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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 29th May 2019, 5:56 pm   #1
jameshunt8082
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Default Need a very low power 500Khz working transmitter to send morse code.

I have purchased a Marconi Atlanta Marine Radio.
I am purchasing a non-working Oceanspan Transmitter.
I want to demonstrate it working.
I need a very low power 500kHz transmitter that I can use alongside the receiver and I need the transmitter to also transmit on another frequency 512kHz to show, how we used to use 500kHz as a calling frequency.

There output power can be in mWatts.
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Old 29th May 2019, 9:46 pm   #2
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

Signal generator feeding a dummy load?
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Old 29th May 2019, 10:27 pm   #3
Julesomega
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

I promise I'm going to stop collecting radios when I've got an Atalanta 2207C
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Old 29th May 2019, 11:33 pm   #4
G0HZU_JMR
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

There's lots of ways to generate the signals, including a 74HC4060 and crystal(s).

However, as you will be running the transmitter right next to the receiver I would recommend running the signal source at twice the frequency (continuously) but key a flipflop divide by two (74HC74?) as the morse sender ahead of the final stage and LPF. That way, there will be 'nothing' heard in the receiver in the gaps between morse characters, i.e. no constant low level leakage tone if the transmitter is right next to the receiver. Otherwise, you would have to screen the low level stages of the transmitter very well in order to avoid a constant leakage tone appearing in the background in the receiver.
The alternative is to key the oscillator on and off but that will probably sound a bit chirpy and unpleasant.
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Last edited by G0HZU_JMR; 29th May 2019 at 11:38 pm.
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Old 29th May 2019, 11:50 pm   #5
paulsherwin
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

It really all depends on what you want to do, which isn't very clear at the moment.
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Old 30th May 2019, 12:13 am   #6
G0HZU_JMR
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

Yes, it's not that clear. I assumed it would be the receiver connected to a regular outdoor antenna and the transmitter sat alongside the receiver running into a dummy load. The risk there is that the radiated morse signal from the dummy load might not be that much greater than the leaked radiation from the unkeyed oscillator inside the transmitter.
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Old 30th May 2019, 8:04 am   #7
G3VKM_Roger
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

Quote:
Originally Posted by G0HZU_JMR View Post
There's lots of ways to generate the signals, including a 74HC4060 and crystal(s).

However, as you will be running the transmitter right next to the receiver I would recommend running the signal source at twice the frequency (continuously) but key a flipflop divide by two (74HC74?) as the morse sender ahead of the final stage and LPF. That way, there will be 'nothing' heard in the receiver in the gaps between morse characters, i.e. no constant low level leakage tone if the transmitter is right next to the receiver. Otherwise, you would have to screen the low level stages of the transmitter very well in order to avoid a constant leakage tone appearing in the background in the receiver.
The alternative is to key the oscillator on and off but that will probably sound a bit chirpy and unpleasant.
I would also suggest the use of crystals and a divide by n chip, as you can then use a crystal around , say, 5120kHz which are quite plentiful and divide that down to 512kHz. I did a similar mod to a Marconi Commandant to get LSB on it and needed a crystal around 100kHz.

HTH

Roger
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Old 30th May 2019, 9:21 am   #8
Jon_G4MDC
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Default Re: Need a very low power 500Khz working transmitter to send morse code.

Jeremy's 4060 divider method provides the answer to on/off keying without spacer wave for free by using the reset line Pin 12.
An 8.0MHz crystal should produce 500kHz at the first output Q4, Pin7. Use an 8.192MHz crystal instead (also commonly available) for 512kHz output.

Last edited by Jon_G4MDC; 30th May 2019 at 9:30 am.
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