soft and isn’t the best
choice of material. When cut it burs, clogs files and
wraps itself around drills, however it’s relatively
cheap and is easy to bend. If possible use an alloy
such as Dural (6XXX series of aluminium alloy)
although harder it’s easier to work but more
expensive. Both copper and brass “work” similar to
aluminium although brass is harder than copper but
it does drill well. All the materials mentioned can
be annealed to soften them which is useful
should you make a mistake while making a bend
(most likely due to poor marking out!). Other
materials include tin plate which is a very thin steel
sheet with a tin coating, harder than aluminium it
works well and can be easily soldered however
watch your fingers when using tin plate as it has
very sharp edges. Another material that many
constructors ignore is Galaveneal, a galvanised
form of steel sheet which can be found on the back
of washing machines, microwaves etc. Needless
to say small pieces of Galaveneal are easily
obtainable from various scrap sources.
The most vital part of making a box is accurate
“marking out”, spending time on this stage saves
time and minimises waste material. To mark out a
sheet you would normally use try squares, rulers
and scribers. Scribed lines leave unwanted
marks on the work so for front panels these are best
avoided. Fine marker pen works well and felt tip
markers can be used to “paint” an area. Pencil can
be used but lines tend to disappear just as you make
the bend or cut the sheet. Marking Blue is the best
aid for marking out your work. When developing
the shape of a box it’s best to draw it out on paper,
creating a template. Figure 1 shows a paper
template drawn
for a small box
fabricated in
aluminium, note
the four small
corner holes
which are
required to stop
the material
“folding” when
the sides are
bent over. Fig. 1
CPREC Newsletter 723 April 2020 Page 3
It’s easy to spoil the appearance of your work with
tool marks such as a vice jaw so it’s best to protect
faces with tape or use wooden strips when the work
is held in a vice. Folding or bending bars are
simple to make with a couple of lengths of mild
steel angle, Jim noted that his are a bit rough for
fine work, see figure 2.
When making a box it’s simple to bend the first
two opposite sides over, however the two
remaining sides are difficult as the bent leaves foul
the bending bars. To make a box you normally
require a former made to the dimensions of the
box. For the box shown in its templated form in
Figure 1, a wooden block former the size of the
internal dimensions of the box was fabricated, the
four sides of the cut aluminium sheet were then
knocked over the block. When bending the sides
of the box a square wooden backing piece was
used, carefully keeping the edges of the backing
piece against the fold lines. The rectangular
backing piece used for this particular job was 5mm
smaller than the top of the box and it was clamped
very firmly while the edges were knocked over.
Whenever sheet metal is hit it stretches and as such
becomes thinner and larger. So when bending
protect the work with a piece of wood, use a mallet
or a Panel Beater’s hammer. An ill judged hammer
blow will leave an unsightly crescent shaped dent
in your material. For thin material it may be
possible to make bends with your fingers.
Drilling and Cutting
A guillotine is best for cutting sheet however it’s
more likely that you have pair of tin snips in the
box. Tin snips come in both left and right handed
versions and can also be straight or curved. When
cutting sheet with snips one side of the cut will
always roll up and it’s difficult to keep the sheet
flat. A sheet metal saw can be used for cutting
larger sheets.
Cutting large holes especially rectangular are
problematic, a scroll saw is good if you have one
else chain drilling and a file may be the only
solution unless you happen to have a suitable Q-
max punch. Hole saws/tank cutters are probably
best avoided as although they are inexpensive they
often give a rough cut. If you do buy a tank cutter
try to get the solid style of cutter.
Drilling small holes is not a problem but when the
thickness of the material is very much less than the
height of the cone point of a drill a roughly
triangular hole is the guaranteed result. This is
because the drill’s point breaks through the
material before the drill’s flutes start to cut. This
issue can be overcome by using a sheet metal drill,
which has a flattened cutting face with a small
central cutting “pip”. A sheet drill can be made by
grinding a standard drill. A useful drill for sheet
metal is the step drill, see figure 3.
Figure 3 - A typical step
drill, these are readily
available.
