Venner TSA 3336 Frequency and Time Measuring Equipment

An interesting piece of antique test gear found at a radio rally (hamfest).

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.  

Dating equipment like this is often assisted by the fact that dates were usually stamped on electrolytic capacitors. The latest date I found was 1967. A calibration label on the front shows that it was still in use in December 1985. Below is a reference to it in the Journal of Scientific Instruments from October 1961

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).
There are mechanical adjustment screws to set the position of the digit on the screen, a meter zero adjust screw, and in addition, the counter had a front-panel pot to set the full-scale calibration - there is a 'reset to 9' button to assist with this. Wires on top are connections to decimal point lamps.

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.