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

    I have both and sb220 and the newer HL 2200 they both have about 2 volt AC on their input when they're keyed up if you put a meter across the input coax to me that seems kind of strange.
    I'm concerned that this amplifier ruined the SWR sensing diodes when I hooked an Icom 706 Mark II G up to it.
     
  2. AA5CT

    AA5CT Ham Member QRZ Page

    Are you measuring this with a 10 Meg Ohm input DMM?

    Try this again with 1 K or 10 K resistor to ground, effectively across the DMM test leads.
     
    KP4SX likes this.
  3. KG7HVR

    KG7HVR Ham Member QRZ Page

    fluke 16 and some other high-dollar Chinese multimeter I'm pretty sure that they're both true RMS reading meters
     

    Attached Files:

  4. KG7HVR

    KG7HVR Ham Member QRZ Page

    Heres what i found Screenshot_20190205-201837_Hancom Office Editor.jpg
     
  5. AA5CT

    AA5CT Ham Member QRZ Page

    More than likely you're seeing stray AC; I don't think the SWR detect circuits are going to be affected in a 706 series radio ... try the resistors, if the voltage is a mere fraction, its just 'stray' AC ... hold one of your meter leads up near a lightbulb - same effect.
     
  6. KG7HVR

    KG7HVR Ham Member QRZ Page

    Here's the other
    I tried the lightbulb trick and I got 0.1 when I did that
    I'll look for a 1K or 10K resistor and try that also I just know that I don't see any voltage across the input in my solid-state amplifier so that's why I was kind of concerned Screenshot_20190205-202108_Hancom Office Editor.jpg 20190205_202311.jpg Screenshot_20190205-202220_Hancom Office Editor.jpg
     
  7. AA5CT

    AA5CT Ham Member QRZ Page

    I should ask - was this with antenna connected? I see about 20 or 30 mV AC using a scope connected to my 2 meter vertical, which is not DC grounded. After the first rain falls that value decreases by quite a bit ... I think I'm seeing stray AC on that antenna from the buried utilities in the alley.

    If I do the same thing with a DMM in the car on a 2 m mobile antenna, underneath power lines I can see 2 or 3 V AC.
     
  8. KG7HVR

    KG7HVR Ham Member QRZ Page

    .076vac with 10k ohm across leads
     
  9. AA5CT

    AA5CT Ham Member QRZ Page

    Ahh ... more like it. All the characteristics of "stray" AC line voltage. Those amps are not terribly well-shielded inside - did you have an antenna of some type connected too?
     
  10. KG7HVR

    KG7HVR Ham Member QRZ Page

    Yep into my tuner then my 300 foot loop
     

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