DX-100/Apache Audio/Modulator Improvements

Discussion in 'Amplitude Modulation' started by AC0OB, Jul 19, 2019.

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  1. WZ5Q

    WZ5Q Ham Member QRZ Page

    Yea, the screen drive really screeeeems!

    Higher Mod percentage was part of the reason for the 370v. The other reason was I only needed 20 watts of carrier to drive the linear to legal limit. Operating conditions of the 809 are 370V on the plate @ 60mA which gives me close to 20 watts carrier output.
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Every design is a compromise and is dependent upon your specific goals. Some are better than others.

    The design of the 12BY7 triode stage is operating Class A with an efficiency of ~ 22%. 300 mW AC only (for practical purposes) is being dissipated across the primary verses 500 mW combined AC and DC heating for the original configuration.

    With the modified circuit, No core saturating DC is running through the inductance, distortion is reduced (never totally eliminated), and the 12BY7A's plate dissipation is less than 1/4th of the max plate dissipation allowed.

    With no cathode bypassing, more localized distortion cancelling can be had.

    The goals here were to simply provide enough secondary voltage and driving current for the grids of the Class AB2 modulator stage for 100% modulation at the least distortion possible. No audiophile designs were even considered nor were they necessary.

    So layout your various designs, run the numbers with simulation and or bench testing the circuits (I do both), then tweak, then decide which one is best for your application.

    Most of all, have fun with your discovery/build process.


  3. K4KYV

    K4KYV Subscriber QRZ Page

    One of the simplest and most effective mods to improve the audio in the scratchy Apache, is to clip out R48 (470k) in the line between microphone jack and pin 2 of V7. This especially true if you are using a crystal mic like the D-104.
    N2EY and AC0OB like this.
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Yep, we did that here:

    https://forums.qrz.com/index.php?th...difications-for-am.594659/page-3#post-4547664 (and no longer for sale)

    and in the DX-100 as well. :cool:


    Attached Files:

  5. K4KYV

    K4KYV Subscriber QRZ Page

    I wasn't aware that the DX-100 had one as well.

    Why the 2.7 meg grid resistor for t he D-104? Astatic recommends 5 (4.7) megs.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Use whatever you feel comfortable with and for the low end audio response you prefer.

    The D-104 I have has good response down to about 75 Hz in both rigs with 2.7 Meg Rcg.

    As we all know, there are specs and there are actual in-circuit measurements.

    N6YW likes this.
  7. K4RT

    K4RT Ham Member QRZ Page

    Thank you Phil. I have saved a copy of your PDF.

    I captured a still shot of a video of my scope showing the strange modulation peaks that seem to be associated with the distortion on 80m. Have you seen anything like this before?


    Attached Files:

  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    As a matter of fact I have and it can one of two sources.

    One source can be RF feedback getting into the audio chain via the mike cord. Try a clamp on ferrite just before it enters the mic jack. In addition, the 220 pF RF filter cap after the mic jack needs to be after the 4.7 resistor and not before it.

    The other source (and more likely) is parasitic oscillations at the modulator plate caused by the Mod. primary ringing, since your loading may be different on 80m. Try inserting a 22-27 ohm 2W CC or CF resistors between the Mod. transformer primary feeds and plates with shrink tubing.

    I hope that helps.


    N6YW likes this.
  9. K4RT

    K4RT Ham Member QRZ Page


    Many thanks for the suggestions. Getting the DX-100B on the bench to check those items will be at the top of my to do list after I finish building the new shack.


    Attached Files:

    AC0OB and N6YW like this.
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Here is an updated schematic but the only change was in the HV B+ power supply. The owner had to put in a set of series resistors to get Rnew* to about 270 ohmish of resistance

    High voltage - CW = 779v, (Was about 815V before he raised Rnew* to 270 ohm resistance)
    with Max modulation = 753v.

    Low voltage - CW = 360v,
    with Max modulation = 360v.

    Bias at "K" - Standby = -69V.
    Transmit = -72v.

    So voltages look good.

    It amazing how much AC voltage variation one sees with those DX-100 T2 secondaries.


    Attached Files:

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