Ron wrote: > By 1980 receivers had grown very selective, but the fact was that a great > many ships still used a regenerative receiver as the "backup" in case the > main receiver was out of commission for some reason.
The auto-alarm (AA) receivers were also designed to be broad enough to detect A2 (MCW) distress AA signals from 492 to 508 kHz. Typically the AA receiver was a dedicated unit of TRF design feeding simple electronics capable of detecting the distress auto-alarm signal of twelve four-second dashes with one-second spacing sent in one minute's time. The AA signal is one of the most interesting portions of the distress signal in the link to the Morse traffic containing the M/V Prinsendam/PJTA SOS. I recommend the web page of N1EA, the assistant radio officer (N1EA) on the ship (Williamsburg/WGOA) that came to the rescue from 120 nm away after hearing the SOS DE PJTA signal: http://www.qsl.net/n1ea/ More details and background are provided, along with direct link to an MP3 version on the SOS traffic by clicking the SOS DE PJTA button on that page. Apparently, the decision to send the SOS was made by the Prinsendam's chief radio officer, Jack van der Zee, and not the ship's master. Very unusual! Also, the QRZ.COM page for the chief radio officer on the Williamsburg has a lot of interesting related information: http://www.qrz.com/db/ns1l Both radio officers of the Williamsburg are hams (NS1L, N1EA). > Some of the signals in the link below sound like they have modulation. They > do. MCW was the norm for emergency traffic so they could be copied even on a > receiver without a BFO. Even the emergency lifeboat transmitters like the SCR-578 and AN/CRT-3 and commercial equivalents, plus all the larger lifeboat emergency receiver and transmitters like the RCMA ET-8053 (AN/SRC-6) and the Mackay 401-A (AN/SRC-6A) sent MCW on 500 kHz. That actually complicated their design and increased the power consumption (generated by a human on a hand-crank) compared to a simple A1 transmitter. (I collect these sets.) > The possibility of an SOS not being heard at all in the bedlam is what > launched the twice-hourly "silent periods" when all ships fell silent and > the R.O.s listened on 500 kHz for three minutes. Yep, from minute 15 to 18 and 45 to 48 each hour. Any Morse traffic being sent on ANY maritime frequency MF or HF would be paused with a "AS SP" when minute 15 and 45 came up on the clock so that ROs on any frequency could listen to 500 kHz. The old MF 405 to 535 kHz Maritime Morse band was an amazing place at night. For years I kept a bedside receiver tuned to 500 kHz. > This link is a real SOS recorded in 1980 when the MV Prinsendam had an > engine room fire and a flooded engine room. It begins with a series of long > dashes. That was the standard opening that was supposed to set off automatic > alarm bells on any vessels whose radio rooms were not operating at that > moment. The bells went off on the navigating bridge and right over the bed > were Sparks would be sleeping. Following the dashes the SOS and emergency > message begins. > > http://mikea.ath.cx/www.n1ea.coastalradio.org.uk/EJM_CD3_Track03_SOS_de_PJTA.zip It has always been interesting to me that the radio officer on the Prinsendam, technically did NOT send the SOS correctly. It's supposed to be sent as as one very distinctive signal of ...---..., yet PJTA sent ... --- ... with definite space between the S, the O, and the S. But in reality, it's those four-second dashes of the AA signal that draw the most attention! Technology did not move very fast in the communications of the merchant marine. When I took my Second Class Radiotelegraph exam in 1981, the exam material was dated 1961 and required schematic drawing and short answers in addition to multiple choice. I never felt the elation of passing any other FCC exam, (especially ham exams) that I got from passing the commercial radiotelegraph exam. 73, Mike / KK5F ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
