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SW-40+ problem

Discussion in 'Homebrew and Kit Projects' started by KB1CKT, Aug 31, 2021.

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  1. KB1CKT

    KB1CKT Ham Member QRZ Page

    Built this kit years ago, not sure why it's taken this many years to "finish" it. All together but on TX it isn't stable. Tuning range is about 7.03 to 7.06, give or take a bit. Sidetone on TX is fine, at both 7.03 and 7.06. But if I listen on my big radio, at 7.03 the TX note is fine, but above 7.05 it gets this raspy note to it. Quick check into a dummy load shows the expected 2W, and it does seem to receive ok. [Been using a simple clip lead on the dummy load to "transmit" so that my station can hear it--doing that as a quick and dirty way to calibrate the dial.]

    Not sure if anyone else has encountered this? Where I built it, my first thought is bad solder joints, followed by improperly sanded magnet wire, followed by wrong component installed. But if someone else has had the same problem, and traced it to ___ then that'd be a big help!

    I plan to hit it with a scope and trace through the TX path, starting at the end to see what the rasp looks like, and also to look at the power supply, just to make sure it's not noise there (same supply that my big radios use but who knows, maybe I lost a cap on this SW40 and it's local noise).
  2. KB1CKT

    KB1CKT Ham Member QRZ Page

    Found a moment. Power rolls off ever so much on the top end. Quick check shows that the right discs are in the LPF (can't read the 1nF mono at the moment) and the turns count looks right too. Not seeing glaring issues on the scope but it doesn't look steady either.
  3. W1VT

    W1VT Ham Member QRZ Page

    That sounds like an oscillator issue. It may be on the edge of oscillation. Maybe a poor solder joint?
  4. KX4OM

    KX4OM Ham Member QRZ Page

    You should be getting more than 30 kHz of range on the VFO. I've built two of those, and I was able to get 7.000 to 7.100 on one of them. I built it from the schematic as a receiver only on a homebrew PC board, stretching the range somewhat.

    The "Elmer 101" that I mentioned on the page refers to a very comprehensive build instruction/circuit analysis that was done of the SW-40+ several years ago. It is currently hosted on the website. Also, see

    Ted, KX4OM
    Last edited: Sep 1, 2021
    KB1CKT likes this.
  5. KB1CKT

    KB1CKT Ham Member QRZ Page

    Haven't found it yet. Almost like a 60Hz hum on the TX note--thing is, it's on a battery.

    Tacked in a bypass cap here and there, just not finding it yet. I've "pulled" the TX frequency by poking around with a finger, so as to make the mixing frequency pull, just to see what would happen; the rasp stays there. Not amount of pushing or prodding makes it suddenly change, so as to give indication of bad joint or component. For a bit I thought it might be in the buffer Q4 but nope.
  6. KB1CKT

    KB1CKT Ham Member QRZ Page

    That's true too, forgot about the elmer courses on this one. Will read through those. But I don't recall them having more than 35kHz of tuning range; the current vendor also states 35kHz of range.

    I pulled out my SW80 and that sounds fine, so it's not my ears playing tricks on me. I put that together years ago, and it worked fine; but this 40m version seems to not like me. Gotta be something I did put together wrong, just have to find it.
  7. W1VT

    W1VT Ham Member QRZ Page

    It could be a wrong component, or one installed in the wrong set of holes. I'd suggest comparing the SW40 against the SW80.
  8. KB1CKT

    KB1CKT Ham Member QRZ Page

    Been looking at the schematic, counting turns, all that. Has me stumped. Starting to think about swapping out transistors next. Something is unstable, I'd think the VFO somehow, just can't find it yet. I think what I ought to do is rig up a small buffer circuit so as to be able to hook up a bit of wire to any point in the circuit and make it radiate--if I can "transmit" the 4MHz mixing signal, perhaps I can hear if that is warbling or clean. Same on the VFO signal, maybe it's the VFO, maybe it's elsewhere. Scope plots aren't showing what I need to see, or I'm just not reading them correctly.
  9. KX4OM

    KX4OM Ham Member QRZ Page

    You are correct. I took a look at the SW-40+ manual, and Dave Benson gave the frequency selection options as a nominal 30 kHz span, as he did for the SW-20+ that I built. On that one, I made it "dual-range" with a switch. The 14.025 to 14.065 range that I had was seriously non-linear, so I created a 14.055 to 14.065 linear range around the QRP calling frequency that was very linear over the span of the pot. That was achieved by changing the tuning by experimenting with adding resistors between the pot and 78L08 and the pot to GND. See the next paragraph for the source for that idea. I did the same thing to modify a friend's SW-40+. On my homebrew 40m receiver, I used a 10-turn pot with the homebrew DL4YHF freq counter and LED readout, so no linearization on that rig.

    I also built a 30m "Trans-Receiver" that used homebrew VFO and receiver boards that I made from my 40m receiver design, and a Vectronics 20m transmitter board kit modified for 30m. That was done as result of a club Christmas party challenge to build a new rig for Straight Key Night. I had bought a few of those kits for $5 each when a local RadioShack affiliate (TechAmerica) closed up shop back in 2003 or 2004. I modified it for 30m and it used switched crystals, 10.106 and 10.121. It has been disassembled for several years awaiting the population of a homebrew SW+ transmit board that I made for it. That project is also on my website. It has a photo of the unpopulated receiver PC board and photos inside the VFO box. It mentions the article from the Adventure Radio Society Sojourner blog by N7XJ about modifying the tuning range of a varactor diode VFO.

    Ted, KX4OM
  10. KB1CKT

    KB1CKT Ham Member QRZ Page

    Worked on it the other night, but this time I used my FT-817 to RX, and boy did the SW40+ sound awful there, was running the '817 wide open for RX. Made the mistake of messing with T2 and T3 IF cans, they form a bandpass filter in the TX chain--I thought it odd that one was near fully turned in, so I went to adjust it and I seem to have nadda now. Probing the filter kills the signal, so it's hard to poke around in there; I have to rig up a simple diode probe I think to see what is going on next. I do see what I think is to be correct coming out of the NE602 mixer, it might well be the 1MHz and 7MHz products that I would expect, so that bit is correct--I'm just losing it elsewhere. [IF cans test as shorts in the coils, so I don't think they have opened up.]

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