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Distorted waveform on Triplett 3432-A sig gen

Discussion in 'Troubleshooting and Repair' started by RodB, Jul 20, 2017.

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  1. RodB

    RodB

    7
    1
    Mar 19, 2017
    I have restored my Triplett 3432-A signal generator but on the 2 lower bands the waveform is distorted, more so on the lowest band. The waveform looks fine at the oscillator but gets distorted at the cathode follower of the 12AU7. I swapped out the 12AU7, no change. I swapped out the bypass cap. on the plate, no change. Replacng the voltage divider in the cathode ckt with a 400 ohm resistor seemed to improve the signal so I'm left with working on the voltage divider. All the resistor calues are in tolerance. I'm not sure where to go from here. Rod
     
  2. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    Is this a recent fault? Is it simply a factor of the simplicity of the circuit design and the none linearity of the circuit?
     

    Attached Files:

  3. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    We don't know what-all your 'restoration' process involved. The first thing I'd do is go back and recheck what you did to make sure all of your grounds are intact, that any parts you may have replaced are the correct material composition (like the resistor-type you use) and value, that your test cables are good.
    If all of your internal solder connections are good, I'd be suspecting a capacitor somewhere.
    Maybe spray some contact cleaner and exercise those variable caps?
     
  4. RodB

    RodB

    7
    1
    Mar 19, 2017
    Thanks for your response. Yes, this is a recent fault. I just obtained the unit and had not applied power until I finished the restore. I figured if the waveform is OK at the higher bsnds most of the circuitry would be fine. The distortion is pretty bad on the lowest band, negative peak is flattened off on the second band, then looks much better at the higher bands. So it looks like the distortion is frequency dependent. That's why I swapped out the tube and plate bypass cap. If the design produces that waveform then I might try to redesign it. Like I said, the signal looks fine on the grid of the cathode follower.
     
  5. RodB

    RodB

    7
    1
    Mar 19, 2017
    Hi and thanks for your comments. I recapped the unit replacing the ecaps and 3 paper caps. All the resistors were within tolerance except one charred 200 ohm across the 2k attenuator pot. The pot measured 1875 ohms. The pot was a little flakey so I cleaned it with deoxit followed with some component cleaner. Now it is smooth and the signal looks fine on the higher bsnds. Like you, I figured some component sensitive to frequency would cause this and that's why I swapped out the plate bypass cap on the cathode follower. The signal on the grid looks fine. The signal at the cathode looks the same as the one at the end of the cable. Maybe something is wrong with the shielded cable that sends the signal from the cathode to the attenuator circuit.
     
  6. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    Good thing to check, possible corrosion on the wire ends maybe(?).
    Before I assumed there was a circuit problem (since it works good on the higher ranges), I'd be seriously looking at the connections; like the shielded cable, solder joints, flakey pots or variable caps.
    Hope this turns out to be an easy fix for your restoration.
     
  7. kellys_eye

    kellys_eye

    4,275
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    Jun 25, 2010
    Got to ask the basic questions so sorry if this sounds 'simple' but what value part did you fit?
     
  8. RodB

    RodB

    7
    1
    Mar 19, 2017
    Problem solved!

    I checked all the connections including cleaning the tube socket and pins but that didn't change anything. So I spent about a day mulling it over and came up with checking voltages. The schematic doesn't have voltages listed so I checked the 12AU7 tube data. For 100 volts of plate voltage the grid bias should be -1 to -3 volts. Measurements showed 118v on the plate and -0.18v grid bias. Not good and it explains the distorted wave. The higher plate voltage is producing more gain in the oscillator driving the cathode follower which is biased on the edge. No wonder it has a flat peak. I looked closer at the voltage divider that provides the bias. The 68k ohm is on the edge of it's tolerance so I replaced it. Now the bias is at just over -1 volt. Some of the distortion is gone. I put a 100 ohm 5 watt resistor in series with the power transformer. This brought the plate voltage down to 105v and the filament voltage from 7.5v AC to 6.35v AC. Now it has a little distortion on the first band and good signal on the rest. Now to check the accuracy. Thanks for all the suggestions.
     
  9. kellys_eye

    kellys_eye

    4,275
    1,147
    Jun 25, 2010
    ....and thank you for updating us on your successful solution.

    Nice job!
     
  10. shrtrnd

    shrtrnd

    3,824
    519
    Jan 15, 2010
    I second that. Thanks for letting us know what you found. Good work on troubleshooting and repair.
     
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