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Help Me Understand the Harbach SK-201 Soft Key

Discussion in 'Amateur Radio Amplifiers' started by WQ4G, Nov 25, 2019.

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  1. WQ4G

    WQ4G Ham Member QRZ Page


    I am trying to understand how the Harbach SK-201 works with the SB-200.

    Here is a drawing of the SK-201 version I have:


    I understand how the relay and bias circuit works in an unmodified SB-200; the key line is grounded to chassis and that causes the relay to close and draws the bias voltage down from about -120vdc to about -2.2vdc, which results in an idle plate current of about 85-90ma +/-. At -120vdc grid bias the tube is in cut off and at -2.2vdc it is just beginning to conduct...

    I also understand the BASIC operation of the soft key in that the optoisolator separates the key line from the bias circuit. Grounding the key line operates the optoisolator which opens the gate of Q1 which in turn operates the relay and draws down the bias voltage.

    What I am not understanding completely is the operation of the optoisolator and the state of the LED when the key line is grounded and not grounded.

    I am confused by the ground shown at the bottom of the (optopisolator's) LED in the drawing...

    It appears to me that the LED would be on when the key line is not grounded allowing current to flow to ground through the LED making it light up. And, when the key line IS grounded it appears to me that the voltage would be drawn down and the LED would go out. But, that does not seem correct. Doesn't the LED have to be on in order to forward bias the transistor and open the gate of the MOSFET?

    Could someone help me understand the state of the LED in the optoisolator and the operation of the SK-201 in the SB-200?


    Dan WQ4G
    Last edited: Nov 25, 2019
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You understand the operation of the LED and the opto isolator correctly, it's normal state is to have the LED lit and the opto isolator output active when the amp is not keyed.

    The trick is the MOSFET, it's a P channel power FET and it does not conduct Drain to Source current when zero bias is applied to the grid but switches to conduct when a negative voltage is applied to the gate.

    In that circuit with an unmodified SB-200 (no changes to the negative bias supply which you'd typically change to a positive cutoff bias supply if you hard ground the grids) the resistor divider feeding the gate of the FET is supplied with a negative voltage from the cutoff bias supply. In the unkeyed state the opto isolator drags the gate to ground which turns the FET off. When the amp keying line is dragged down the LED turns off, the isolator switches off, the gate voltage is driven down to a negative voltage which turns on the power FET to key the amp's relay.

    Here's the datasheet for the IRF9610, look at the Drain to Source current vs Gate voltage curves:

    Here's the key chart in terms of understanding how the MOSFET switches:
    Last edited: Nov 25, 2019
    WQ4G likes this.
  3. WQ4G

    WQ4G Ham Member QRZ Page


    Thank you for that explanation....

    BTW, beautiful station there. Hope to hear you on some time...

    Dan WQ4G
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page


    [edit] Ooops, just saw this typo:
    That should read gate... aint no grids in a FET :)

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