Assembling a Heathkit HW-99

Discussion in 'Homebrew and Kit Projects' started by N5OLA, Jul 31, 2019.

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  1. N5OLA

    N5OLA XML Subscriber QRZ Page

    On a related question, while querying Mouser and others to find these 50 pF N75 caps, I'm asked what is the wattage. This isn't on the caps or on the schematic. I have no clue where to find it.
  2. N7EKU

    N7EKU Ham Member QRZ Page


    It's for a transistor VFO run off 12V -- any value you buy will work since the voltage will not be that high, nor the power.

    Have fun cap hunting,

  3. N2EY

    N2EY Ham Member QRZ Page

    I don't think you need to replace those capacitors unless they test bad.
  4. N7EKU

    N7EKU Ham Member QRZ Page

    The VFO inductor in this kit apparently with heating, drifts in value a lot in the positive directionl So he's following a correction plan someone came up with that employs some negative coefficient caps...
  5. N2EY

    N2EY Ham Member QRZ Page


    But, IMHO, it would be better to build the thing first, and then, if the drift seems excessive, to seek out the fix.
    WN1MB likes this.
  6. W9GB

    W9GB Ham Member QRZ Page

    N75 : ±60 ppm/°C max. capacitance change from +25°C reading over temp. range of -25°C to +85°C
    Craigslist, eBay. ... these are 25 pF

    OR contact SP International in Burnaby, B.C. (majority of their products are SMT parts)

    SP International
    650 - 3711 North Fraser Way
    Burnaby, British Columbia, V5J 5J2
    Phone - 604-324-4811
    Fax - 604-324-3184
    E-Mail -
    Web -
  7. N7EKU

    N7EKU Ham Member QRZ Page

    For sure,

    And if the inductor is the problem, then fix that first.


  8. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Caps generally aren't rated for wattage.
    N2EY likes this.
  9. N5OLA

    N5OLA XML Subscriber QRZ Page

    If anyone is still following this thread, I appreciate all of your "pre-flight" advice on the HW99.

    It's moving along. I checked all the resistor values (replaced a few) and got new electrolytic caps to replace these 30+ yr caps in the kit. The boards are all assembled, and now I'm running the initial multimeter tests and hitting some sour notes. It's hard to know where the culprit(s) could be, as the parts are so old. My soldering is pretty good, no bridges or cold solder joints.

    I'm posting these queries in the technical forum "Radio Circuits, Repair & Performance" if anyone wants to come visit. Thanks again for all of your invaluable suggestions.
  10. KA1LHJ

    KA1LHJ Ham Member QRZ Page

    I built one circa '85; still works, but disappointing vfo. Drifts a bit when turned on as to be expected; doesn't return to the original starting point when off. The accumulated error is about 22 Khz.
    Artifact from my youth; wouldn't part with it. One day soon I'll do a couple of contacts with it for old time's sake.

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