Re: U1 fried

Mharpen@...
 

Hello. The LM386 audio chip in the BITX40 is powered via the RX rail thru a 100 OHM resistor

R111. Under normal operation (in my shack 12 V gelcell) this resistor drops about 4 volts with moderate audio. Thus, the chip is powered by about by about 8V. This of course would fluctuate with audio lever if not for the 470 uF capacitor (C110) which should smooth out small fluctuations.

 

The DTA2822 in the uBITX is powered directly via the 12 V power line without the dropping and smoothing RC circuit. Thus, it would see the full 12 or (13.8) volts. This is done, presumably, to facilitated CW (reduce CHIRPing). This is probably why the less robust DTA2822s fail. I'm not particularly interested in CW so when I finally get my uBITX I will replace the the WX with an ST chip and install the same RC circuit as in the BITX40.

 

Mike N6CMY

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