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Progress with Tablet Instruments in 2020

Malcolm Kitchen has sent us this report on the work to recreate Tyers No.6 Tablet Instruments.

At the start of 2019, the L&B had five Tyers No.6 tablet instruments restored to working order and ready for use in the Phase 2a extension. More operational instruments would be good to enable ‘long-section’ working and for subsequent extension phases.

Also in 2019 work started on the refurbishment and assembly of a further instrument from the collection of spare sub-assemblies and miscellaneous parts that are held. This instrument was completed at home during lockdown #1 (picture below).

Fig 1 – Completed Tablet Instrument ready for testing.

In order to verify its functionality, a small test box was built so that the signals from a similar instrument located at the other end of the section could be replicated. The test box incorporates a Morse key and meter I inherited from my father and which possibly date back to WW2.

Fig 2 – Test Box.

During summer 2020, attention turned to the remaining stock of spares, to see if any more instruments could be put together. A cursory examination of the spares suggested that most of the parts to assemble another two instruments were to hand, along with about half the parts needed for a third.

However, there are some additional complications. Tyers No 6 instruments were manufactured over a long period of time, and several design changes were introduced over the period. Some of the dimensions of the main castings also changed quite significantly – presumably when patterns were replaced. Rather than being made to conform accurately to a drawing, some smaller parts were made so as to fit a particular instrument. Holes for many of the set screws used in assembly seem to have been drilled in situ. All of these things make the interchange of parts between instruments more difficult, so a significant amount of time has to be spent assessing the compatibility of various combinations of components.

The spares themselves can vary enormously in quality. Shown below is a brass plunger casting riddled with air bubbles which may well have been a ‘spare’ right from new.

Fig 3 –  Plunger casting.

And in contrast, a near perfect light alloy casting showing little evidence of its age.

Fig 4 –  Commutator Rotating Rod Casting.

Each instrument incorporates 7 solenoids and surprisingly, despite some probably being over 100 years old, no failures have been found in either the windings or the insulation. There is an abundance of some non-critical components. For example, we have been able to rebuild/refurbish ten of the mahogany side panel/inspection covers, whereas at most, only 6 of these will be needed.

Fig 5 – Refurbished side panels.

On the other hand, we are desperately short of some key components – notably polarizing relays and tablet slides.

Fig 6 – Polarizing Relay (before refurbishment).

Notwithstanding the lack of critical parts, it was decided to commence construction of a further two (later three) instruments, in the hope that any component shortages could be solved along the way.

Fig 7 -  Indicator Panel base units  on an ‘assembly line’.

The indicator panel base on the RHS of the above photo was initially considered to be beyond repair, as the iron casting that forms the base and joins the front panel to the back board had been broken in half. However, it was subsequently repaired by bolting 1/8” steel plates to each side of the casting across the line of the fracture.

Fig 8 – Repair to Indicator Panel base casting.

After making replacements for a few other missing parts, all three indicator panels have been restored to working condition. The only task remaining is to letter the front panels.

Fig 9 – Three more finished indicator panels.

It is hoped to be able to start work on the lower halves of at least two of these three instruments during 2021, but first, the shortage of polarizing relays has to be addressed. No modern equivalent appears to be available, so it was decided to tackle the problem head-on and make a new relay to the original Tyers design. If all goes well, its construction will be the subject of a further news article in a few months time.

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

Malcolm Kitchen

The 2nd part of this project (uploaded on 26th January 2021) can be viewed HERE.

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