An interesting piece of antique test gear found at a radio rally
Anything with a not-immediately-obvious type of digital display always attracts my attention.... This frequency counter uses an ingenious digital display I had a vague recollection of seeing in the London Science Museum.
It uses a projection display, but instead of the more conventional multiple lamp
arrangement, it had "E.A.C Digivisor Mark.2" modules, containing moving-coil
meter movements, projecting the light from a single lamp through a mask clear-on-black
pinted digits. The current through the meter movement determines the digit displayed.
I have also seen a reference to an advert in The December 1961 issue of Wireless World (p.22)
Related links : Advert for a frequency counter with a different unusual display
Pics of the display module, with close-up of the movement and mask. AVI of units digit moving (34K).
An interesting feature of the display was that in addition to the coil connections, it had a not-fitted pair of terminals marked 'lock'. I wonder if this was for an electrically operated shipping lock to make the movements more robust during transport, or maybe some way to latch the displayed digit.
As well as the interesting display device, the construction of the rest of the device was also unusual. Most of the electronics were housed in plug-in logic modules - pretty much the predecessor of the standard TTL ICs that would be used in later devices. These were hand-wired on matrix board, using mostly Mullard germanium transistors.
Insides of unit with display unit in place (left) and removed (right)
Underside of chassis, plus detail of circuitry wired on tag-boards.
Inside some of the modules
Power supply module - note selenium rectifier ( centre) and vintage style power transistor (top left), type V15/20P.
Thanks to James Larsson for sending this picture confirming my memory of having seen something similar in the Science museum - this appears to be later model which abandoned the modules in favour of printed circuit boards, but uses identical display modules.