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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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2 comments:

  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.

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  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

    73
    Tony

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