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Wednesday, October 16, 2013

Yikes! Screeching Transistors Throwing off Blue Light!

I found a quote from Doug DeMaw that I wanted to share.  This one seems appropriate as I struggle with unwanted oscillation (and as Halloween approaches). From the January 1986 QST:

"Self -oscillation occurs not only in the low-frequency and high-frequency spectrum, but it often takes place at audio frequencies!  I have actually heard the transistors "screeching" when strong audio oscillations were taking place in a homebrew transistor power amplifier. On one occasion I could see a bluish glow coming from within the transistors (visible through the ceramic heads of the devices) during a period of instability.  Needless to say, the transistors self-destructed."

I've been having some instability problems with my BITX 17 IRF510 final.  But I think I have the problem identified.  I was getting low frequency oscillations (around 100 KHz).  Tonight  tried putting the IRF510 on a separate power supply.  The oscillations ceased and I am getting a nice clean 3 watts out.   So I'll go back and beef up the decoupling on the power supply lines.

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  1. Also, you could consider adding some ferrite beads on the supply lines between your circuit blocks to help make the local decoupling caps more effective for their particular subcircuits.

  2. Try as I might I could not get more than 2.5 W RMS from an IRF510 on the BITXV3. Now and again it would take off and self-oscillate. Bill - it would be more expensive but try the RD16HHF1 type devices. They are reported to work OK at 12V

    Usual story -
    If you want to build an amplifier you get an oscillator. HI HI HI



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