Homebrew VFO unusual two-tone chirp question

Discussion in 'General Technical Questions and Answers' started by KC0QWE, Oct 10, 2019.

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  1. KC0QWE

    KC0QWE Ham Member QRZ Page

    I have a Heathkit keyer HD-1410, keying a homebrew transmitter fed by a homebrew VFO (permeability-tuned).

    I have used this VFO for years. Lately I've been struggling with VFO drift. Letting it warm up, powered on, minimizes that problem.

    But lately it has just started "chirping" in an unusual way...not a glissando or a whooping sound, but simply a distinct two-note combination of low and high, most noticeable on a "dah". It doesn't do it on every keystroke...maybe 10 percent of the time. It's not like any chirp I ever heard.

    It started when I temporarily and carefully packed the open areas inside the VFO enclosure with weatherstrip foam. It continued after I removed the foam.

    Last night I cleaned the keyer contacts and that seemed to help some, but not 100%.

    Q: Could dirty paddle contacts even cause something like this?

    Thanks
    John KC0QWE
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It's unlikely the paddle contacts themselves cause the chirp since it's not the paddles themselves that key the rig but rather the output of the keyer's digital circuitry.

    I'd try a straight key or even a couple of wires that you can touch together to see if they chirp symptoms are the same when the rig isn't driven by the Heath keyer. If it does appear to be the keyer causing the problem there's a resistor change you can make in the HD-1410 that drags the keyer output closer to ground potential. Some rigs need a fairly hard ground to key and the HD-1410 manual describes how to change the value of one resistor to support that kind of rig. It's quite possible that your rig needs to be pulled a bit harder to ground to avoid the two step chirp that you're describing.

    I guess the obvious followup question is whether you're directly keying the VFO itself as opposed to running the VFO continuously and keying the transmitter further downstream. A lot of chirp problems can be resolved by keying the signal after the VFO instead of initiating oscillation directly from the key input. Perhaps your rig's architecture requires direct keying of the VFO or perhaps you're already keying downstream and you have a VFO output loading issue that leads to chirp but if you are keying the rig via the VFO itself that can easily lead to chirp problems.
     
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  3. N2EY

    N2EY Premium Subscriber QRZ Page

    Pictures and schematics would be a big help.

    Reading the OP, I don't even know if it's SS or hollow state.
     
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Last edited: Oct 10, 2019
  5. KC0QWE

    KC0QWE Ham Member QRZ Page

    Thanks.

    I forgot to mention that I tried it with a straight key and got similar results last night. I'm going to use a simple old knife switch (cleaned with sandpaper) next. If that chirps, then I'll be looking at VFO and/or the transmitter.

    Because this two-tone chirp started after I had temporarily added insulation in the VFO enclosure, I may have rubbed something the wrong way on the circuit board. I've poked around with a chopstick while the paddles were clamped together making constant dahs. No smoking guns there.

    I'll follow up here with whatever I find.

    The VFO is per WA60TP and I don't have the schematic in a file to send. The transmitter (aka exciter) schematic is attached. It's based on a 1967 QST article by W1FB. The chirp happens whether the output goes to either antenna or dummy load.

    Steve Smith WB6TNL helped me debug this rig back in 2003 or so and it has been reliable....for what it is. It's shown in my profile picture. I've learned a lot from him and others. I never had this particular problem before, but I have a better idea of where to look now. I have stubbornly neglected to upgrade beyond this cw-only, 40m-only rig, because its problems are always interesting to me for some reason.
     

    Attached Files:

  6. WR2E

    WR2E Ham Member QRZ Page

    Does it chirp in the “tune “ position?
     
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    No need to bother with the knife switch. You have already proven the problem is the VFO.

    Chirp in a previously stable circuit means something has changed or is changing value.

    -Visual inspection: Since your stuffed the box, look for bent components, component that are out of place, touching or being affected by another. Broken or partial contact.

    -Pull the IC's, clean the pins and sockets. Spray some contact cleaner.

    -Check components for changed values.

    -Check your power supply for static voltage and voltage drop under load.
     
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  8. KC0QWE

    KC0QWE Ham Member QRZ Page

    Thanks for all replies. Now I have some work to do. I don't know if it chirps in the TUNE position, but I'll bet it does.
     
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Is the VFO keyed?

    I would also have a look at the 10 uF capacitor in the transmitter. It's there for shaping....if it goes bad, all sorts of odd things could happen.
     
    KC0QWE likes this.
  10. N3PM

    N3PM Ham Member QRZ Page

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