The design of most of
the following mustn't have changed much for years when they were
made. Some look twenty years older than their indicated dates.
Apart from the first and last items, the following came from
Christchurch recycling centre and are marked "Hawker Siddley
Dynamics" and may have been dumped by British Aerospace
who now occupy the old Plessey site nearby. In the 1970s test
equipment began to look "high tech" instead of "quaint".
One has to have very small fingers to operate the latest gear
but the biggest advance is the use of transistors which do away
with long warm-up times. The advice, "switch on at least
an hour before measurements are taken to allow the equipment
to stabilise" is no more.
Heathkit home built
tester made from a kit of parts. It uses a "magic eye"
to indicate resonance
picture to see more details
Gavanometer or "Scalamp" made by Pye
This strange shaped instrument
uses a filament lamp and mirrors to provide extremely sensitive
current measurement. I think the scale reads +/- 7uAmp FSD. At
least it's calibrated in units to 7 either side of the centre
line and there are 10 subdivisions making it possible to measure
changes of 0.1uAmp. There's a multi-position switch allowing
the moving coil to be shunted to provide various sensitivities.
It is claimed that current changes down to 1 nanoamp can be detected
by similar instruments.
One use of this instrument is
to measure magnetic field, when it is known as a "Fluxmeter".
A coil is connected to the input terminals and this can be used
to measure changes in field strength. This technique pre-dates
measurement of magnetic field by nuclear magnetic resonance which
is a technique used in archaelogical searches.
Bridge by H.W.Sullivan
A design dating
back to pre-Victorian times this example was made in 1956 but
looks much older. It was used for making accurate measurements
of a component by comparing its parameters with known values.
Who invented Wheatstone's Bridge?
Not Mr.Wheatstone but a chap
called Christie or so an old Physics book in my library tells
me. None of my other books gives this information and I cannot
find another refenece to Mr.Christie. So who was he? Wheatstone
was the first to apply the bridge for practical measurements.
It's like saying a chap by the
name of Flintstone invented a useful looking round disk and a
Mr.Wheel first nailed them to the side of his cart.
Hands up anyone that knew that
Mr.Wheatstone invented the Telephone. Well he did, and in 1821,
no less than 55 years before it was patented by a Mr.Alexander
Graham Bell! Can anyone explain that conundrum!
Wire Potentiometer, Cambridge Instruments
equipment employs an external "Standard cell" which
can be from 1.017 to 1.019 volts and a 2V accumulator connected
across two pairs of its terminals. I haven't had time to study
it... perhaps someone would let me know what it might have been
used for? I guess it's a precision Wheatstone Bridge and it can
provide readings to 0.0002V which I think is 0.2millivolt or
Top, a basic model, Croydon
RBB5 supplying from 0.1 to 111,110ohms. Below, a Sullivan &
Griffiths Dual Dial equipment made in 1959 claiming zero reactance.
The latter can be achieved by using non-inductive resistance
elements or perhaps bifilar wound components. This model with
three knobs is interesting. One can switch in multiples of 100k,
10k, 1k, 100, 10, 1 : the last three by swinging the lower half
of the dial to the top. This saves panel space whilst keeping
the knobs and scales a handy size. It covers from 1 ohm to 1,110,000
Made by Taylor, this must
be one of their last valved equipments before transistors took
The simple analogue tuning dial
enabled receiver alignments to be made very easily, the engineer
being virtually oblivious to the sort of accuracy imposed by
later digital equipments.
Tuning drift was irrelevant
and in fact for most applications the accuracy provided by this
equipment was fine. The mathematics of superhet ganging alignment
does not require much in the way of pin-point accuracy when it
comes to setting up medium and long waves on an ordinary radio
Below, another similarly styled
equipment from roughly the same date. This is a Grayshaw Instruments
Model SG50. I've used it recently and it provides a decent range
of output signals but alas, using modern analysing equipment,
one can see the output is not very clean, carrying lots of harmonics.
The dial is very simple but very clear and easy to read. Below
the picture is an advertisement for the SG50 in the December
1954 issue of Radio Constructor.
Lyons PG-73N Pulse Generator
This not-so-ancient signal
generator has been sitting in my rack of test gear for maybe
20 years being used very rarely but plugged into the mains as
it has a proper on/off switch. The other day I was rearranging
things and absentmindedly switched it on. There was the sound
of a mains surge (don't ask but I hear this regularly) and the
front panel lamp failed to illuminate. I looked at it and decided
to deal with it later.
The same day I extracted
it from the rack and realised it hadn't been listed in my Radio
website and hadn't got a user manual so looked on the Net and
found one which I cleaned up and is to be seen by clicking
on the picture above. Now, when something fed from the mains
goes pop the power supply is often responsible and turning to
the PG73 power supply circuit I spotted a mains transformer which
is a good start as a failure will be straightforward. Sometimes
its a mains filter capacitor but then again it might be a short-circuit
in the transformer secondary circuitry. I looked at the rear
mounted fuse and it was open circuit. Only 500mA and not blackened,
just a "medium-sized" failure.
The top and bottom covers
are held in place by four square-section aluminium bars which
neatly hold in place the front handles. The top slides out sideways
but the springy underside cover needed to be flexed slightly
to release it. You can see in the two pictures above what the
innards look like. Early 74 series logic and obviously new to
the designers as they fitted the chips in holders, although I
think this might be partly due to the use of non-plated-through
holes. The power supply is mostly contained on the smaller circuit
board mounted vertically behind the front panel and below is
For a change the fault
only took a minute or so to discover. The smoothing condensers
measured OK (at least they weren't short-circuit) and a check
on the bridge rectifier showed it had a dead short across one
of its four diodes. Fortuitously the rectifier could be removed
without detaching the circuit board although I discovered the
constructional practices harked back to WW2. Nowadays components
are merely poked through the printed circuit holes and solder
applied but in this case the rectifier wires had been cut and
the ends bent tightly across the solder pads. To free up the
rectifier required the bent leads to be straightened whilst simultaneously
applying enough heat to prevent the pads lifting. I managed three
but one fragile connection broke away and the repair needed a
short length of wire to be added.
You'll note the long leads
at the new rectifier. On the off-chance a hidden fault had caused
the original to fail I left the leads long for ease of a second
repair. In the event the repair was successful. Why the old rectifier
failed is a bit of a mystery. I should check its rating and see
it was close to its limit. This brings to mind a long-standing
puzzle. Why do manufactuers make a whole range of diodes rather
than just a fully rated one? Take for example the 1N4000 series.
This range covers operating voltages of 50 to 1200.
I plugged in the mains lead
and the front panel lamp came on so I reassembled everything
and the PG73 can be put back on its shelf. Why is that mains
transformer so big and heavy?