Hi Brent;
? ? ?Gotta say; I share your glee with this radio. I¡¯ve been heads down on ¡°fixing some of the more serious woes of this rig the past couple of weeks, while resisting the urge to alter the as-is configuration. ?It¡¯s not useful to others to do otherwise. I need to spend some time documenting my findings though; both to defend them, as well as my fix recommendations.
? ? ?It¡¯s been helpful that this radio is so simple to simulate and analyze. The fate and fixes almost jump out at you. :-) I¡¯m still new to this project and list though, and I haven¡¯t yet found the repository of sanctioned upgrades and fixes. Much of what I¡¯ve discovered and conquered has likely already been addressed; so my apologies in advance for not doing the research to find them.
? ? ?The issue you describe is related to the PTT pop issue. The hard answer is that it¡¯s inherent in the current design. DC Xmt power is applied in advance of the decay in the Rcv DC supply. Hence; both are simultaneously enabled for the duration that the Rcv DC supply holds up. This is exacerbated by D10, and the sequential switching of K1 followed by K2 which keeps the audio preamp connected to the PA... albeit briefly (up to ~ 7 ms) following enabling of the mic amp.?
? ? ? I¡¯ve conquered the transmit pop, but in full disclosure, I¡¯ve not yet implemented the following ?receive side fix that I have in mind. ?It should address your problem nicely, but you¡¯ll be on your own, as there¡¯s always the risk of a snafu I failed to spot. If you wish to stick your neck out though, give this a try:
? ? Observe that the right side of K2 does the T/R antenna ?switching, While the top section of K1 grounds the now disconnected wire, and thus serves no purpose. Sever the connections at K1 pins 12, 14. Jumper K1 pins 12 and 3 and your done. The keying of K1, now dumps the receive supply to ground before the transmitter circuits become enabled.?
? ? ? I¡¯m guessing that it¡¯s not easy to sever the connection at K1 pin 16; otherwise the top portion of relay K1could be used to dump transmit power to ground when switching to receive as well by simply grounding pin 12. That would be the preferred fix.?
? ? ? While at K1, shunt the coil with a diode. D7 does nothing as wired.?
? ??
? ? ? Swap D18, and R127, or simply jumper across the old D7 location. A diode in lieu of R127 greatly accelerates the power-up of the mic amp. Also add 20 to 100uF from the top of C-123 to the top of C-124... plus side to C-124. Together, these changes nullify the PTT carrier spike (pop!) at the beginning of a SSB transmission.
? ? ?Finally; place a short jumper between the VFO input side of L4 and the emitter of Q8. This bypasses the first two VFO Impedance buffers that band limit the Raduino clock and starves the first LO in USB mode. No need to remove the unused buffers, as they don¡¯t interfere. This fix yields +6 to +7 dBm into the first mixer at both 5 and 19 MHz, and results in full transmit power out, good sensitivity (0.1uV) and RF dynamic range to +25 dB over S9 in all modes.?
? ? ?All but the relay wiring fix and miswired D7 were derived via simulation, and verified on my BITX-40. I also added a 1uF ceramic cap across the PTT to suppress switch bounce noise from chattering the relays.
? ? ?I zapped the SPOT port of the Rudino¡¯s Nano, so I¡¯m limping along until I can get it replaced. I always seem to fat probe stuff when I¡¯m eager to get stuff done. <sigh>
? ? ?Good luck with the mods. I hope you¡¯re having as much fun as I am having.
72 es 73
Gary, ?N3GO
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