I am working on getting my bitx40 into a housing & have mostly succeeded. I thought I'd check the transmitter into a dummy load, so I set my "big" radio to 7.150, had a 300w dummy load on the bitx (I know, a bit overkill...) and tried transmitting while watching the ammeter. Hm... the current is staying around 1 amp, and I don't hear anything in the big receiver. Then, I noticed smoke from the PCB. Yikes!
I shut everything off, and started looking. I finally found that R42 was burnt. This is a 10 ohm part in the TX amp chain. OK, if it is burnt, there's a reason why. I measured it, and it showed about 14 ohms - a little high, but should still be workable.
Using my scope, I determined that Q4 had input, but no output. I put an MMBT2222A in its place, and did some resistance checks before powering it up. To my surprise, I found that the R41 side of R42 showed 10 ohms to ground. That can't be right - further visual examination showed a hard solder blob short between R41 and R45, and a little "whisker" between R45 and R46. This explained the 10 ohm to ground, and also why this stage was dead & had a burnt resistor. I cleaned up the solder blobs, Fired up the rig, and KAZAAM! 25V peak across my 50 ohm load, which works out to about 17.6 VRMS, giving me about 6W into my dummy load. The modulation on the 'scope looked decent, too, so I'm going to try an actual antenna and maybe get a QSO?
