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Wednesday, September 17, 2008

Beacon Final Amp Goes Bad

All of a sudden the power output meter on my 30 meter beacon (MEPT) transmitter stopped jumping up and down. The oscillator was still running, and the keyer was working, but output had obviously dropped considerably. I started troubleshooting and my attention started focusing on the 2N2222A in the final. The stage runs Class A, with long QRSS key down periods. The emitter resistors get a bit warm. I replaced the transistor with a new 2N3904 and -- BINGO -- output was back up in the tens of milliwatts.

I did some tests on the 2n2222A. With a VOM I looked at forward and reverse bias resistance across both junctions, comparing the part from the rig with a new part from the same batch. In the transistor that went bad there is a lot more reverse bias current across the base-collector junction.

So, what happened? Any ideas?

This was one of those very satisfying repairs in which you quickly zero in on the problem, change one part, and then put the gear back into action. The victory was made even sweeter, when, a few minutes after I put the rig back on the air, I noticed my QRSS CW signal on I2NDT's online grabber.

I'm almost finished with my 10.140070 frequency standard. Armed with that, I'll be free to experiment with FSK keying (without fear of being unable to find my way back to the 100 Hz -wide QRSS frequency band). 73 from Rome

1 comment:

  1. Bill, in SSDRA, Chapter 2, pg. Wes and Doug mention a similar phenomenon however their example is for Class 'C' amps.. If the drive voltage is too high, the reverse bias placed on the base-emitter junction during the negative transition of the signal may exceed the transistor's reverse breakdown voltage. According to Msrs. Hayward and DeMaw, this does not cause immediate failure but a slow degradation of the transistor's Hfe.

    You say that your PA runs Class 'A'. Are you sure it's biased properly? If the bias is such that the negative part of the input signal drives the transistor into cutoff and exceeds the reverse breakdonw voltage, that might be the answer. Or maybe the bias is right but your simply over-driving the transistor? Have you put a 'scope on the input to the PA? I'm anxious to hear what you discover. 73.......Steve 'neuron' Smith WB6TNL "Snort Rosin"


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