Diagnosing Negligible Power Output

General discussion and support for the Peaberry SDR V2.

Diagnosing Negligible Power Output

Postby ka8kpn » Tue May 02, 2017 12:03 am

I've been building a Peaberry SDR V2 kit off and on for the last while. I got to the TX Amplifier Circuit step and when I set up the test I initially got no power out. Upon investigating, I had a problem with one of the transformers in the transmit chain where two of the windings were shorted together and another problem with T5 where two of the windings hadn't been soldered properly. (It looked okay until I took a closer look under the microscope and discovered that the wires weren't wetted at all.) After resoldering T5, I was getting a tiny amount of power out, like 30 mW (measured by putting an oscilloscope on the output while driving a 50 ohm resistive load) and it was varying from maybe 50 mW to 20 mW even though there was no audio going in to the computer.

I figured that maybe the BS170's were damaged, so I ordered a bunch and replaced them. I got maybe 0.5W out at first, and then it dropped off and although it's more than it was, it's still only about 200 mW and it still varies significantly. The thing is, I just discovered that even though I was driving a dummy load, I had forgotten to put the tuner into bypass so it certainly mismatched at first. After bypassing the tuner, I'm measuring anywhere from about 1.5 V RMS to 3.5 V RMS. It's like being modulated by maybe a 1 Hz sine wave. I'm not looking forward to replacing the BS170's again, so before I do that again, I thought I'd ask and see if it's behaving like I actually did blow up the finals. If I did, it's not really a problem. I've got six more sets before I have to order more MOSFETs.
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