Drake TR-4 External DDS VFO

Discussion in 'Homebrew and Kit Projects' started by K0OKS, Oct 8, 2018.

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  1. K0OKS

    K0OKS Ham Member QRZ Page

    I want to home brew an external DDS VFO for my Drake Tr-4

    I understand all the DDS and Arduino stuff. That is easier on my end. I am trying to figure out how Drake implemented their external VFO with the TR-4 through the built in connector.

    Please see the attached schematics. It looks like when the external VFO is in receive and transmit mode (thus replacing the built in VFO) there is no change to any of the pins other than the signal going out pin 6 due to the V1 filament turning on. Am I misunderstanding the switch diagrams?

    This pin 6 is attached directly to the internal VFO Q2 buffer through R148 (middle of schematic). I am not seeing how the internal VFO gets “turned off” when the external is being used. The signal goes in to the grid of V1b. Anyone know how this works? I need to be able to tell the TR-4 to use the external signal on pin 6 of the connector (middle of the second schematic, top left of the first schematic).

    Thanks.

    FF9E1DFD-DE9F-4057-AC91-230698553EFF.jpeg 6D51126E-E157-481D-A023-C83E0778BF35.jpeg
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    At first glance at the schematics it appears as though pin 2 into the rig enables or disables the internal VFO through R150. The switch inside the remote VFO applies different control voltages to pin 2 depending on the selected mode and the RCVR CATH and XMTR CATH lines supplied from the rig side of the connector.
     
  3. K0OKS

    K0OKS Ham Member QRZ Page

    Thanks for the reply.

    Yes, that is about all I am getting, and it is tough to figured those voltages out without an actual RV-4 to stick a meter/scope on. I figure the OA-2 will be around 150V. I can measure the voltage on pin 1.

    This is making more sense now that I see the xmit or recv cathode lines are alternately grounded by the T/R relay.

    So it seems I can “read” the cathode lines to determine if we are transmitting or receiving. Then I can “set” the pin 2 voltage to disable the internal oscillator as needed. I can generate these control voltages with a voltage divider from pin 1 as the source/high voltage.

    Starting to make more sense. Thanks for getting me to look at it differently. Please let me know if anything I said above sounds misguided.

    Thanks.
     
  4. K0OKS

    K0OKS Ham Member QRZ Page

    Anone out there have a RV-4 and would be willing to stick a meter on pin 2 to give me voltages with the RV-4 switch in all four positions both transmitting and receiving? I could make an educated guess, but theory and practice are often quite different :(
     
  5. K0OKS

    K0OKS Ham Member QRZ Page

    From the schematic (with my new understanding) it looks like pin 2 would be about:

    Code:
    Pos        R     T    (Volts)
    OFF        0     0
    RCV      150     0
    X/R      150   150
    XMIT     0     150
    
    So the 150V from the OA-2 disables the internal VFO.

    So that should be pretty easy.

    Sound right?
     
  6. K0OKS

    K0OKS Ham Member QRZ Page

    I also think if I take the incoming +250 and then use a 150V Zener and a 22k resistor I should be able to supply the 150V to pin 2 with a relay driven from the micro-controller (ideally through an opto-coupler).

    And given it is a diode that does not have to warm up like a tube, I can just switch the incoming voltage so it’s not sitting there drawing a watt for no reason when off.
     
    Last edited: Oct 8, 2018
  7. K0OKS

    K0OKS Ham Member QRZ Page

    Anyone out there have a Tr-4 with the external VFO able to take some measurements?
     

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