Upon completion of my QMX (#331) build I found that the sensitivity on 20 meters to be quite poor, about -92 dBm (5.6 microvolts) for a Minimum Discernible Signal. A topic soon appeared on the forum regarding this as a design problem so I entertained myself otherwise and waited for someone to find a solution.
Since there has yet to be a solution proffered I decided to try the obvious solution of replacing C403, a 22 pF capacitor in the band pass filter, with a 16 pF value. That seemed to be what a ratiometric guess would suggest. Nope! The band pass response did indeed get closer but the 14 MHz frequencies were still highly attenuated. A 10 pF capacitor did little better but confirmed that the tapped inductor ( L401) and the associated parasitic capacitances formed an effective trap for the entire 20m band.
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It occurred to me that the simplest solution would be to just bypass the 20m band pass filter entirely. I live in a rural area with no broadcast stations or other strong HF signals. If I needed a band pass filter then I could add one externally or try to wedge in one separate from L401.
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I removed the tap at the 19th turn of L401 from the solder connection, soldering the two conductors together so to effectively remove the tap from the coil. I then removed C403. I installed a 1 nF (.001 uF) 0805 sized capacitor from the input signal side of C402 (the end shared with C403, C401, and C404) to the previously vacated solder pad of the removed tap. I positioned it a bit diagonally, reflowed the solder at the C402 pad so that it fastened the new capacitor. The other end was then soldered to the pad where the L401 19th turn tap used to reside.
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Progressing to the Step 8 tests, I confirmed continuity from IC3 pin 9 to pins 3, 4, and 5 but not 6. Visual inspection confirmed that no potential short circuits had been created.
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The results have been satifactory. MDS is now -121 dBm (0.2 uV) which is comparable with the other four bands. No new birdies or images. No overloads. Just a 29 dB improvement in receive level. Terrestrial, solar, and galactic noise are now all stronger than the weakest signal that I can detect. Operation on all other bands were unaffected.
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If new builders want to duplicate this fix might avoid making the 19th turn tap on L401 and not install C403. The value of the new capacitor is not critical, it just needs to block the DC bias but pass the 14 MHz receive signal when that band is selected. It could be a leaded part, placed from the hole at the input end of the C403 position to the hole where the 19th tap was supposed to go.
72, Don
Don, ND6T