User menu

Progress with Tablet Instruments (Part 2)

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


Tablet Instruments – New Parts

During lockdown #1, work started on the assembly of three more tablet instruments from spare parts and sub-assemblies that are in the S&T store. It was recognised from the outset that none of these instruments could be brought to an operational condition without acquiring or making some additional components. In particular, we have no suitable tablet slides and only a single polarising relay. Making new tablet slides is way beyond my capabilities, but I thought I could have a go at making a new polarising relay to the original Tyer’s design of 100+ years ago. I started this in lockdown #2, and it was completed in lockdown #3, along with some other work described below.

Polarising Relay

The polarising relay is a safety critical component that lies at the heart of the Tablet instrument. Its operation is controlled by the identical tablet instrument to which it is connected at the other end of the section. It is a double throw relay with a centre-off/neutral position, and usually just a single pole. Although only required to switch ~12V and ~1A, the relay is substantially built and is a sizeable item at 20x15cm. The design comprises two orthogonal magnetic circuits; a permanent magnet circuit that maintains the relay in the neutral position when it is unpowered, and an electromagnetic circuit that effects the switching once power is applied to the coils. The permanent magnets are needed to prevent the relay switching if the tablet instrument were simply to be shaken or tipped over, and also to overcome any residual permanent magnetism that could build up in the electromagnetic circuit. Except for these magnetic circuits, all the materials used in the relay construction are non-ferrous.

Fig 1 – Tyer’s Polarising Relay in original condition. This is a double-pole example that incorporates some optional components.

The massive Tyer’s design does not demand components machined to fine tolerances, which is just as well as I do not possess a lathe or a pillar drill. Several of the magnet pole pieces are castings in the original, but in the copy they were fabricated by bolting together steel sections. 

Fig 2 - The permanent magnet circuit in the new relay. The U-shaped mounting for the adjustable end-stops is an original part that was to hand. The horseshoe magnets themselves are mounted behind the backboard so are not visible in this photo.

Fig 3 - As Fig 2 but with the relay contacts added.

I thought I would be able to source some electrically and mechanically compatible solenoid coils to use in the relay. This turned out not to be possible, as modern relays and electromagnets all seem to use much smaller coils with higher resistance. The next plan was to try and buy an old instrument that contained suitable coils and could be cannibalised. I set up a search on Ebay, and after a few weeks wait, I managed to secure a ‘spares or repair’ Tyer’s instrument that contained 4 suitable looking coils - all for a reasonable £40. There then followed another change of plan (explanation given below), which brought me back to square one. The eventual solution came from re-assessment of the existing stock of tablet instrument spares. This revealed that we probably have a surplus of the electric locks that control the withdrawal or admission of tablets. Each of these locks has two coils, and although they are not exactly the same size or resistance as those used in the polarising relay, they are close enough. Moreover, no modifications are required to the coils to adapt them to the new role, so there is always the option of returning them to the electric locks if this should ever be required in the future.

With this problem solved, the new relay was quickly completed.

Fig 4 - The brand new relay (compare with Fig 1).

Heater

The Exmoor climate is not very kind to tablet instruments - especially their veneered wooden cases. The expectation must be for fairly frequent overhauls of any instruments housed in buildings that are unheated for most of the time. With a view to slowing down the rate of deterioration, a small 15W heater has been installed in one of the refurbished Tablet instruments as an experiment. The heater is simply a wire wound resistor around a ceramic core and runs off the standard DC supply to the instrument.

Fig 5 - Heater installed in the base of a refurbished tablet instrument.

Gong

Tyer’s No. 6 Tablet instruments use an external gong or bell to ‘vocalise’ the bell codes. The L&B already possesses enough gongs and bells to meet the immediate needs, but another gong has now been acquired, more or less by accident.

As mentioned above, an instrument made by Tyer’s was purchased off Ebay in ‘spares or repairs’ condition.  At the time of purchase, I did not know exactly what I had bought. The photos on Ebay included a bell that obviously didn’t belong, and clearly showed that other key components were missing altogether. Once it could be inspected properly and compared with images on the internet, I discovered that I had a gong (albeit minus all the actual gong parts) mounted on a common base with a line-interface relay.


Fig 6 - The ‘Spares or Repairs’ gong as purchased.

Removing the paint from the wooden base revealed the following:-

  • Layers of Southern Railway/Region green paint, underneath a coat of BR grey.
  • An early build date - 3rd Sept 1888.
  • The full name of the person who made the gong (G Beech) - rather than just initials.

Fig 7 - Detail of the gong base.

With this provenance, I didn’t have the heart to destroy the gong just for parts, so I decided to restore it instead. A new armature and striker were fabricated, along with some other small parts, to replace those that were missing. Finally, the gong from a recently deceased 1950’s clock was appropriated and fitted. Although it is probably not as large a diameter as the original gong, it makes a satisfactory sound.

Fig 8 - Gong after restoration (internal).

Fig 9 - Gong after restoration (external).

The tandem line-interface relay (the upper and lower coils on the left in Fig 8) is a sensitive low- resistance device that enables the gong to be activated by a telegraph line (say), but powered from a local supply. This facility will probably not be needed for installations on the L&B, but the relay can easily be isolated.

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

Malcolm Kitchen

Corporate Supporters - Where to Stay

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