Heath DX-40: two different modulator circuits?

Discussion in '"Boat Anchor" & Classic Equipment' started by K5DH, Aug 11, 2018.

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  1. K5DH

    K5DH Ham Member QRZ Page

    Okay, DX-40 mavens, let's see if you can help me. I've seen photos of the underside of various DX-40 chassis and it looks like there were two different circuits, mainly in the modulator section. I'm fixing up a DX-40 whose modulator section differs quite a bit from the schematic that I have, but the circuitry looks just like a bunch of the photos I've seen on line. I recall seeing a reference to there being two different circuits, but I can't find it. Can someone shed some light here? If there are two different circuits, which one is better and why?
  2. N2EY

    N2EY Premium Subscriber QRZ Page

    I'm no DX-40 maven, nor an expert in anything.....but I'd like to help if I can.

    Can you show us some photos and trace out the as-wired circuits?


    The DX-40 uses a form of controlled-carrier screen modulation. The basic circuit operation is explained in the manual. It's possible that some folks didn't like the controlled-carrier system and changed it to straight screen modulation, despite the fact that controlled carrier gives more power output.

    73 de Jim, N2EY
    Last edited: Aug 12, 2018
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    There was a change made by Heath with the modulator in the DX-40 sometime in the production run. However, I have no documentation as to just what was changed and exactly when the change was made.

    There has been some information on this posted on one of the boat anchor E-Mail reflectors quite a ways back. But, I just do not know on which reflector the information appeared.

    The only thing that I remember was that certain component values were changed to increase the audio power and to improve the frequency response.

    Glen, K9STH
  4. AC0OB

    AC0OB Subscriber QRZ Page

    These are the latest upgrades I use for a DX-40:


    Attached Files:

  5. HAMHOCK75

    HAMHOCK75 QRZ Member

    Not sure if this helps but I do restorations and have restored a DX60B and a DX35 but not the DX40. If some earlier versions of the DX40 used a modulator similar to the DX35 then it will have a problem achieving AM modulation of 90%. My notes show that I discovered both the DX40 and DX60 achieve high modulation % with a high pass network at the output of the cathode follower that drives the screen grid of the 6146 final.

    I tested that on the DX35 and it was able to achieve the same modulation depth as the DX60B. I then returned the DX35 to its original state.

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