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Neutralization Question

Discussion in '"Boat Anchor" & Classic Equipment' started by W8TMT, Feb 11, 2019.

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  1. W8TMT

    W8TMT Ham Member QRZ Page

    Granted I don't have a ton of experience at this but I have seen it at least 10 times with Heathkit Hw-16's, Hw-101's and my latest project, an SB-102, is no exception. Here's the scenario: I neutralize the finals using the method of removing plate and screen voltage then adjust the neut capacitance for minimum RF pass through. Then I go back and check. Plate dip and output max DO NOT occur at the same point. So what gives? The method leaves me with a dilemma. Do I tune for a Ip dip or max RF? Am I doing something wrong? I've done it using different equipment like and oscilloscope or relative RF indicator. Now I'm blessed with a Motorola R-2001D service monitor which has a spectrum analyzer function. Note: in all cases I've terminated the radio in a 50 ohm load.
     
  2. K2XT

    K2XT Subscriber QRZ Page

    Try this, after you have done it by your current method.
    Dip it, load it to rated current.
    Then rotate the plate tune control ever so slightly looking for peak output and notice which direction you had to turn it.
    Then ever so slightly adjust the neutralizing cap and write down which direction you turned it.
    Repeat, using what you learned (keep doing it if it improved, or vice versa).
    You will get it to where max out and dip occur simultaneously.
     
    KB4QAA, N4MTB, AI3V and 3 others like this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    I know there are various suggested methods for neutralisation . . .

    Personally, I have only EVER adjusted the neutralisation trimmer (on hundreds of SSB rigs of all makes) so that the PA Current Dip concurs with Maximum output.

    Roger G3YRO
     
    KI4AX likes this.
  4. W8TMT

    W8TMT Ham Member QRZ Page

    Thanks Roger and XT. I've been led to believe that the other method is superior, but my experience and yours sure says otherwise. Would have been nice - it's quick and no trial and error.
     
  5. WA5VGO

    WA5VGO Ham Member QRZ Page

    Cold neutralization is an excellent way to get things very close. After it is done, it’s typically necessary to slightly touch things up to get it exact. That entails adjusting so that maximum output occurs exactly at the plate current dip.
     
    N2EY likes this.
  6. WD0GOF

    WD0GOF Ham Member QRZ Page

    Where you disconnect the HV and screen voltage will make a difference. In the sb102 the plate choke and its bypass needs to remain in the circuit. In the screen R924 and C941 needs to remain.
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Normally I leave plate voltage "on," screen voltage "off," to assure all components that should be in service still are.

    Kenwood hybrids and some others have a screen voltage on/off switch right on the rear panel just for this, so you don't need to disconnect anything.

    Always worked fine for me.
     
    K7TRF likes this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yup, that's exactly how I do it as well. I've never pulled the plate voltage nor the filaments though there are advocates for neutralizing completely cold finals.
     
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    Sometimes the change in dimensions due to heat can severely affect the internal capacitance. This is especially true of high power-sensitivity tubes like the 6146. I always used "warm" neutralization.
     
    N2EY likes this.
  10. W8TMT

    W8TMT Ham Member QRZ Page

    Hmmm. Interesting point. I lifted the red wire under the chassis. It passes through into the RF cage and I did not look to see if there was a bypass cap at the base of the choke. Will look tonight. BTW for others - heaters were on. Accessory plug removes the screen voltage.
     

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