I have an old tube-type receiver that is designed for use with a companion transmitter. The receiver has a DPDT Potter-Brumfield 12VDC relay with the relay coil attached to a terminal strip labeled "REMOTE" on the rear panel. This relay provides +250V B+ to the IF circuits through one set of NC contacts of the relay , and connects the receiver front end (common) to the antenna connector (NC contact) through the other set of contacts.

When 12VDC is applied to the relay coil, the relay operates and does two things: 1. The B+ voltage is interrupted to the IF circuits, muting the receiver. 2. The receiver front end is switched to ground (NO contacts), protecting the receiver front end from RF overload.

My goal is to eliminate the mechanical relay. I'm planning to use an N-channel depletion mode FET (IXTP3N50D2) to feed the 250VDC bus to the IF circuits in receive mode. Upon application of 12V to the rear terminals (transmit mode), I want the -12VDC to turn off the FET, disconnecting the IF circuits from the B+ bus. I think this will work, although I don't have all the component values figured out yet.

On the other hand, I think I may have to use a reed relay for the antenna switching, because I don't see any way to emulate a SPDT with FET(s) where there is no DC voltage on any of the "relay" contacts. It's strictly a passive switch function.

Does anyone have any ideas? Am I crazy? The main reason I'm doing this is to enable the receiver to switch from receive to transmit mode and back at Morse Code (CW) keying speeds of say 20-35 words/minute. A mechanical relay can't do this.

Thanks for your input, advice, diagrams, etc.
Mike Harmon, WB0LDJ
mharmon at att dot net

 

enable the receiver to switch from receive to transmit mode and back at Morse Code (CW) keying speeds

What's the reason for that?
Don't you leave it in the transmit mode while sending a message in code.

 

What's the reason for that?
Don't you leave it in the transmit mode while sending a message in code.

It's called "QSK" or "break-in keying". The way CW operation works is that the transmitter is only producing a signal when the key is down. For example, when sending the letter "a", the transmitter is only generating a signal during the duration of the "dit" and the "dah". The receiver is operating continuously during the entire time. The idea is to blank the receiver only during the millisecond or so that the transmitter is actually producing a signal. When several operators are on the frequency at the same time, this allows one operator to interrupt the transmitting operator between words or even letters. That's all happening too fast to get a conventional relay to do the T/R switching.

Go out to Google and type in "break in keying". Read the wikipedia article on QSK operation.

It will explain what I'm trying to accomplish.
Mike

 

To maintain isolation, you may want to use a DC output solid-state-relay (SSR).
Here's an example with two SPST, NO, 350V, 120mA MOSFET switches.

Is 120mA sufficient for the B+ current?

So you could conceivably use one switch for the B+ and the other to ground the receiver front-end (is a 25Ω on-resistance sufficient to ground the signal?).

You would need to add a simple transistor inverter to invert the input signal to one of the switches, so that one is on when the other is off.

 

Thanks for the info. You've given me some ideas for my project. I'll do some more research and see what I can do.

Mike