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1.9v ac on input of sb220 when keyed. Why? Is this normal?

Discussion in 'Amateur Radio Amplifiers' started by KG7HVR, Feb 6, 2019.

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  1. KG7HVR

    KG7HVR Ham Member QRZ Page

    Both amps have the exact same amount of ac voltage when you key it up. I'm not sure what exactly the changes were anymore that I made to these two but they're both direct ground grid. one of them had a bunch of BS on it from the recommendations of the stuff that rich measures published. I took all that crap out of there. the bias was changed from the original. Simply switching a couple of wires on the relays and I don't remember what the actual name for the type of bias change. I'm wondering if that has anything to do with it whatsoever? the HL 2200 has the upgraded relay kit to isolate the power switch from handling anything more than a few volts to power the relay. they've both been exceptionally good amplifiers to me except for the fact that I had a bad solder pin on one of the tubes because of the socket being loose. They don't seem to work well with solid-state radios however the input is not quite at a optimal 50 ohm but they work great with my boat anchor radios
  2. AG6QR

    AG6QR Premium Subscriber QRZ Page

    The question isn't whether they're true RMS, or accurate, or anything of the sort. It is whether they're high impedance, which they are, because that's almost always a good thing for a voltmeter.

    Except it's not such a good thing when measuring a line that may have stray voltage, sometimes called ghost voltage. This can produce a high impedance voltage source that will register on a high impedance meter, but will vanish when any moderate resistance is present.

    I own a Fluke 16. It has a standard 10 Mohm high impedance mode when the selector is in the voltage setting. But put the meter in the resistance setting, and if a reasonably stiff voltage is present, the meter will switch to Lo-Z voltage mode and display the voltage. (The ham Technician test pool has a question that says exposing a meter to a voltage when it's in resistance mode is a common way to damage a meter, but the question authors apparently weren't thinking of a Fluke 16. The technique of measuring a voltage in resistance mode is documented in the owner's manual as the way to measure voltage in low impedance mode. The Fluke 16 is guaranteed to safely handle up to 600 V on its input terminals when in resistance mode. Don't try this with other meters unless you verify that it's supported in the user manual!)

    So try measuring that voltage with the Fluke 16 in resistance mode. What happens? For fun, try measuring an AA battery the same way.

    For more explanation of ghost voltages and why a LoZ mode in your meter can be helpful, see here:

    For explanation on the way the Fluke 16 implements its Lo-Z mode (called V*Check in Fluke marketing speak), allowing you to measure a voltage with the meter in resistance mode, see here:
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't trust the Fluke, either.

    Mine does the same thing when probing anything with a big power transformer in it.

    Wrong instrument for the job.
  4. KG7HVR

    KG7HVR Ham Member QRZ Page

    Well is this normal or not? Could this damage modern solid state radios?
  5. AA5CT

    AA5CT Ham Member QRZ Page

    My SB-220 plays well with my Icom IC-756ProIII.

    I have zero concerns, and the SWR circuitry still functions as expected ...
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I doubt it. If you want to hold the voltage down further and have a static bleed, solder a 100k 2 watt resistor at the input connector.
    Last edited: Feb 7, 2019
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Measure the voltage once the solid state radio is connected to the amp input by a coax patch cable.

    Bet you'll see that voltage drop to zero or very close to zero.
  8. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Currently, there are 3 SS HF radios here, 2 Yaesus and one ICOM from 3 different decades. They all have an inductor across the antenna jack. There would be millivolts at most unless that coil was open.
    Last edited: Feb 8, 2019
  9. KG7HVR

    KG7HVR Ham Member QRZ Page

    I see what your saying
  10. G3YRO

    G3YRO Ham Member QRZ Page

    This is why I never use any of those digital multimeters !

    Give me a decent Analogue meter any day !

    Roger G3YRO

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