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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

    The top one is the better circuit but it will only get you 267V +, - for both supplies.

    Last edited: Oct 12, 2021
  2. N2DTS

    N2DTS Ham Member QRZ Page

    I would use none of them.
    You want 600 to 700 volts for the HV and 250 to 300 for the low, and -120 for bias if you use a screen mod design that calls for that,
    plus 12 volts dc.

    I like choke input supplies, better regulation, easier on the diodes, and you can use a lot of capacitance.
    Hammond sells some nice small chokes...

    I would rather do the 700 volts and 250, the 700 will deliver more peak power and the 250 will make life easier on the low level tubes.
    Best would be a 700 to 800 volt center tapped transformer and use a bridge for the HV and use the center tap for the low voltage.

    Screen modulation does not increase the plate voltage on the final so whatever the power supply HV is, is the most the final will ever see.
    I run my 4X150's at 2500 volts (rated for 2000) as the peak power goes up.
    I think some radios ran 6146's at 800 volts or more.

  3. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    thanks for the comments guys.

    The two schematics in my previous attachment are for an addition to the chassis that houses my copy of the UT-1 supply. Another LV B+ and a bunch of biases: one variable by pot, and two regulated by gas.

    For my copy of the UT-1 (see below) I am using a 300VAC 0.66A toroid instead of the 260VAC 0.6A xfmr called for in the UT-1. I think under load I will get 300VDC and 600VDC for the Cheyenne.

    I want to use the Cheyenne to excite my 812A rig on 20-15-10, i.e., on the bands not covered by the present MOPA exciter.

    Attached Files:

  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    A 300Vac source will raise the Keydown voltages at least 50V over the 260Vac original transformer.

    Here is the circuit I use when revamping the Voltage Doubler FW PS.


    Attached Files:

  5. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    Phillip, what is the current draw for those key-down voltages on your schematic ?
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Approx. 200 mA.
    KC2ZFA likes this.
  7. N2DTS

    N2DTS Ham Member QRZ Page

    On the power supply topic, WB2RON told me I had hum on my signal and sure enough I did.
    It did not show up on the mod monitor (REA) and I wonder how low in response that goes.
    It did not show up on the power meter, I could see it on the sdr if I went narrow on the spectrum display.
    It turned out to be in the power supply for my home brew vfo/exciter.
    If I power the exciter unit with a Heathkit adjustable supply its clean.
    I found a bad rectifier assembly and replaced that but no change in the hum.
    Choke input power supply, full wave center tapped HV setup, I checked the transformer, both sides putting out the same voltage to the center tap, choke measures ok, adding more capacitance does nothing???
    The hum is 30 to 35 db down.
    Tried running the filaments off DC, no change.
    The supply is remote from the vfo/exciter part and I re did the cable between, shortening it up and increasing the ground wire thickness, no change.
    Something very odd going on here....

    For the moment I am using the 7610 to excite the RF deck, but want to fix this problem as the setup has always worked so well...
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If the voltages are too high for your particular project increase R18* and R19* to 220 ohms or even 270 ohms 20W.

    With the voltages present in the above circuit your bleeder current alone averages about 26 mA without any external loads.

    For R20 I use a 30k 50W WW Variohm with the slider tap to adjust the LV supply.

    Last edited: Oct 13, 2021
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Which, what transmitter are you describing that has the hum problem? Could it be a ground loop problem or RF getting into the power supply?

    For the bias supply, one only has to use a dropping resistor for the total current required to get the proper voltage, with a filter capacitor after it to minimize bias supply hum.

    Yes, the HT-37 and Valiant ran about 800 volts on the plate.

    Last edited: Oct 13, 2021
  10. N2DTS

    N2DTS Ham Member QRZ Page

    The transmitter is one of my homebrew ones in which I can run 811 plate modulator or a screen modulator, and I can put in the 3x 4D32 rf deck (for plate modulation) or a pair of 4X150 rf decks for plate or screen modulation.
    I tried both setups and all had the hum so it was not in the modulator or rf deck.
    That left the HV supply or the vfo/exciter.
    I pulled both and checked them and they were ok.
    The vfo/exciter has a remote supply that sits on a shelf in the rack.
    The hum is on the hv output of that supply.
    280-0-280 vac to diodes, ct grounded directly, output of diodes to a 12 henry choke that measures 52 ohms, 4 series parallel caps of
    100uf at 450 volts, for a total of 100 uf at 900 volts, 470k bleeders across each cap plus the output goes to a wirewound pot and two big VR tubes in series (also acts as a bleeder.

    This is the vfo/exciter:

    The psu:


    That diode assembly has been replaced as I think it went bad, the ground connection was off.


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