Heathkit DX-35

Discussion in '"Boat Anchor" & Classic Equipment' started by K1OIK, Jan 17, 2022.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I have a couple dummy loads much like that design that are available inexpensively on ebay: https://www.ebay.com/itm/265351908998?hash=item3dc834ce86:g:EtMAAOSwNWxaS~Bu

    I find these very hard to beat for the price. I've swept mine up past 1GHz and it's a very good 50 ohm load up through any of the ham bands I'm likely to use. I keep these on my test bench for use when working on rigs. No relationship with the seller other than I've sent him some cash to buy these but at this price there's not much excuse not to have a dummy load in the shack.

    Here's what they look like, they come equipped with N connectors and a SO-239 to N adapter:
    Screen Shot 2022-01-19 at 2.16.41 PM.png
    N0TZU and N2EY like this.
  2. W9GB

    W9GB Ham Member QRZ Page

    N2EY likes this.
  3. N2EY

    N2EY XML Subscriber QRZ Page

    Very nice - thanks!

    But a bit overkill for ~100 watt strictly HF use, wouldn't you say? N0SS's method costs a lot less if you have the heat sink and a connector.

    73 de Jim, N2EY
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, if you've got the heatsink and don't mind a bit of simple metal working to put it all together. But for a lot of hams that don't have a parts bin with things like heat sinks and connectors it's a pretty good deal. That said, I'm a big fan of home brewing gear and the N0SS design is clean and an easy first home-brew project.
  5. K1OIK

    K1OIK XML Subscriber QRZ Page

    I have a Heathkit Cantenna. Is that adequate?
  6. N2EY

    N2EY XML Subscriber QRZ Page

    What are its ratings?
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    W1BR likes this.
  8. N2EY

    N2EY XML Subscriber QRZ Page

    The point of my question was to teach someone how to fish rather than cooking them a seafood dinner.

    The owner of a Cantenna should know how to find the specifications and determine whether it is a better dummy load than a light bulb. If not, they should learn.
    W1BR likes this.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

  10. W5RKL

    W5RKL Ham Member QRZ Page

    I agree with you Jim.

    The HN-31 Cantenna manual has a specification page that would list the specifications including power and impedance
    ratings. The operator has to take time to read the manual. If they don't have the HN-31 manual then do some searching to find and download
    it. I have it on my website (click link below)


    The HN-31 first page, "Specifications", lists the maximum power, VSWR, and impedance ratings of the HN-31 Cantenna dummy load.

    Yes, the DX-60/60A/60B manuals show a light bulb as a dummy load BUT if you read a bit more in the DX-60B manual, it
    clearly states the following:

    "The light bulb type dummy load may not work properly on all bands and therefore is not recommended"

    The HW-101 manual page 119 step 2 states

    "Do not use a light bulb as a dummy load as they present an impedance which varies with power and frequency".

    The HW-16 manual page 39 step 2

    "A 50-ohm nonreactive dummy load that is capable of 100 watts dissipation such as the Heathkit Cantenna HN-31"

    If an output indicator is needed/desired then buy a wattmeter or use an external SWR meter with the SWR meter switch set to FWD. The Heathkit
    AM-2 and HM-11 1KW SWR meter works well as an output indicator by setting the meter switch to FWD. Some transceivers and transmitters have
    a "REL PWR" meter switch setting which is simply a relative power output indicator NOT a wattmeter.

    Mike W5RKL
    N2EY likes this.
  11. K1OIK

    K1OIK XML Subscriber QRZ Page

    I get it about the light bulb. I tried a resistor dummy load and the RF and/or oscillation continued. I believe it is an oscillation problem as it continued with RF defeated.
    It is hard to know what voltages are off as none are given on the schematic, but I had +20v on Pin #7 on the 12AU7 which seems high for a grid.
    The DX-35 was bought at a flea market. I don't assume it ever worked properly.
    It is on 80 meters.
    The audio source is a D-104.
    The mic input circuit was replaced as I thought the "feedback" was a result of a problem in that circuit.
    Last edited: Jan 22, 2022
  12. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I trust you have the complete DX-35 manual and can follow the schematic, otherwise none of this will make sense. Pages 27 and 28 of the manual do have some voltages to check.

    I don't think you understand the 12AU7A stage's circuitry. It is a being operated as a dual cathode follower, but first:

    With the Function switch in AM and the second 12BY7A tube (Driver) and the 6146 tube (Final) removed, check for about 425 volts on Pins 1 and 6 of the 12AU7A. Scope Pin 8. There should be no oscillations. If there is, check the complete 12AU7A circuit against the schematic for some 'uneducated' modifications.

    Back to the theory of the 12AU7A stage. Pin 2 is given audio from the speech amplifier via a .001uF cap. Pin 3 has a 2.2Meg resistor from its cathode to ground which means Pin 3 and Pin 7 are being pulled up to the same DC voltage. So check the voltages on those pins,

    Pin 3 is setting the grid bias for Pin 7 via a 1Meg resistor. This means Pin 8 should have at least 40V DC on it. Pin 7 is also being fed audio from the speech amplifier as well via another .001uF cap. Thus, the first 12AU7A' stage is setting the bias voltage for the second triode. This second stage's cathode Pin 8 will be modulating the screen voltage in normal operation. Since both stages are receiving audio, Pin 8 will have a varying DC voltage in sympathy with the audio from the speech amplifier plus a DC bias component above ground.

    If you are using an unamplified D-104, the 1Meg resistor coming off the mic jack should be removed. If you are using an amplified D-104, you can leave it in.

  13. N2EY

    N2EY XML Subscriber QRZ Page

    How was "RF defeated"?

    As pointed out by @AC0OB, voltages are given in the manual.

    Did you read the manual?

    Have you traced out all the circuitry to insure that it is wired per the schematic?

    I understand that you want a magical quick and easy fix that will permit you to resell the unit for many times what you paid for it.

    Unfortunately, sometimes fixes aren't that easy. Particularly with kits, where it might have never worked properly in the past.

    It's also possible that more than one part has gone bad, or there is more than one wiring error.

    As previously mentioned, the first step is to read the manual and understand how the circuit actually works.
    W1BR and G8XQB like this.
  14. G8XQB

    G8XQB Platinum Subscriber Platinum Subscriber QRZ Page

    Sage advice...
    N2EY likes this.
  15. K1OIK

    K1OIK XML Subscriber QRZ Page

    Pins #1 and #6 on the 12au7 have about 325vDC, pin 8 with the plate cap 6146 removed has an oscillation in excess of 50 V P/P. Pin# 2 on the 12AU7 has 0.78VDC, Pin#3 30VDC, Pin#8 48VDC, Pin #7 19VDC.

    Almost no voltages are given in the manual on pages #27 and #28.
    I have spent many hours on this and that is OK.
    I do sell equipment and the profit goes to St. Jude, my schools Sunshine Fund and children's education. The donations exceed $40,000 a year(most from investments) although recent turns in the stock market may lessen that a bit. The sale of ham equipment is a pittance. If I were to donate to amateur radio it would be QRZ, not the ARRL.
    St. Jude's send me lots of memo pads, address labels and calendars. Worth a fraction of what I send them.

    st judes.jpg
    Last edited: Jan 22, 2022

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