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-   -   Mimco Crusader Transmitter (https://www.shipsnostalgia.com/showthread.php?t=294035)

R651400 5th July 2019 19:59

Mimco Crusader Transmitter
 
Follow-up from the Globespan here

sparks69 5th July 2019 21:36

Every time I turned that big knob in the middle I held my breath !

Varley 6th July 2019 00:47

Why on earth the 'big knob' was not interlocked with the HT switch will remain a carbonised mystery.

DickGraham 6th July 2019 11:32

Sailed with one on the Fleetbank which was a bit of an upgrade from the 'Span VI I'd sailed with on the previous trip on Weybank (LOL)

sparks69 7th July 2019 22:35

I did like the power though. The fluorescent tubes in the radio room would flash when keying on 4 & 6 MHz.

Paul Braxton 7th July 2019 23:48

... and the metal parts of the earphones would often burn your ears a bit on the old 'Serenia'...

Ron Stringer 8th July 2019 08:44

Quote:

Originally Posted by Paul Braxton (Post 2990667)
... and the metal parts of the earphones would often burn your ears a bit on the old 'Serenia'...

It wasn't just the use of 'open' antenna feeders (copper tubing) between the transmitter and the external antenna that caused such phenomena. Satisfactory grounding (earthing) of the transmitter was a cause of many problems that technicians, including those working in the installation planning department of MIMCo, didn't fully appreciate. For the Oceanspan range, tinned copper wire 7/.029" connecting the transmitter cabinet to a brass bolt screwed into the metal deck of the radio room was deemed adequate and, with the low powered Oceanspan range, did not exhibit any problems. This continued with Worldspan and Globespan and I can confirm that in hot weather, sitting at the key wearing only a towel around the waist, you had to be damned careful to keep the headphone cord away from your sweaty body if you wanted to avoid a painful reminder that RF does burn skin.

For the Crusader, after many initial problems, the earthing arrangements were improved somewhat to 6-inch wide, 0.4mm thick copper strip replacing the wire. With the higher power of that transmitter however, problems persisted, made worse by the changes in ship design that enforced the use of short, inefficient antenna arrangements. On some ships the length of copper tubing within the radio room was a significant portion of the overall antenna length.

To try to improve matters when Conqueror came along, I won a long struggle to convince the installation planning department, and others, that a minimum 12-inch strip must be used for the grounding. The transmitter cabinet was provided with appropriate connection arrangements for the wider strip and a specific grounding diagram was produced and included in the installation manual. Despite the power increase of Conqueror over Crusader, we had very few reports of operational problems with unwanted effects. A couple of years after the Conqueror entered service, I received a grudging acknowledgement from the head of the department that the transmitter had given them fewer problems than its predecessor.

Of course the real solution was to re-locate the transmitter's antenna tuning/matching arrangements out of the radio room, to the base of the antenna, being connected to the transmitter's 50-ohm output using co-axial feeder. Then all was calm within, as the transmitter's RF output was conducted into the antenna instead of bouncing around the radio room. This had become the standard for most manufacturers by the time that the radio room disappeared from ships and communication was transferred to the Bridge and the cabin.

J. Davies 8th July 2019 11:30

Slightly off topic, a party trick which one captain particularly enjoyed was tuning up on 512 KHz or so with knee down on key, holding a lead pencil with the thumb on the blunt end towards the copper tube antenna feeder. A gap of a few inches would produce a long hissing orange RF arc in which with the other hand you could introduce a slip of newspaper. This would burst into flames and made a good captain's fag lighter when he lost his matches.
The only downside was a small but painless RF burn the size of a pinprick on the thumb.
It is interesting to note that at higher frequencies up to HF the colour of the RF arc turned progressively purple and the audio hissing tone increased. Someone should make a study of this phenomenon .

Varley 8th July 2019 11:56

Send me at once a Crusader, an appropriate gap and a good Captain. I will start the research upon receipt (might we also need a planck?).

R651400 8th July 2019 12:22

Where's our Troppo to enquire if this corona discharge was actually radiating and causing Class B QRM on the international distress frequency of 500 kc/s?

CrazySparks 11th July 2019 23:37

I think it was on the Riverbank in around 77 that keying the main Tx (Conqueror or Crusader - I don't recall which) that keying on 22MHz literally stopped the ship - interfered with engine room electronics somehow. Good thing it wasn't any other band! My first trip as solo R/O!

Devans47 12th July 2019 00:22

Crusader transmitter.
 
2 Attachment(s)
Found this MIMCO Crusader in the back room of a maritime museum in Port Chalmers NZ last, while on a Holland America cruise.
Attachment 191157

Attachment 191159

Varley 12th July 2019 00:25

That looks in cracking condition.

IvortheEngine 12th July 2019 12:35

Cracking or crackling?

aussiesparks 18th July 2019 04:34

Quote:

Originally Posted by J. Davies (Post 2990719)
Slightly off topic, a party trick which one captain particularly enjoyed was tuning up on 512 KHz or so with knee down on key, holding a lead pencil with the thumb on the blunt end towards the copper tube antenna feeder. A gap of a few inches would produce a long hissing orange RF arc in which with the other hand you could introduce a slip of newspaper. This would burst into flames and made a good captain's fag lighter when he lost his matches.
The only downside was a small but painless RF burn the size of a pinprick on the thumb.
It is interesting to note that at higher frequencies up to HF the colour of the RF arc turned progressively purple and the audio hissing tone increased. Someone should make a study of this phenomenon .

Same trick was used at the Northern Counties Radio with an Ocean Span, however most pencils have paint etc over the end of them and this stopped problems however one day a student tried it with a pencil with a bare end , this was the same as touching a bare antenna and he was thrown across the radio room, luckily unhurt

J. Davies 18th July 2019 21:07

Quote:

Originally Posted by Devans47 (Post 2991371)
Found this MIMCO Crusader in the back room of a maritime museum in Port Chalmers NZ last, while on a Holland America cruise.
Attachment 191157

Attachment 191159

I never thought I could get all bleary-eyed and nostalgic over an old but rather magical lump of British technology.


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