Triumvirate by ‘Theorist’
Things have changed a lot recently. The Pulse is
under new management. The club has shut down
along with museums, pubs, restaurants, and my
local library. The supermarket shelves have been
stripped. Although it’s not a fair comparison I am
reminded that soon after August 1665, when Isaac
Newton received his degree from Cambridge, the
university closed to combat the spread of the
plague and he went home to Woolsthorpe in
Fig.2
CPREC Newsletter 723 April 2020 Page 4
Lincolnshire. Whilst there he developed his theory
of optics, the theory of gravitation and the laws of
motion, and invented the mathematics of the
Calculus (or ‘fluxions’ as he called it) to solve
problems relating to these theories. Later in life
Newton related the story that his work on gravity
was inspired immediately on seeing an apple fall
from a tree in his garden. If I had seen it I would
have immediately eaten it. Anyway perhaps I’ll
get a burst of inspiration about something while I
avoid social contact in the next few months, but I
have a suspicion that my laptop and Netflix will
prove too great an attraction. That’s my excuse
anyway.
You undoubtedly came across the three laws of
motion [1] and the law of gravitation at school.
What you may not know is that a huge
mathematical structure was developed around these
laws which broadly speaking now goes under the
name of ‘classical mechanics’ (CM) although I
assume that it used simply to be called
‘mechanics’, omitting the word classical. In it
there are three main approaches to solving
mechanical problems. The first was developed by
Newton, whose approach was really to calculate
the forces on everything and then use the second
law to work out the motions. This is F = ma, or F
= dp/dt in calculus terms (and as Newton wrote it),
where p is momentum.
This approach works well for some problems, but
generally more complex problems are better solved
using techniques developed by the French
mathematician Lagrange and the Irish
mathematician Hamilton. Lagrange's approach
involves using the total difference between the
kinetic and potential energy of the particles
involved: L = T – V where T is kinetic energy, V is
potential energy and L is the ‘Lagrangian’. Using
this makes some otherwise difficult problems
rather easy. A reasonably well-known textbook
example involves a large sphere on the top/north
pole of which is placed a small ball bearing. This
obviously starts to roll down the side of the sphere
and at some point leaves its surface. The problem
is to find the point at which it does so. Using
Lagrange’s approach this seemingly intractable
problem becomes a few lines of maths. Another
problem that yields an easy or easier solution is
that of the motion of a pendulum, where the object
from which the pendulum is suspended is free to
slide from left to right (and vice-versa) horizontally
along a rod as the pendulum swings
Better though, and the best ‘go to’ solution, is to
use Hamilton’s approach which considers the total
energy involved, i.e. the sum of the potential and
kinetic energies of the particles/objects forming H
= T + V, where H is the ‘Hamiltonian’. This is the
best method overall and is particularly good at
solving problems with larger numbers of objects
where you would need a very clear head to work
out all the forces using Newton’s method. It used
to be used when calculating the motions of the
planets to work out their orbits and predict solar
eclipses, and such like. It may still be, although I
imagine most of the work is now done by
computers, possibly using brute force calculations.
The trouble is that although CM forms the basis of
all mechanical engineering enabling skyscrapers,
bridges and aeroplanes to be built as well as the
astronomical uses, it is completely incorrect. It
works brilliantly for macroscopic (i.e. large)
objects but at atomic scales when applied to
electrons orbiting the nucleus of an atom it fails
totally. For this a new mechanics was needed,
quantum mechanics (QM), so I assume that this
was when the word ‘classical’ was needed to
describe the old theory.
I sometimes hear people on the TV or radio
correctly say something like ‘you need quantum
mechanics when you are dealing with very small
things like atoms’. However this is misleading.
Just as classical mechanics was meant to apply to
everything, no matter how big or small, so it is with
QM – it is meant to work at all scales, and as far as
we know it does. You could work out the orbits of
the planets and so on with QM if you wanted, but
CPREC Newsletter 723 April 2020 Page 5
nobody would. This is because first of all it would
give the same answers as classical mechanics
(because after all, both are supposed to work at all
scales and CM gives the right answers for the
planets), and secondly because the maths would be
more difficult. In fact it is easily seen from the
governing equations of QM than when the masses
of the particles increase the equations soon
approach those of CM so must give the same
answers.
