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Progress with Tablet Instruments (Part 3)

Further to his previous Part 1 posting dated 7th December 2020 (can be viewed HERE) and Part 2 posting dated 26th January 2021 (viewed HERE), Malcolm Kitchen has sent a further report on the work to recreate Tyer’s No.6 Tablet Instruments.

Tablet Instruments – Peripheral Items

Work on the tablet instruments themselves came to a stop during lockdown #3 due to the lack of any more parts at home to work on. The time has instead been spent making some peripheral items that will hopefully facilitate installation of the instruments in the future.

Tablet Instrument Trolley

A fully assembled tablet instrument is extremely heavy and extremely difficult for one person to move – even sideways  for just  a short distance. To enable an instrument to be re-located around the house and garage, a simple trolley was built in 2019 using industrial castors that had come off something in my father’s garage many years ago.

Fig 1 – Initial design of tablet instrument trolley.

Unfortunately, two design flaws revealed themselves very quickly. The major castings of the tablet instruments are held together by through-bolts that pass vertically down each corner and which are secured by a nut beneath the instrument bedplate. Both assembly and disassembly require each corner of the instrument to be moved over the edge of a table to enable access. As mentioned above, this task is very difficult for one person so the solid top of the trolley therefore made the instrument sitting on top very awkward to work on single-handed. The second flaw was the small wheels on the castors made it impossible to negotiate even the smallest step once loaded. The trolley was therefore completely redesigned with cut-outs in the top to allow access to the bottom end of the through-bolts and with new larger-wheeled castors. All the necessary wood came from some heavy-duty pallets that had been used to deliver concrete paving slabs a couple of years ago.

Fig 2 – New design of table instrument trolley.

Having successfully assembled a tablet instrument on this new trolley, I was rather reluctant to then remove it, as it would be difficult to get it back on without dismantling again. So in the hope of being able to re-start work on further instruments later in 2021, a couple more trolleys have been made to the same design.

Tablet Instrument Stand

The tablet instruments are too heavy for normal shelves, so are intended to sit on a separate stand or cabinet that is designed to bear the weight. I understand that the L&B currently possesses two of these stands, one ‘original’ and one made by Jim Price a few years ago. More will be needed for use on the planned future extensions, so I decided to make another as a bit of a diversion from working on the instruments themselves.

A search on the internet revealed a variety of designs for these stands, so I am guessing that few, if any, were actually supplied by the instrument manufacturer Tyer & Co. A common feature seems to be that the stand dimensions are carefully matched to the size of the instrument baseplate such that the weight is carried vertically down the frame members of the stand. I set out to make a reasonably close copy of the example that is in the museum at Shillingstone Station, Dorset.

View the Somerset & Dorset Railway (Shillington Station) Visitor Guide HERE.

Fig 3 – Tyer’s No. 6 Tablet Instrument and stand in Shillingstone Station Museum.

Some of the more minor dimensions were changed in my replica to suit the wood that was to hand which came from yet another pallet, a retired model railway baseboard, and an old picnic bench which had been dumped near to our home.

Fig 4 – New tablet instrument stand under construction.

When it came to the cabinet top, it was decided to leave this open in the centre similar to the trolley platform, to enable a tablet instrument sitting on top to be dismantled in-situ.  If this feature is deemed undesirable at some point in the future, it should be possible to fit a panel across the opening that can be unlocked and removed from below.

Fig 5 – Completed tablet instrument stand in undercoat.

Power Supplies

I believe that tablet instruments were originally powered by 12V accumulators. In anticipation that mains power would be an option for the future, a couple of metal-encased 12V 2.5A transformer/rectifiers were purchased. I was not happy with their modern utilitarian appearance though, especially if they are destined to be on view alongside vintage instruments. I, therefore, built a mahogany case to go around them and added an old ammeter and voltmeter to provide the necessary reassurance that power was being delivered. The mahogany was mainly re-cycled, but with the top covers made of new wood. The operation of the tablet instruments involves both the positive and negative 12V supply lines being pulled to earth potential at different points in the operating cycle. My understanding is that each instrument then needs to be powered by its own isolating transformer, or at least through separate secondary windings. Two power supplies were made – enough for one intermediate signal box. With hindsight, it would have been quicker and more space-efficient to have mounted both transformers in a common housing.

Figs 6 & 7 – 12V power supplies for Tablet Instruments.

If you have any parts we could use, please email us.

Malcolm Kitchen

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The Denes Fox and Goose Highfield House Lynton Cottage Moorlands North Cliff Sinai House