Saturday, August 21, 2010

813 AM Transmitter Accessory: External PTT Control

My 813-based AM transmitter (see posts here, here, and here) sits about 8 feet behind me in the shack. I wanted to put the TX/RX antenna relay near my operating position (rather than at the transmitter which would have meant running an extra length of coax over to it), and I also wanted a conveniently-placed TRANSMIT switch next to my operating position so that I could easily put the transmitter into XMIT mode.

I also wanted to use my Flex-5000 transceiver as the receiver because it has, to my ears, a very good Synchronous AM detector and sounds great on AM. But I needed a way to "Mute" the 5000 via external control (sure, I could use my PC's mouse each time and click on the Console's "MUT" button, but what a pain -- I wanted automatic control). Unfortunately, the 5000 lacks an input that can be used as an external mute control for the receiver. But all was not lost ...

On the 5000's back panel there is an RCA connector for an external PTT input. I normally use this to place the 5000 into XMIT mode. But why not have it serve a dual purpose? That is, why not let it act as either a PTT control (its normal function) or a Receiver MUTE control?

I modified the Flex Console code so that the 5000's PTT RCA jack can be used for either of these two functions. And to select which function this RCA jack would perform, I added a new "button" to the Flex Console.

Let's start first with the circuitry. Here's the schematic:

(Click on image to enlarge)
Design notes:
  1. Everything came out of my junkbox, which is why you'll see a 26 volt relay (the Antenna Relay) mixed with 12 V relays, and powered from a 24VAC transformer. I used what was at hand.
  2. J7 connects to the PTT RCA on the 5000's back panel, and can either be used to place the 5000 into XMIT (via a pushbutton attached to J6), or as a 5000 RECEIVER MUTE control when I'm transmitting with the 813 AM transmitter.
  3. J5 connects to the first set of contacts in the 813 Transmitter Sequencer (JP5 pins 1 & 2, per page 3 of the 813 AM Transmitter schematic). This keys the Antenna Relay as well as the 5000 Receiver Mute.
  4. The military Antenna Relay internally shorts the unused RF port, as I've shown in the schematic.
  5. Switch SW2 is a toggle switch and, when ON, it places the AM Transmitter into XMIT mode.
  6. Originally relay K3 did not exist and switch SW2 connected directly to the AM Transmitter's PTT input (via jack J4). I quickly discovered, though, that if I toggled SW2 ON to transmit and if I'd forgotten to turn on power to my External PTT circuitry (that is, the circuit above), the Antenna Relay wouldn't switch-over during XMIT, and I'd transmit into a short. Not good! So relay K3 was added as a safety interlock. If power isn't ON, relay K3 is OFF, and switch SW2 cannot place the AM Transmitter into XMIT mode. SW2 can only act as a PTT switch if power is ON.

Here's the finished box (shown in Receive mode):

And here's a snapshot showing the new "button" I added to the Flex Console.

If this button is depressed (as it's shown above), the 5000's PTT RCA acts as a Receiver MUTE input. The software (modified by me) MUTES the 5000 and this button will turn RED whenever the PTT RCA is shorted -- that is, whenever the AM Transmitter is transmitting.

If this button is not depressed, the PTT RCA operates normally, that is, as an input for an external PTT control. In this mode, my external PTT push-button puts the Flex-5000 into transmit.


Standard warning applies: I may have made mistakes when writing this post or in my design. I cannot guarantee that everything is correct. Use at your own risk.

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