Re: BITX40 Power question???

Jerry Gaffke

Here's Raj's post 19105 of Dec 16, 2016:

At pin 12 of K1 relay solder 2 4148 diodes in parallel but in opposite polarity. This will limit the receiver from getting high RF voltage. 
Also if you cut the track between K1 pin 12 and K2 pin 14 and insert a 12v 60 or 40 ma miniature incandescent bulb it would be better. This is common protection method in commercial transceivers and amps. This track seems to be on the component side. 
cheaper still! 

73 Raj vu2zap 

The track between K1 pin 12 and K2 pin 14 is only used during receive, not during transmit.  I'd never heard of that incandescent lamp trick, wasn't sure I wanted a fragile bit of tungsten between me and my antenna.  But AA1OF flagged me that the resistance goes up drastically with temperature.  The first line of the document in the link that follows reads: "Tungsten-filament incandescent lamps exhibit a very-high positive temperature coefficient of resistance with the cold filament resistance being approximately 10% of the hot filament resistance."   Wow, very cool!

Those little mini 12v bulbs won't have much of a filament, likely fine through all of HF.  

Somewhat off topic:  When I was going to school at Oregon State in the 1970's, the EE dept storage room still had boxes of ancient Edison bulbs with carbon filaments.  Had used them as RF dummy loads back in the day.  The coiled up tungsten filaments of 120v household bulbs just didn't work as well.  But a 120v 40w tungsten bulb had worked fine as a dummy load on my 40m 35w 6146 single tube crystal oscillator as a novice back in high school.

Jerry, KE7ER

On Sat, Feb 11, 2017 at 03:39 am, Raj vu2zap wrote:

>   Q13 blew due to other transmitter!
Put two diodes 1N4148 or similar parallel but opposite direction from Relay K1 pin 12 to ground. This will prevent voltages greater than about 1V from going to Q13.  Q13 is connected to antenna in RX mode.


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