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 built it in 2004, and although it's quirky, it has been quite usable until the recent drift issue. It always had a bit of drift, but nothing that got in the way of QSOs. A few days ago, letting it warm up for 30 minutes, VFO on constantly, eliminated both the drift and the chirp. I expect the problems to appear again at some point, and then I'll get out the magnifier, chopstick, etc.
     
    Last edited: Oct 15, 2019
  2. KC0QWE

    KC0QWE Ham Member QRZ Page

    I used Manhattan construction, and some of my component leads are really long....much longer than what is shown in the photo. I have shortened a lot of them, which helped with the drift problem I haven't found the time to get in there and rebuild things as I'd like. I may build another one eventually. It nicely covers the entire 40 meter band, and I use a really really big knob from a 1920s radio, which helps in setting the frequency.

    Also, the board included in the kit was double-sided board. I only used one side. Recently, before the chirp started, in an attempt to tackle the drift problem, I etched all the copper off the unused bottom side of the board. That was a chore, and didn't help the drift. I had gotten the idea that the double-sided board was contributing, and I was doing a lot of trial-and-error grasping. My troubleshooting skills aren't the best. I still think there's a poor connection somewhere.
     
    Last edited: Oct 15, 2019
    KA0HCP likes this.
  3. KC0QWE

    KC0QWE Ham Member QRZ Page

    Very true, and I clean and inspect them routinely. They're not ideal. The power connector is a mini plug/jack, and the output is RCA. Poking at these connections while listening to the oscillator has convinced me they're not the problem, however.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Not sure Chinese food will help.

    But Good Luck. Read your fortune cookie. :cool:
     
    KC0QWE likes this.
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    How are you connecting the VFO to the transmitter that you posted previously? Are they laid out on a common PCB with a shared ground plane and good power bus or are they strung together with fly wire connections or something else? From what you describe I'd take a careful look at layout issues like lack of a good low impedance ground plane or lack of a hefty power bus that supplies both the VFO and transmit portions. High impedance paths in either of those circuits could easily cause the VFO to pull in frequency as you key the transmitter.

    Personally I wouldn't have etched away the ground plane and instead would have used that other side of the board to make sure you had a very low impedance ground plane and would probably drill small vias and solder wires through them to the solid ground plane for all critical VFO and transmitter ground connections. IOW, I'd use that other side of the board to create a low impedance ground and tie critical RF circuit grounds to it using the shortest practical lead lengths.

    In terms of drift I'd take a close look at what capacitors you're using and consider temperature stable capacitors like NPO ceramics or silver mica capacitors in all RF tuned circuits, especially in the VFO.
     
    G4COE and KC0QWE like this.
  6. KC0QWE

    KC0QWE Ham Member QRZ Page

    The VFO connects to the transmitter via RCA connectors and some sort of extra-thin coax; I don't really remember where the coax came from. This has always seemed to work well enough, but if I continue to have problems, one of the first things I will do is to replace that with better coax and connectors.

    The VFO and transmitter are on separate power supplies. Doing that solved a lot of problems earlier this year when I got this rig out after 6 years of inactivity. The power cable to the VFO could be heavier, I reckon. That's another thing on the to-do list.

    When I etched off the copper, I had never heard of using the bottom side of two-sided board for ground plane. The kit seller never mentioned doing that. I continue to be amazed at how well this VFO works, for what it is. It is really easy to find the needed frequency anywhere in the 40 meter band, with a flick of the wrist. My old bakelite knob is like a big flywheel. Since the VFO (PTO) uses long brass screw through the coil instead of a variable cap, that's all the knob I need.

    The transmitter itself is hard wired, as specified in the 1967 QST article by Doug DeMaw. No boards. It uses coil forms instead of toroids. This transmitter is used as an exciter for a small amp that brings it up to about 8 watts...the Miniboots amp, which also filters the output nicely.

    I agree about the caps. Where I go with it now depends on how it will perform now, with the warmup time method. But I would like to branch out to other bands, and this VFO and transmitter are 40 meters only.

    In 2013 I had a confirmed QSO with Czech Republic with this rig. I think the guy at the other end had a whopper of an antenna, though.

    Details of transmitter construction...
    http://www.zedrovision.com/kc0qwe/kc0qwe.html

    KC0QWE journal, last updated on 7-9-19:
    http://www.zedrovision.com/kc0qwe/journal.html
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That's fine for the signal path.
    That can bring up some interesting issues if the grounds aren't well bonded and if there's any DC coupling between the stages. If the only signal path coupling is coupled through a series capacitor then it should work out but any DC path between them can cause funny problems with independent power supplies.
    Yeah, in general with higher frequency and RF work it really pays to have a good ground plane to work against. It avoids things like small ground traces or ground wires that can create voltage drops across separated ground connections under load which can lead to all kinds of strange problems.

    You can effectively replace the missing ground plane by mounting the board(s) on a sheet of metal and tying your ground connections to that underlying conductive plane. That was real common in pre-PCB and even the early days of PCBs before double sided boards became popular. Manufacturers would just tie all ground connections to the metal chassis. That could still create some unusual problems like the way board edge screws in some older Heathkit and Kenwood gear can oxidize and create differential ground potential problems but it's a lot better than using individual wires to carry grounds around your boards.
     
    KC0QWE likes this.
  8. KC0QWE

    KC0QWE Ham Member QRZ Page

    Thanks. I'll see how it behaves in the near future, and I'll keep your suggestions on file.
     

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