A feature of QM is that something can be in two or
more different states and even two or more
different positions at the same time. This may
sound impossible and is of course completely
counter to our normal experience of the world. Yet
there is ample experimental evidence that
something like this does indeed happen. There are
optical devices known as beam splitters. These can
be very lightly silvered mirrors, so that when a
beam of light is shone at them a half (or some other
fraction) of the light is transmitted and the rest
reflected. What happens though if a single photon
encounters a beam splitter? QM tells us that the
photon, even though it is a discrete particle, both
passes through and is also reflected, existing in two
places simultaneously – until its position is
measured. Then the photon has to ‘decide’ where
it is, and will ‘choose’ to be either at the position it
would be if it passed through, or at the position
where it would be if reflected. This can be tested
in the laboratory via experiment and found true.
Equally important is that particles can become
‘entangled’. If, say, two atoms or particles interact
in some way then they become entangled. This
means that they become connected in some way so
that no matter how far apart the particles become,
disturbing one particle will automatically disturb
the other. You could liken this to two disturbances
in a pool of still water. The ripples are combined
as they pass through one another and affect one
another even as they spread further and further
apart.
Einstein definitely did not like QM despite being
one of its founders, especially the idea of
entanglement, refusing to accept that it could
happen. Suppose two photons interacted and then
went off in different directions 180 degrees apart.
After two years they would be two light years
apart, yet disturbing one of them would
instantaneously affect the other which would
appear to break the idea that ‘nothing can travel
faster than the speed of light’
These ideas are behind the quantum computers that
are now being developed and will revolutionise
computing if and when technology allows.
[1] Actually the first law is just a special case of
the second law where F = 0 so that the acceleration
must therefore be zero. Two laws would suffice.
Bench PSU Repairs by Bob G3OOU
I have owned and used a Daiwa PS-120M power
supply for some years having purchased it from a
silent key estate. When it failed, I had problems
obtaining replacement parts so decided to re-
engineer it with easily obtainable parts. The
original specification was 3-15v at 10A mean and
12A peak current with a front panel analogue meter
and rear panel heat sink.
Design 1 used an L200 regulator and two external
TIP3055 power transistors as current amplifiers.
However, it proved very difficult to prevent the
output devices from failing under short circuit
conditions so an alternative solution was sought.
Design 2 used an LM723 regulator with a BD131
driving two TIP3055 current amplifiers. As the
internal voltage standard of the LM723 is 7.5v it
must be reduced to about 3v using a resistor
network so that the minimum output voltage may
be reduced to 3v. Two 0R1 wire wound resistors in
parallel provide a fixed current limit of about 13A
subject to the tolerance of the current limit circuit
in the regulator.
An output fuse was provided to protect against
failure of the current limiter and the analogue meter
was replaced with a low cost dual readout digital
meter providing voltage and current displays which
can be seen in the nearby photo.
Some basic RF decoupling was provided together
with high peak current diodes to protect against
reverse output polarity and the output voltage
holding up longer than the input voltage.
A circuit diagram of the power supply is attached
at the end of the newsletter.
CPREC Newsletter 723 April 2020 Page 6
Snippets
Morse
The US company McElroy were often promoted as
the best maker of mechanical bug keys. Eric
G3IIR, a silent key CPREC member and RSGB
President, had one and I watched him use it on HF
field days but I never managed to get my hands on
one. Now they are like gold dust and cost a mint so
getting a left handed one is probably impossible.
T R McElroy held the morse receiving record for
many years at 77wpm helped by his ability to type
at 150wpm. Not sure if that CW record has ever
been broken!
See www.telegraph-office.com/pages/mcelroy.html
- Bob G3OOU
2M Net
The club net on 18th March attracted 8 participants
which must be a near record. Andy G2RW in
Tooting Bec was running 5W and Dave G0PAR in
Dulwich was also on air, both using indoor aerials.
Bob received their signals 5 and 9 at his QTH in
Cousldon.
Events
Needless to say in the current climate there have
been many events postponed or cancelled.
The Kempton Park Rally has been rescheduled for
Sunday 15th November.
CATS Bazaar will now take place on Sunday 6th
December.
If you have an article or anything of interest for the
newsletter please contact me at
Take care and be mindful of government
guidelines.
Alan
CPREC Newsletter 723 April 2020 Page 7