TS-520 VFO Doesn't stay on freq through range

Discussion in '"Boat Anchor" & Classic Equipment' started by K2HNL, Jan 12, 2018.

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  1. K2HNL

    K2HNL Ham Member QRZ Page

    Another TS-520 question...

    If I calibrate the VFO movable ring at lets say 175khz, then I move to 350 khz, I see a up to 3 khz difference. That difference appears to change as I go through the range, as in it might be high at 250, then low at 350 compared to the 25 khz cal reference. Does that make any sense? And is this something that can be adjusted?
  2. KX4OM

    KX4OM Ham Member QRZ Page

    That is not unusual for a rig of that age to be out of calibration, and it can be adjusted. Do you have the Owners Manual? Section 6.10 briefly gives the procedure on setting the VFO using TC1, a capacitor in parallel with the VFO main tuning capacitor. If that does not get the VFO frequency linear enough over the 500 kHz frequency range, the TS-520 Service Manual section 3.21 has the procedure for other adjustments of the VFO, namely, adjusting the variable inductor, L1. The TC1 and L1 adjustments interact to set the starting end point of the range and the 500 kHz span. I performed similar adjustments to my Heathkit HW-101's 5.0 to 5.5 MHz VFO, and I was able to get it to within 1 kHz over the 500 kHz range.

    Before doing any of that, make sure that the crystal calibrator is zero-beat to WWV.

    Both manuals can be downloaded for free; Google "TS-520 Service Manual", and the site that comes up has PDF versions of both.

    Ted, KX4OM
    Last edited: Jan 12, 2018
  3. N8YX

    N8YX Ham Member QRZ Page

    Ted nails it here.

    The TS-520 and TS-820 VFO assembly is for all intents and purposes the same part; I've been able to get all of my '820-series equipment to within 1KHz linearity across the tuning range by careful adjustment.

    For that matter, everything of the period which uses an analog 5MHz VFO of similar design will use these calibration and adjustment procedures.
  4. KE4OH

    KE4OH Ham Member QRZ Page

    That's not too bad for a TS-520. This is why they come with a 25KHz calibrator. You can probably improve it, but you must have great patience to do the adjustments per the manual. That sort of thing isn't worth it to me. I just recalibrate when i move 100KHz or so.
  5. K2HNL

    K2HNL Ham Member QRZ Page

    I just checked that WWV and the 25 khz calibrator were zero beat together and they are within a few Hz, as close to spot on as I would ever be able to get it.

    Then I went from 0 to 350 khz listening to the 25 khz calibrator. What I saw was that when I zerod at 0 khz, I was off by between 2-3 up until 350 khz where I was back to 0 khz deviation. Is that normal to see a deviation from zero and than back to zero again?

    When you adjust TC1 do you just keep checking different points along the range until they are all close to 0 deviation?
  6. K2HNL

    K2HNL Ham Member QRZ Page

    Looks like I'd need to remove the whole VFO assembly from the unit to get to the board where TC1 resides, is that correct?
  7. K2HNL

    K2HNL Ham Member QRZ Page

    Well I found that there was a foil tape seal over the cut outs in the VFO cover. There are a total of 3 holes, two that were under the seal. Not 100% sure which is which since there appear to be no labels. From the service manual it looks like the ones that were under the seal were probably L1 and TC1. Can anyone explain which one is which? Thank you.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Get a DG-5 or one of the more modern equivalents and you can ignore the dial.:)
  9. K2HNL

    K2HNL Ham Member QRZ Page

    I found TC1 and adjusted it. I was able to get it so that there is no more than a 1.5kHz deviation from the 25 khz calibrator in either direction, which still means there is a total range of 3kHz that it can vary throughout the range of the VFO.

    Not sure if that is well within expectations or it should get better than that.
  10. K2HNL

    K2HNL Ham Member QRZ Page

    If I was going to do that I'd probably just spend the money on a modern radio, this works for what it is. Just trying to fine tune things that are able to be fine tuned.
    N5PNZ likes this.

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