Heathkit Cheyenne power supply project

Discussion in 'Amplitude Modulation' started by N2DTS, Sep 2, 2021.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Nice construction.

    Some things you might check.

    Is the hum 120Hz (or is the waveform at a 8.3 ms repetition rate or some other freq./repetition rate)? Isolate your scope if you can with an isolation transformer.

    Lift your AC ground(s) with an isolation plug. I had a garage door opener that was sending conducted noise along the house ground.

    Place a 0.22 uF (1kV) cap across the choke and see if that at least attenuates the hum.

    Last edited: Oct 13, 2021
  2. N2DTS

    N2DTS Ham Member QRZ Page

    I measure 240mv AC on the HV output no load.
    I tried various caps across the choke and only got higher ac output.
    I then put a 2nd choke in line with the one in the supply and it went down to 137 mv.
  3. WZ5Q

    WZ5Q Ham Member QRZ Page

    If you don't already use it, here is a great simulation tool for modeling power supply's from Duncan Amps.
    It allows you to make circuit changes and see the results, it is very accurate.
    They even have a support forum.

  4. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    just finished the heathkit UT-1 part of my supply, for use with an MT-1. It puts out 300 VDC at 80 mA and 620 VDC at 120 mA.

    Onwards to the +/- low voltage part of it.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    collecting parts, will start work after my recovery when both arms work.

    Got a big variac for the HV section also.
  6. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    ouch, hope you get better quick.

    the LV ps involves the rectangular xfmr, the choke behind it, and the gas regulators now built per the schematic below.

    Tested the +150/75, works fine, BUT when I connect the bias bridge I blow the 3A fuse (one 3A fuse for the whole supply. I guess inrush current is too big due to the 100uF electrolytics.

    I can put a small choke after the bias bridge, would that tame inrush ?

    Attached Files:

  7. W8KHK

    W8KHK Ham Member QRZ Page

    Look carefully at the schematic. There is a critical flaw. You have two bridges connected to the same transformer secondary. One has the positive output terminal connected to ground, and the other has the negative output terminal connected to ground. You are effectively shorting out the entire output of the bridge, causing the fuse to expire immediately.

    I suggest you consider a voltage doubler, using just two diodes, one for positive, and one for negative, if you wish to use the same transformer winding for both positive and negative.

    Alternatively, you could retain the bridge for the positive rail, and capacitively couple a diode to generate your negative voltage, but you cannot parallel the inputs to two bridge rectifiers, and indiscriminately ground the positive and negative outputs simultaneously.

    PS. I can assure you that this is NOT the first time this circuit has been tried. When I was around six years old, my dad had a 15 volt 30 amp transformer that we used primarily as a battery charger. I attempted to connect the output of two separate selenium bridge rectifiers in series to double the output voltage from the single transformer secondary winding. I quickly learned how bridge rectifiers function, but the results of my experiment were much more spectacular than blowing a fuse. Anyone who has ever had to replace a selenium rectifier will understand, no explanation necessary!
    Last edited: Oct 29, 2021
  8. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    doh! me stooopid !

    so the attached should work ? I’ve only see what Ivshowctge done with center-tapped secondaries.

    Attached Files:

  9. W8KHK

    W8KHK Ham Member QRZ Page

    That will not quite work either. When your stand-alone diode conducts (bottom of secondary negative) the bridge diode between the top of the secondary and the positive output would be conducting, thus virtually no output from your bias supply section.

    Break the lead between the bottom of the secondary winding and your diode, and add an electrolytic capacitor in series with the secondary and the diode. The positive side of this capacitor connects to the transformer winding, and the negative side connects to the diode.

    Next, add another diode (anode end) from the junction of the existing diode and capacitor, and then connect the cathode end of the new diode to ground.

    This additional diode will cause the add-on capacitor to charge to peak AC voltage, and at the bottom of the capacitor you will have an alternating AC signal with a negative bias, causing the original diode to conduct on negative transitions, charging you bias filter capacitors.

    This is basically a capacitor-coupled section of a voltage doubler, and, under load, it will not provide as much output voltage as a standard half-wave rectifier supply, but it should meet your needs for the bias supply.

    If you are using the bias supply for class B or C stages that draw grid current, this, and any bias supply for that purpose, should be loaded with a low resistance that will prevent the bias voltage from rising significantly when grid current flows in the biased stage(s). The resistance is typically much smaller than that of a bleeder resistance, and its purpose is to regulate the rise in bias supply voltage to an absolute minimum, while keeping in mind the amount of output current available from the supply.
  10. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    thanks for the help 'KHK. Don't the VR tubes act as low-current bleeders ?

    Does the following reflect what you wrote ? 47uF or 100uF or something else for that first cap to the xfmr secondary ?

    Attached Files:

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