As careless consumers were forever popping the HT fuse due to sputtering rectifiers and valve overloads, Mullard hit upon the idea of introducing an HT Overload cutout for all 1954 production models of the MHSVT. It was based on a polarised relay which would trip at overload condition but could be reset at the press of a button. The cut out trip current was 200mA.
Another handy wheeze was what Mullard termed "the MHSVT pre-heat device" where a toggle switch was added which allowed the instrument circuitry valves to have heaters lit without a valve setting card inserted and the gate handle in the "Off" position. To avoid CRT wear, no EHT - just LT was applied. On inserting a card and valve, the CRT indicating spot appeared virtually instantaneously which was considered a boon for the busy commercial workshop.
These enhancements could be retrofitted to earlier instruments on their return for routine overhaul - Mullard were very specific in that kits of parts could not be sent to users for self installation.
The cost of upgrade was as follows: -
Pre heat device: £2 10/-
Overload cut-out: £2
Both pre-heat & cutout: £4
The diagram below shows the circuit amendments that were made - does your MHSVT have them?:-