Re: receive works great, but no trans power, #ubitx-help

Gordon Gibby

That would make for a very useful experiment!   If you try it, let us know what you find out!

It’s possible that different transistors have different reverse (base-emitter) break down voltages, which would dramatically change the amount of energy deposited on the base-emitter junction by a strong signals.... I just don’t know enough about the devices to say for sure.

On Jul 17, 2018, at 23:30, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

We have had a couple reports of Q90 frying, but not the mixer.
Perhaps as Gordon notes, these were due to a strong VHF signal.

I believe Gordon has suggested in the past the the failure mode might be 
due to reverse voltage at the BE junction, not forward current.

I have no idea if the back-to-back diodes would significantly contribute to the spurs.
Doubt it would cause trouble where I'm at, as there are no strong local broadcasters.
I should try it, include a strap to remove the diodes if curious about some new spur.
We've got plenty of electrical storms.


On Tue, Jul 17, 2018 at 01:28 PM, ajparent1/KB1GMX wrote:
It the signals are strong enough to fry Q90 the mixer will soon be gone too.
The BE junction is as robust as the mixer diodes.  so whatever you are seeing
is quite strong and could even be ESD.

For ubitx (V3 or 4) a pair of back to back 1n194/4148 diodes from K3 Pin14 to ground
is the best bet as then its only there for RX and has no impact on TX.

Bitx 20 and 40 the solution is different.

Its easily done on the back of the board.


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