Linerality across the dial

Discussion in 'Tuners' started by CJVx, Apr 27, 2017.

  1. CJVx

    CJVx AK Subscriber Subscriber

    Messages:
    78
    Location:
    NJ
    any tips or tricks on how to achieve this? I'm talking about stations lining up with dial pointer from end to end, not just shifting the band with the oscillator dial. It seems to be easier when there are a series of those bendy copper oscillator coils to get things lined up, but on units with only one coil or potted coils it gets tricky, is threre anything else that can be done?
     
  2. CJVx

    CJVx AK Subscriber Subscriber

    Messages:
    78
    Location:
    NJ
    I have a fisher 125 receiver and the stations line up perfectsly from 88-to about 98, then they seam like everything shifted to the left 10mhz. Any adjustments, if any, might correct this?
     
  3. FauxHall

    FauxHall Super Member

    Messages:
    1,445
    What manufacturers used to do on their top-end models was adjust the tuning dial indications to match the frequency actually tuned. The frequency band is seldom a straight line and perfectly linear with the string that moves the indicator.

    Obviously, this is a costly refinement that can't be done at home without repainting your dial scale.
     
  4. steerpike2

    steerpike2 Super Member

    Messages:
    3,199
    Location:
    Transvaal, South Africa
    usually this is done by bending the vanes of the tuning capacitor gang. They are slit into several 'fingers' so different sections can be bent individually.
     
  5. jlovda

    jlovda Things I loved from the 60's and 70's

    Messages:
    3,122
    Location:
    NE Ohio
    You cannot just go to the middle of the band and tweak the osc. First, the frequencies on the dial (usually slightly non-linear) are positioned by the factory design. The service manual usually states the following procedure for analog tuners (I'm generalizing here); Assuming you don't have an RF signal generator, at the low end pick a medium strength station at a known frequency of about 90 mHz and set the dial to the correct freq. Adjust the oscillator COIL to peak the output at the correct spot. Then, move the pointer to a station with a known frequency of about 106 or 107. Adjust the trimmer screw on the oscillator section of the tuning cap (not the RF trimmers) to max the output at the correct location. Unless they were accidentally bent, you should never have to touch or bend the actual capacitor plates. The low end will shift slightly. Go back and do the low end portion again. The high end will shift slightly but move closer to the correct point. Then go back and do the high end section again. Go back and do the low end. The location should be closer than before. After about three passes at both ends the dial pointer should line up quite nicely. Remember, align the low end with the LO inductor and the high end with the LO cap trimmer. It's a little more work if the inductor is air wound without a ferrite slug. You will have to squeeze the coil to move the freq down and spread the coil to move it up. You have to use a non-metallic nylon hex head tool of the correct size to adjust the LO inductor ferrite core (although some use a straight bladed slot) and plastic screwdriver tool with a tiny metal end to adjust the cap trimmer. This same procedure is used to align AM radios by using appropriate high and low stations or (preferably) an RF generator.
     
    Last edited: May 7, 2017
    Pio1980 likes this.
  6. Tube Radio

    Tube Radio Super Member

    Messages:
    3,386
    Some tuning caps do have vanes which can be adjusted that way, but some do not.

    When doing the alignment the instructions may give a tolerance for the dial pointer which as long as it falls in that tolerance it is good.

    Have a Yamaha receiver that specifies a 2mm tolerance for the dial pointer. Personally I don't agree with a dial being off like that, but it is what it is.
     
  7. chazix

    chazix AK Subscriber Subscriber

    Messages:
    981
    Location:
    Groton MA
    This description strikes me like way more than a bit of non-linearity. Might try cleaning the slip contacts in the tuning capacitor. Check the sticky in this forum entitled "How to clean a tuner (analog)". Might also check that the vanes of the tuning cap are not contacting something that they shouldn't when tuned above 98.

    Cheers,

    chazix
     
  8. jlovda

    jlovda Things I loved from the 60's and 70's

    Messages:
    3,122
    Location:
    NE Ohio
    It could be dust that is lodged between the plates but this usually causes a loud scratchy noise when tuning. Same with the ground slider. Has the tuner ever been tampered with? It sounds like one of the plates (most likely a moving one) has been bent and half way through its rotation it is touching an adjacent plate. This would change the capacitance as long as they are touching. A change in the capacitance of either the RF or mixer cap would detune the input and cause the input to the mixer to be the wrong frequency. Look for bent plates there also. Usually the shift in those places is not that great and the gain of the desired frequency just drops. There are fewer FM plates and they are spread further apart than AM.
     
  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    If the plates of the tuning capacitor for the local oscillator touch it will likely stop the local oscillator and reception would stop.
     

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