Marine Receiver – Polish Receiver OK 102

Written by Dr. Marcin Marciniak (SP5XMI) Technical journalist

The Polish OK-102 is one of the less known maritime main receivers. The OK-102 was a derivative of older OK-2 (OK stands for: Odbiornik Komunikacyjny – eng. communication receiver). As some other receivers the old OK-2 was derived from pre-war National HRO that was considered one of the best receivers of its era. The diagram looks similar, uses the same principle but the overall receiver design is quite different. Instead of precise micrometer scale tuning and changing whole tuning coil set during band change (a HRO “trademark”), Polish receiver was tuned by a two stage reduction gear and bands were changed by switching fixed coil sets. There was neither bandspread tuning nor noise limiter circuit.

OK-102/I had all descriptions in Polish as it was built only for Polish ships.

OK-102 was designed by R.Żebrecki and made by MORS*) factory from 1958 to the middle 60s. It was one of the very few valve maritime receivers that could be powered from 24V battery without voltage converter (for the anodes as well, jolly good idea!). The old OK-2 used loctal valves (eg. UCH21, UBL21) and newer OK-102 used noval valves (ECH81, EF85). Anyway the circuit still resembles the old and good HRO.

OK-2 and OK-102/I awaiting restoration by a radio amateur.

The first version of OK-102 could be used as a direction finder. When used as such the input tank was connected to goniometric DF antenna set. In fact the OK-102/I with RG-141 antenna was the first maritime direction finder made in Poland.

The second version OK-102/II had support for higher HF bands but lacked the DF circuit and the 200-550kHz band was dropped.

The OK-102/I restored to almost mint condition by radio amateur SQ8LHI.

Both receivers were powered from 24V DC for filaments and 110V DC for anodes. At this time most of the Polish ships were DC powered. In emergency the receiver could be powered from 24V DC batteries for anodes as well and it worked fine albeit at heavily reduced sensitivity (less than 70µV) and low output power (suitable for headphones).

Bands were divided this way:

For old OK-2: only two bands, probably similar to 220-550kHz/1,2-3,5 MHz,

For OK-102/I : 1) 220-550kHz, 2) 1220-3500kHz, 3) 3,4-8MHz, 4) 7,7-16,5MHz.

For OK-102/II : 1) 1200-3500kHz 2) 3,4-8MHz, 3) 7,7-16,5MHz, 4) 16-32MHz.

The first stage – the RF amplifier – was made with single EF85 valve (HRO had two 6D6 pentodes), second stage was the mixer and oscillator on ECH81. Two stage pentode IF amplifier used ECH81 valves (triodes were used for BFO oscillator and audio preamplifier), detector used Polish germanium diode DOG58 (similar to 1N68, the HRO used diode from 6B7 valve). Intermediate frequency was 660kHz (much above 500kHz band to avoid interference, the HRO used 456kHz and it was probably too close). OK-102 had single crystal narrow filter for CW almost the same as HRO had. It was simplified and lacked “phasing” control though.

The audio final stage used PL84 pentode. Filaments for E-type valves were connected in series (4×6,3V to be powered from 24V power supply) and PL pentode filament was connected by a clever circuit that dropped excess voltage on three resistors and two lightbulbs that lit the scale.

schematic diagram of the OK-102/I (a copy of original blueprints).

Nowadays this receiver seems somewhat old as it lacks bandspread or any precision tuning knob, neither is stable enough for long SSB reception nor selective enough for crowded amateur bands. But it wasn’t that bad in the 50/60s. On some ships the receivers were used later as a reserve when OMNK or imported receivers came to use.

radio room of the m/t LUTJAN/SPZQ – from left to right: OMNK-2 (main shortwave receiver), OK-102/I (medium and shortwave receiver with DF set), OA-1 500kHz emergency receiver. OK102/I is set to 515kHz or 3,25MHz or 7,45MHz or 15,6 MHz. Photo from a Facebook profile.

During the transition to SSB the old receivers and CW/AM transmitters were replaced by combined radio station that had a Mewa-class 1kW main CW/SSB transmitter and a pair of EKV solid state main receivers with frequency synthesizers.

Footnote about the company that made them:

*) MORS (pol. mors – eng. a walrus – but the name was derived from Morska Obsługa Radiowa Statków – eng. radio service for ships) – was a small company that made and serviced maritime radio gear. In the 60s/70s MORS was the only company in the Comecon (Eastern block) countries that produced electronic gear for yachts and small vessels like the taffrail log, electronic windex, and echo sounder. It grew and split and later became bigger RADMOR company (the RADMOR name had also something to do with maritime business as it was derived from RADio MORskie – ang. maritime radio). RADMOR still exists, produces rugged VHF/UHF radios for military and professional use. Morska Obsługa Radiowa Statków MORS also exists as a maritime radio service shop within Remontowa shipyard that services GMDSS gear in Gdańsk.

Below a youtube clip of the receiver OK 102