Tablet Instruments Update – July 2022

Two of the L&B Tyer’s No 6 tablet instruments were missing both their tablet slides and polarising relays. This post concerns the making of replacements.

Another Polarising Relay.

Having already made one new polarising relay incorporating some components from the spares box, fewer parts were available this time to use as a starting point for the new relay – just the coils (borrowed from a spare electric lock), the moving contact strip, one fixed contact post, and a wire wound resistor. The pictures below show stages in the construction.

Fig 1 - New fabricated pole piece

Fig 2 - The permanent magnet circuit and primary contacts

Fig 3 - Electromagnet added

A second pole was added to the relay so as to be the same as a relay on a previously refurbished instrument. This was done in case there were to be some requirement in the future for a matching pair with extra switching capability.

Fig 4 - Completed relay mounted on the instrument backboard.

The only thing remaining thing do is to source and fix suitable permanent magnets to the rear of the backboard. The original magnets are to hand, but so old that their magnetism is insufficient to reliably return the relay to its neutral state when current to the coils is switched off.

A Tablet Slide.

The tablet slide is a large casting approx. 600 x 170 x 16mm, machined on almost all its faces, and containing a recess towards the front in which the tablet sits when it is either being extracted from, or returned to, the tablet storage cylinder.

Fig 5 - Tablet slide from a Tyer’s No 6 Instrument.

As I possess no machine tools, let alone the necessary skills to use them, Plan A was to have two new tablets slides made elsewhere. However, a plan B also came to mind - making use of part of a slide that had been broken in half at some time in the distant past (perhaps as a result of an instrument being dropped?). Although the front half of this slide was completely missing, for some reason the rear portion had not been scrapped and was hanging around in the S&T stores.

Fig 6 - Remaining rear portion of broken casting (already squared off at the break)

The idea was to fabricate a replacement front half of the slide from two pieces of sheet steel sandwiched together and spliced onto the broken casting. All three parts to be fastened together with set screws. The bottom plate is 6mm thick steel and the top plate 10mm - the thicknesses chosen so that together, they form a tablet pocket of the correct depth. Although plan B sounded simple, I completely underestimated the difficulty in execution, and now rather regret embarking on it.

Fig 7 - New bottom 6mm plate spliced onto the broken casting

A recess to house the brass configuration ‘pin’ also had to be created in the top sheet (a pin for configuration ‘B’ tablets is shown mounted in this recess in the picture below)

Fig 8 - Top 10mm plate added - creating the recess to hold the tablet (compare with Fig 5)

Work still to do includes creating the angled bearing surfaces down the edges of the slide, and general finishing around the front handle. Having got this far, I am looking forward to being able to resurrect another complete tablet instrument.

Malcolm Kitchen