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CPREC Newsletter 723 April 2020

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


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


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


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


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.


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

[email protected].

Take care and be mindful of government

guidelines.

Alan

http://www.telegraph-office.com/pages/mcelroy.html

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