Originally the Warbler is designed for 80m using color burst crystals for TX and RX ladder filters. For reasons of W1AW, the Warbler is using a b.f.o. above the filters' frequencies. Given that the Warblers are supposed to be used for PSK, this is an option, PSK-software can be set to operate on the lower side band.
The modification described here is supposed to get the Warbler running on 40m, with as few changes as possible. The modified Warbler should also serve as a WSPR transceiver. Actually, this was the main point on doing this mod in the first place.
This is what I did (please grab a circuit diagram):
- the b.f.o. xtal changed to 7.038MHz
- all filter xtals changed to 7.040MHz
- C13 -> 47pF
- L3 -> 10µH
- C10 -> 100pF
- C11,C12 -> 470pF
- L2 -> 1.1µH (17 turns on T37-2)
- C3 replaced by 2.2µH and a polyvaricon-trimmer in series
Points 1. and 2. are somewhat obvious, I believe. The crystals are available from Rich N4ESS.
Points 3. and 4. change the passband frequency if the C13-L3 series resonance circuit of the receiver to the 40m band.
Point 5. matches the coupling between driver and final to the operation frequency. This part of the mod may have to be revisited.
Points 6. and 7. bring the low pass filter response to something above the 40m band.
Point 8. allows for variable pull of the b.f.o. to 7038600Hz. I used a combination of a polyvaricon-trimmer (4...18pF, I believe) and a 10pF NP0 capacitor in parallel. This is certainly the part of the modification in which some experimentation is required, since tolerances and different cuts or series capacitances of the b.f.o. crystal play a role in that game.
As I mentioned before, WSPR is certainly a main point for the little great rig. The first night out, RX only, at a hidden G5RV-jr, the following are among the decoded stations: VK6POP (14190km), VK6BN (14188km), W4PJZ (6781km), NB3N (6137kM), K4YO (5954km).
I did TX, however, only once for testing, with very low power. For regular WSPRing, an enclosure would be required. Presently, the Warbler is laying open on the table, next to the grabber computer.
Next to WSPR many other modes were detected on the spectra, e.g. several types of PSK and even JT65. I am sure that the 40m Warbler will be a big hit! Dave K1SWL may put more on his webpage, stay tuned!