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Johnson Valiant grid current with plate switch off.

Discussion in 'Amplitude Modulation' started by AG5CK, Jan 25, 2021.

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  1. AG5CK

    AG5CK Premium Subscriber QRZ Page

    I wasn't sure if this belonged in the boat anchor section but I figured I may have better luck here.

    My Valiant recently started blowing the 8 amp fuse due to a shorted plate transformer and I have connected an Antek 8T-800 toroidal transformer in its place. The 866a rectifiers have been replaced with solid state plugins long ago. The Chernobyl resistor in the VFO has also been taken care of.

    As I was typing this post the below issues have disappeared and all is normal. I have a feeling this isn't the end of it though.

    Once tuned up on 75 meters I had about 5ma of grid current with the plate switch off. The rig makes 150 watts on 75m with 275ma of plate current and seems to be ok other than drawing grid current in stand by.

    On 10 meters I couldn't get it to load up properly. At 300 ma plate current the best output I can get is 50 watts. With the plate switch off the grid current meter flutters around 2 ma.

    Does anyone have an idea as to what could be happening?
     
  2. N1BCG

    N1BCG Ham Member QRZ Page

    You will see grid current in standby if either the Oscillator switch is set to “Zero” or the Mode switch is set to “CW”. Both are normal.

    As for low output, it could be that you are on the wrong “dip” in plate current, the buffer is mistuned, or the driver tube is weak. How high does the Drive pot have to be set in order to get 7.5mA of grid current?
     
    WQ4G likes this.
  3. AG5CK

    AG5CK Premium Subscriber QRZ Page

    Hi Clark. I was seeing grid current with the oscillator switch set to VFO, mode switch set to AM and plate switch off. It would vary with drive setting and would increase when the oscillator was peaked.

    I didn't take note of the drive control position. The pot was bad when I got the rig a few months ago and has been replaced. I don't think the drive control ever needed to be past 1:00 to get 9 ma of grid current with the oscillator switch set to zero or when transmitting.

    I never had this issue before the plate transformer crapped out. I thought something else may have failed when I lost the transformer but all of a sudden it started working normally.

    I had to pull the HV rectifiers to shoehorn the new HV transformer in. I need to finish up with that and will put it back on the air to see what happens.

    Thanks for any input you may have. A plate modulated transmitter for dummies book would be nice. 73
     
  4. W2VW

    W2VW Ham Member QRZ Page

    Maybe the tx relay was stuck from heavy current flow on the high B+ xfmr feed and was keying RF Drive all the time.
     
    AG5CK likes this.
  5. N1BCG

    N1BCG Ham Member QRZ Page

    It may also be that the keyer setting (R39) is out of adjustment as it sets the bias point on the keying tube (V11). R39 should be adjusted slightly past the point where the oscillator drops out when unkeyed. It could be very close to that threshold, thus the intermittent issue.

    Low drive on 10M may be due to L5 (to the left of the VFO) being out of adjustment. Set it to maximum grid current on 28.6 mc.

    When operating on the upper bands, it's recommended that the the Auxilary loading control be initially set to 9 and the Fine control set to "0" (assuming the knob was installed properly). The Valiant uses a frequency multiplier so this will help avoid tuning to a lower frequency than desired.
     
    Last edited: Jan 26, 2021
    WQ4G, W2VW and AG5CK like this.
  6. AG5CK

    AG5CK Premium Subscriber QRZ Page

    I did try the keyer adjustment and it didn't make any difference. I didn't think of a relay being stuck. It may have opened up after I keyed the thing a few times. Time will tell.
     
  7. K5UJ

    K5UJ Ham Member QRZ Page

    Last fall when I was turning a Ranger into a 160 m. driver, I had a weird keying problem that drove me up the wall. I don't remember much about it except that I eventually discovered that the tube on the elevated keying circuit chassis had gone bad on me all of a sudden. I replaced it and everything worked correctly.
     
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  8. AG5CK

    AG5CK Premium Subscriber QRZ Page

    It's alive again! Now I just have to wait on the furniture guys to finish up so I can bring it back inside and connect the modulator.

    It took some rearrangement but it still fits in the case. The plate relay contacts looked pretty rough and finished it off when my screwdriver slipped. Luckily Mouser had what I needed.

    Plate voltage is now 715 volts. Hopefully it doesn't start arcing and sparking with modulation.

    20210213_142153.jpg 20210213_142433.jpg 20210213_152337.jpg
     
  9. W2VW

    W2VW Ham Member QRZ Page

    Do you have enough step up there in the mod xfmr?
     
    AG5CK likes this.
  10. AG5CK

    AG5CK Premium Subscriber QRZ Page

    I have too much. I did the math with 715 volts and 300 ma. I need a 1:24.4 and have a 1:29.
     
    Last edited: Feb 14, 2021

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