Re: uBitx v5 issue

Jerry Gaffke

I think Iz is right, that DC path does help with static on the antenna, 
which can get quite substantial with nothing more than the wind whistling past.
Evan is also right, the DC path does not protect Q90 from something like
a nearby FM or TV broadcast station due to the 30mhz LPF not allowing it through.

HF will get through the 30mhz LPF, and likely get at least somewhat snubbed out by the diodes in the first mixer.

Gordon GIbby has pointed out that the base-emitter diode of Q90 will conduct
only on positive RF peaks from a nearby (VHF?) transmission, causing C80 to charge up.
The resulting negative voltage on the base during negative peaks can exceed the Vebo spec
of a 2n3904, 6 volts max, and blow out Q90.  He suggests a 1n4148 or similar, anode at Q90-emitter,
cathode at Q90-base, to prevent Q90-base from going more than a diode drop below the emitter, and thus
avoid a violation of Vebo.

Farhan has suggested that there might be more birdies in the receiver if the back-to-back diodes are
added, due to the wideband nature of everything between antenna and first mixer.  The diodes will
create lots of harmonics, creating lots of new mixer products that could sneak through the IF filters.

Most uBitx owners haven't bothered worrying about protecting Q90,
but a few have been bit.

Jerry, KE7ER

On Mon, Nov 23, 2020 at 04:39 PM, Evan Hand wrote:
The dc path does not really help in protecting the rig in my opinion.  Q90 is the first driver stage in the transmitter, it is connected before the receiver low pass filter, and has been known to fail when the antenna is connected and there are strong nearby RF signals.  There is a preventative measure that I and some others have taken to protect that device.  Back to Back 1n4148 diodes across pins 12 and 16 of K1 would be some protection from strong RF.  Lightning would be beyond the simple diode protection.  A lightning arrestor on the feed line is a better solution to protect against damage from large static discharges, or better yet ground all antennas when storms are near

My opinions so there could be better solutions.  Feedback always welcome.

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