Dave GW8GLO sent some details of his LF U3S project. I was so impressed by the incredible high standard of construction and attention to detail that I decided to put these photos and details on their own page here! Here are some other pages about the Decca 5501 and its conversion for use on the 136kHz amateur band:
Details of Decca 5501 LF Transmitter, by Mike G3XDV
Decca 5501 Modifications for 136kHz, by Jim Moritz, M0BMU
Decca 5501 Transmitter Conversion, by G0MRF
Once you have seen these pages, you'll appreciate even more, how much work Dave GW8GLO has put into his project.
In Dave's words:
The U3 keyer is built in an old modified pye receiver 19inch case and uses the 27v aux supply from the Decca 5501. There is a card power supply to get 5v and 12v from the 27v with additional filtering and an interface board to protect it from the outside world.
The power supply is a much modified "blade" power supply that now gives 10 to 75 v at up to 50a. This makes the tx output variable from a few watts up to its predicted max of over 1.5kw. Unable to confirm the actual value as I do not have a bigger dummy load! The power supply was originally 50v fixed at 3kw. It does give the setup efficiency to about 90%.
The 27v supply is a small linear supply in the same case. The swicth mode supply is also keyed which reduces any noise to zero on receive.
The Decca 5501 has been extensively modified to improve its reliability and OP.
The new board is the new alarm and shutdown control and synchronous keying circuit with a constant level amp to give a wider range of input to the Decca IP circuits. The swr is set for 200watts of reflected power to trip and the guard circuit in the Decca makes it almost indestructible with any bad load.