600 watt HF Linear Amplifier Project

Discussion in 'Homebrew and Kit Projects' started by KD8UYQ, Feb 9, 2017.

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  1. N1OOQ

    N1OOQ Ham Member QRZ Page

    Take a look at the 78xx data sheet. Those caps you added are pretty much mandatory to avoid oscillations.
    Just go to Digikey's site and punch in 7805, click any of the data sheets it offers, and scroll down to the apps section.
    Same thing with 79xx negative regulators. I had to add the same caps to one just yesterday.
    AF6LJ likes this.
  2. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Sometimes that isn't even enough if they get real hot.
  3. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello Bill,

    Thank you for the tip and hello Tomas. I don't know if it was the decoupling capacitors or the combination of the decoupling capacitors in combination with the 220uH RF choke on the input of the of the 7812 that did the trick but 10 meters works fine now.

    This evening I found the source of my 160 meter relay chatter problem in Auto mode. It is something on the Icom interface board. On a whim I unscrewed the four screws that secures this circuit board to the "U" shape metal bracket holding the band selector rotary switch and moved it to this position.


    Normally that board is mounted in such a way that it is within an inch or so of the 160 LPF section of the Diplexer board. However, with it kinda resting on the edge of the bracket I could drive it almost to full power before you could hear the relays starting to click on and off. Next I tried to mount the board such that the left edge of the interface board used the right two set of holes in the bracket. This moves that circuit farther way from the LPF diplexer board. It improved the situation verses the interface boards original location but wasn't anywhere as effective as when I hung it on the edge of the mounting bracket.

    OK, maybe that short length of ribbon cable is acting like antenna so I wrapped it in copper foil with the end closest to the rotary switch grounded to the chassis via a short length of wire. That action actually made it worse with the board still positioned as shown above. At about 200 watts of output power the relays commenced to chatter as opposed to 450 watts without the copper foil wrap. However, I left the foil ribbon cable shield in place and mounted the interface board back in its original location.

    Now this is the weird part. My fourth problem disappeared. Remember the increasing SWR reading and decreasing indicated output power on the LCD display as I moved up the bands? All of that went away. Now the indicated power is linear across all bands and the SWR reading agrees pretty much with what my Telewave is telling me. What???


    Above is the output power and SWR reading on 40 meters. Before it use to show 562 and with a SWR of 1.4 whereas my Telewave indicates 500 watts to the dummy load and a SWR of 1.1:1

    How could wrapping copper foil around the ribbon cable that goes between the band selector switch and the Icom interface board fix the issue with this display?

    I fear this is the work of dark magic ;)

    Back to the interface board, when I get back from Hamvention I will make a shield box to enclose this board and see what happens

  4. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello Sue,

    No mam, the regulators are operating just warm to the touch.

    AF6LJ likes this.
  5. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    I didn't that much of a load on them, I was just making a general comment in regard to those three terminal regulators. :)
  6. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello again,

    I turned on the amplifier again this morning to see if the power and SWR display was still showing linear reading across the bands and they were. I am still puzzled as to why the copper foil tape around the band selector ribbon cable apparently fixed the problem with this display.

    My truck is packed and I am ready to head out to Dayton in the morning.

    AF6LJ likes this.
  7. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Have fun and good work. :)
  8. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Thank you.

    I am looking forward to the new venue.

    AF6LJ likes this.
  9. WB2GCR

    WB2GCR Ham Member QRZ Page

    Bill, please explain the issue with a "single return"??
    And what possible effect does the gauge of the wire have?
  10. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    A single return will provide a point for all the currents in the return path to add, thereby raising the noise in the system. This is not to be confused with "Single Point Grounding" used in low level, low noise circuits.

    Small wires have higher series inductance than larger gauge wire, even if it is only DC on those wires they make better "antennas" than larger gauge wire.
    SWL37632 likes this.

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