What’s a good in circuit cap tester?

Discussion in 'Radio Circuits, Repair & Performance' started by WA7F, Nov 26, 2020.

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  1. WA7F

    WA7F Premium Subscriber QRZ Page

    Tuning Caps and I just verified on the schematic that they are all connected to ground and the other side goes to a n/o relay contact. Testing should not require removal and I venture a guess that it’s safe to use a megger in the RF circuit. Please correct me if I’m wrong about the megger. I’ve long since replaced this ATU but, I could use a second one. My options are to send it back to the mfg or try to fix it.

    Moving away from the ATU and back to test equipment. I think I’ve narrowed capacitor test equipment down the BK Precision 890C Capacitance meter or the 879B LCR. I’m about a step away from pulling the trigger on the 879B. Is there something else that I should look at before pulling the trigger?
     
  2. KD0CAC

    KD0CAC Ham Member QRZ Page

  3. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks. Looks like there is a video and forum for everything these days. Thanks for sharing the links.
     
  4. W9WQA

    W9WQA Ham Member QRZ Page

    id get a cheap dmm with cap value check. do value and short check on ohms. then make next move. they are either shorted or open id guess

    btw you and others; i found a NICE dmm i added to my collection, "haiweets"ht118a.
    very nice,freq,caps and most others. check it out
     
  5. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks for the tip.
     
  6. G3YRO

    G3YRO Ham Member QRZ Page

    If you're building Antenna Traps, you don't really need a meter . . .

    Start with the fixed capacitor you know will give you the desired L/C ratio (doorknob capacitors are best for high power . . . for 100 watts high voltage Disc Ceramics are OK) . . . then you can simply calculate the inductance required to resonate it on the desired frequency.

    There are various online calculators that will estimate the number of turns you need on your chosen former . . . BUT the whole point is that will only be an estimate.

    In practice it's EXTREMELY critical . . . even half a turn on 40m will move the resonant frequency out of the band . . . and moving one turn a tiny bit closer will move it 100 kHz !

    But in terms of Test Equipment to measure them, all you need is a Transmitter that will go anywhere . . . hang the Trap up in the air (away from any metal) with your rig connected to one side . . . connect any RF sensitive meter to the other side to measure the voltage, and tune around on your Tx to find the dip - that will be the resonant frequency. (I have made several Traps sucessfully that way)

    Roger G3YRO
     
  7. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks for the trap building tips, Roger. Since most of the doorknob caps are old military surplus I’d like to check them out first. Perhaps even match pairs with similar performance. I need to add a cap tester to the bench anyway.
     
  8. GNUUSER

    GNUUSER QRZ Member

    using a megger on electronics is asking for a disaster!
    even at low voltage settings a megger produces a minimum of 500 volts and can easily destroy transistors and diodes.
    while you can use a megger to test capacitors its better to remove them from the circuit.
    Im a retired industrial electrician and have used meggers and earth resistance testers a lot .
    an lcr meter can test your caps and display the values within their tolerance but a regular capacitor tester can test for leakage.
     
  9. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks. The caps are basically isolated but, I was thinking about this and decided that it could potentially damage the sensing circuits.
     
  10. GNUUSER

    GNUUSER QRZ Member

    back when i was an apprentice we had a journeyman demonstrating a megger for us under the guidance of the master electrician for testing the conductors in a customers house.
    He never checked to see if everything was unplugged and burned up an old ladies oxygen concentration machine!
    he had a heck of a time explaining it to the insurance adjuster!
    Ive had to instruct others in the use of meggers and earth resistance testers after I hired in the factory.
    (Earth resistance testors!( just consider them as a megger on steroids)):D

    once doing a job for a cell tower we set up the barrier tape and charged the electrodes and boy did the worms fly up out of the ground! one of the guys thought to grab them for fishing and did one hell of a funny dance when he stepped into the test zone:D
    he wasn't hurt but he wont step into a test zone again.
     

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