More Laboratory Testers

Valve voltmeter, Dawe Type 613C

Valve voltmeters such as this were employed when only a very high impedance load could be tolerated by the item being tested. In practice there was a problem. If the signal being measured had a pure sinewave waveform the reading would be true RMS otherwise the reading would need to be judged as relative i.e. One could peak something up or tune something out but you couldn't guarantee the voltage reading to be valid because many waveforms met in practice would have a degree of distortion typically because of harmonic content. This example made by Dawe Instruments measures voltages in 6 ranges from 1V to 300V and, via a toggle switch the ranges change to cover 1mV to 300mV. Usefully the ranges are also marked in dB with 3.16 Volts marked as 0dB and 3.16mV as -60dB. It probably dates from the late 1950s.   The red coloured label, top left, would be illuminated and serves as the ON indicator.         The method of detaching the case is unusual. There's a small dzus fastener at the lower edge (where its says "DAWE") at the front and a similar one at the back panel which when unscrewed half a turn allows the lower edge of each bezel to be pulled forwards, lifted upwards and removed and so releasing the side covers. A clever idea but it must have given the drawing office and metal fabricators a headache and been relatively expensive to produce. There are some 11 valves which surprised me somewhat. These are five EF91, two A2134, and one each EF86, ECC83, EB91 and EZ80.

The reverse side includes three aluminium covers which are detached as below.

I don't have the circuit diagram or the specification of the voltmeter.

Audio Signal Generator, Solartron OS101

This piece of equipment is built like a battleship. It weighs something like half a hundredweight and according to little markings on the dial of the meter dates to 1955. Frequency coverage is 25c/s to 250Kc/s and of course predates the move to "Political Correctness" of scientific units. Inside reside four small B7G valves and a larger 5Z4G rectifier. Although these equipments were very stable and accurate it used to take some time for them to warm up and meet their specification. Usually they were switched on at the start of the day otherwise an engineer would have to make the tea before proceeding with his work while the meter stopped wandering up the scale. This example is a 19" rack mounting type fitted in a standard case. Unfortunately it's been stored in damp conditions resulting in deterioration of the finish. This impressive piece of British Engineering was used by a friend of mine who makes microphones in the neighbouring village of Burley in Hampshire. The "Muirhead" style slow motion dial has a standard cursor at the top plus a vernier at the bottom. The latter could be used for resetting to a specific dial reading so that batches of items under test could be checked at the same frequencies.

AVO "All Wave Oscillators" I seem to have picked up three different AVO Signal Generators

Before cleaning   After cleaning

This one (above) is an AVO wide range Signal Generator and has a turret tuner. Nowhere on the case is the model identified but, like the first king of a particular name, the type number seems to have been allocated when the second version was introduced. It's advertised as merely "AVO Signal Generator" in a 1949 Instrument Manual together with a Mk7 AVO meter. The one below must have referred to as a Type II and that above a Type I ?

Oddly one (unattenuated) RF output is accessed down that metal tube on the right labelled "Force". The tube presumably helps to stop RF leakage affecting the attenuated output.

After removing 14 screws the metal cover can be removed

Two valves are evident, an L63 modulator left and an EF91 oscillator inside the RF box.

As you can see this instrument covers from 50Kc/s to 80Mc/s in 6 wavebands and below is the circuit diagram. I checked the electrolytics and they measured 8uF 0.2 ohms and 9uF 0.24 ohms so must be in excellent shape.

The third AVO signal generator at the Radio Museum is a Type 3 (or Type III... take your pick!)

Compared with the Type II there are several changes, in particular the instrument has been considerably cheapened by doing away with the diecast chassis, saving a lot of weight at the expense of little by way of practical performance. The thing is aimed at the budget end of the laboratory market or more at the retail equipment service engineer. You can see perhaps evidence of pressing the designers and pushing it into the marketplace as quickly as possible by differences in labelling. Is it a Type 3 or a Type III? It also displays one of my pet hates.. namely a poor choice of colour versus labelling, although the yellow might have been a little whiter when first printed.

Inside, the construction is cheap and cheerful. The message on the can is a bit mystifying and may reflect a design glitch? The tin plating is clearly pretty poor and the tin itself may have had its origins in a confectionary firm. Below.. the can removed (without the help of a tin-opener)...

Cost-cutting so gone... the diecast box and turret tuner and in their place a sweetie tin and a yaxley switch. A double triode ECC81 serves as oscillator and modulator but without the expensive modulation transformer, instead using a small LF choke. The bridge rectifier is now a half wave affair. I didn't see much purpose in removing the front panel as this would be time consuming although underneath you'd see some mechanical detail regarding the tuning and attenuator switches.

Frequency coverage now extends to cover TV Band III but no longer covers lower frequencies common in early superhet receiver IFs.

Marconi TF868/1, Universal Bridge

This is used for measuring Resistance, Capacitance and Inductance Somewhat larger and more consuming of power than the latest hand held LCD multimeters but in the 50's, 60's, 70's and even much of the 80's you didn't have a lot of options. It was either the trusty AVO, or one of these if you wanted to look the part. It came from the same place as the OS101. Ranges were quite decent and like other Marconi instruments the design is well thought out; Resistance:- 0-10/100/1k/10k/100k/1M & 10Mohms Capacitance:-0-100pF/1000pF/0.01uF/0.1uF/1uF/10uF & 100uF with tan delta Inductance:- 0-100uH/1mH/10mH/100mH/1H/10H & 100H with Q The principle of the bridge requires the meter to be dipped using appropriate delta or Q settings with capacitors or coils. Measurements of resistance are made at DC whilst C and L are measured at either 1KHz or 10KHz and measurements to 3 significant figures are readily made. Not that easy to use otherwise why would they written all the instructions on the top! Compare this and other ancient stuff with the modern Peak instruments; below in use in an R107 refurbishment.

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