Some Laboratory Testers

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 Condenser Checker

Heathkit home built tester made from a kit of parts. It uses a "magic eye" to indicate resonance click the picture to see more details

Spot 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.

Wheatstone 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!

Slide Wire Potentiometer, Cambridge Instruments

This complicated-looking 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 200microvolt.

Resistance Boxes

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 ohms.

RF Signal Generators

Made by Taylor, this must be one of their last valved equipments before transistors took over. 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 set. 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 the schematic.

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?

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