DS-35 digital stopwatch was produced by ZPA Pragotron concern company (in Čakovice) in the first half of the 1980s, using mainly Czechoslovak Tesla components.
These stopwatches are used to measure time with a resolution of 0.01s up to 99min 59.99s (after reaching this value, it does not stop, but continues counting from zero again).
It uses six RTF Z573M nixies with 13mm digit height and a right decimal point.
The device is powered from 220V 50Hz mains, the indicated consumption is 9VA. I measured 7.8W at 220V and 8.5W at 230V with a wattmeter.
Wired remote control FK1 is used for control. It is equipped with a Start - Stop switch and a Zero button for resetting.
It is connected to the DS35 stopwatch from the back using a 5-pin DIN connector. The 5V supply voltage of the TTL logic is regulated using MA7805 circuit from Tesla.
The nixies are powered by a bridge-rectified non-smoothed (pulsating) voltage.
These stopwatches do not have a crystal - they are clocked by the mains frequency. They contain TTL integrated circuits from the 7400 and 5400 series manufactured by Tesla:
6 MH74141 circuits (BCD to 1 out of 10 decoder with open collector outputs),
6 MH5490A circuits (like MH7490A, but wider temperature range, decadic counter),
1 MH5437 circuit (like MH7437, but wider temperature range, quadruple 2-input positive NAND power logic elements),
1 MH5410 circuit (like MH7410, but wider temperature range, tripple 3-input positive NAND logic elements)
My stopwatch is missing the original remote control, so I had to build my own.
The MH74141 circuit for tens of minutes was faulty - I had to replace it.
The MH5437 circuit looks like it has been changed in the past. The original capacitors still work.
The nameplate seems to indicate this piece is made in 1983. The component date codes are between 1975 - 1981.
