Need help from amp builders

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Nov 16, 2018.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    I tried this again using the quality 10k ohm pot that I have. I’m assuming it doesn’t matter which side of the dc voltage source it is wired to so long as current flows through the pot.....? I then very carefully adjusted this pot until until the needle showed full scale. The pot actually came out of this amplifier and stuck out the back for adjusting the plate current meter......? I think. It’s labeled Ip adjust on the chassis. Anyhow, after adjusting this pot on my bench for full scale, I measured the resistance of it out of circuit with my LCR meter. It reads 138.76 ohms. I then attached the short leads from the LCR meter to the terminals of the meter movement and re measured it (the first time I measured it, I used my cheap dvm). It measures 3.522 ohms directly across the terminals. Now I’m even more confused. I thought the resistance of the pot was supposed to be equal to the resistance of the meter movement at full scale using this setup. The LCR meter I’m using to measure this is a brand new DER EE DE-5000.......the same thing I measured all my capacitors and inductors with
     
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    Also, my dvm is very old and has mostly been used for checking continuity (audible beep) and for 12v automotive applications working on cars.....so I’m unsure of its accuracy measuring resistances.
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Watch W2AEW's video tutorial about "Basics of Analog Panel Meters:

    Alan uses a fancy power supply but, ANY variable low-voltage power supply will get the job done.
     
    KN4CTD likes this.
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I watched it but I have limited stuff here to work with......question.....if it takes 37mv (.037v) to deflect this meter to read full scale, and the resistance of the meter (according to my lcr meter) is 3.5 ohms, then can I not work this as an equation using ohms law Vmeter .037 divided by Rmeter 3.5 =Imeter .01057. Assuming some variation in the accuracy of the dvm reading that low of voltage, and it’s actually .035 volts.035/3.5 =.01. Making this meter a 1ma movement needing 35mv for fsd? That still doesn’t explain why the resistance of the pot doesn’t equal the resistance of the meter coil at full scale
     
  5. KN4CTD

    KN4CTD Ham Member QRZ Page

    ???....anyone?
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It sounds like your meter is actually a 10 mA FS movement.
     
  7. KN4CTD

    KN4CTD Ham Member QRZ Page

    Please explain why you think this. I’m just trying to understand and I’m terrible at math
     
  8. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    I have always used 2-potentiometers to measure the resistance of a meter.

    Connect the first potentiometer in series with the meter and the battery. Adjust for just full scale on the meter. Then, connect a 2nd potentiometer with one of the outside terminals connected to 1-terminal of the meter and then the center terminal to the other terminal of the meter. Carefully adjust this 2nd potentiometer until the meter reads exactly half scale. Remove the 2nd potentiometer and, without touching the shaft, using an ohmmeter (DMM, etc.), measure the resistance between the outside terminal, that was connected to the meter, and the center terminal. This value will be the value of the internal resistance of the meter.

    Using the first method will produce the correct answer. However, using 2-potentiometers makes the adjustments a lot easier. Well, at least for me!

    Glen, K9STH
     
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  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    Glen, am I correct in wanting to know how much voltage (mv in this scenario) it takes to provide full scale meter deflection? Anyone else for that matter also? It seems to me that having really accurate measuring devices for these small voltages etc is important at this point.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Because you said 37mv and 3.5 Ohms.

    37mv/3.5 Ohms = .01057A ~ 10 mA.
     
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