Power Supply Voltage Drop

Discussion in 'General Technical Questions and Answers' started by N5XJT, Jun 12, 2019.

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  1. N5XJT

    N5XJT Ham Member QRZ Page

    My Yaesu FP-757HD Power Supply shows 13.9 volts no load but drops to 11.1 volts under transmit load. I know most power supplies have some voltage drop under load but 2.8 volts seems excessive. My first thought is to replace the capacitors but may be just guessing. Any suggestions appreciated.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If your load does not exceed the power supply current rating (?) I’d suspect the regulator itself more than capacitors.
  3. KU3X

    KU3X Ham Member QRZ Page

    Are reading the voltage on the display on the radio or confirm the voltage drop by putting a good voltmeter across the leads on the back of the power supply?

    My FT 897D always showed what I thought was a high voltage drop but my power supply was actually rock solid.

    There a moticable voltage drop on the power cable and that drop is very noticable after the two fuses.
    SM0GLD likes this.
  4. K8ERV

    K8ERV Ham Member QRZ Page

    Long shot-- always try a different voltmeter. Could be an RF problem.

    TOM K8ERV Montrose Colo
    K3RLD likes this.
  5. W9WQA

    W9WQA Ham Member QRZ Page

    read my posts about hot fuses and comments on the astron 35.
    fuses are a big problem and cheap fuse holders.
    my favorite connectors,bananna plugs loose tension and also drop volts under load, they gotta go too.they are fast to use but loose contact..

    my astron dropped almost a volt, all in connections/fuses.
    the back terminals read 14.02 no load and,,,14.02 at 18 amps! zero drop inside.the regulator is rock solid...
    SM0GLD likes this.
  6. WD8BWW

    WD8BWW XML Subscriber QRZ Page

    In my opinion, the voltage on the readout of the radio should be your best indication that there is a problem worth checking. For example 897 readout is calibrated to 13.8 vdc and then set in memory in the radio as part of alignment. If that meter shows a lower voltage by a couple of volts it should be checked to prevent damage to the radio. I say no matter what is the cause don't transmit if you are seeing significant voltage drop.

    In my opinion-experience with good power supply, low gauge wire and good connections the drop should be measured in tenths of volts. Checking the voltage at the power supply and seeing 13.8 when the radio is showing a much lower voltage, simply means there is an issue between the supply and the voltage inside the radio, as posts mentioned above. I repaired an FT900 with only a 50w TX power output because the six pin connector had black tin corrosion on the radio side pins. In my experience, major voltage swings due to bad connections, especially during SSB transmissions, can damage the finals, and even the main processor.

    While I'm on my soapbox be very careful buying power cables both AC and DC off Ebay, especially if they are coming from an overseas seller. Read the fine print! I have seen titles like "10 Gauge DC Power Cable" but when you read the description it says 16 gauge wire. I purchased a dozen computer/radio AC power cords off of Ebay once. They got "very" hot pulling just a few amps. Even though they were the normal outside diameter, I cut them open to find about 26 gauge of copper. I guess copper costs more than plastic.
    K3RLD likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, if (and only if) the rig's internal voltmeter reflects what's really going on.

    Measuring at the power supply means nothing at all, if the power supply is working; measuring inside the rig is certainly the place to measure to determine if all is okay.

    But if the voltage is dropping at the power supply, there's a problem with the power supply.:)
  8. KC8VWM

    KC8VWM Ham Member QRZ Page

    You didn't specify if this is a new problem that suddenly sprung up, or if it's always been that way all along.

    This information will tell us if it's a recent component failure or something else going on.
    AI3V likes this.
  9. AI3V

    AI3V Ham Member QRZ Page

    He also neglected to mention if he was using a dummy load or not.

    KC8VWM likes this.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    A hit & run from more than two days ago...
    KC8VWM likes this.

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