On Mon, 2009-09-28 at 23:40 -0400, Mike Hyder --N4NT-- wrote:
> Seems in my younger day what you describe was often caused by a bad cathode
> bypass capacitor. Probably there are similar things that could go wrong
> today.
>
> 73, Mike N4NT
>
Nah, an open cathode bypass just cuts the gain, makes the stage more
stable from the inverse feedback of the unbypassed cathode resistor. Its
power supply bypassing going low in value that lets the radio motorboat
from multiple stage feedback where the capacitor should have been 10 mfd
but is now 1 mfd in series with considerable resistance. That makes it
bypass OK at 30 Hz and higher, but not at 5 or 10 Hz so the radio
oscillates at the low frequency which sounds like a motorboat going put
put... There are still bypass capacitors on the B+ lines (now 12 volts
or less) on the modern radios and likely more stages for more effective
motorboating when the miniature electrolytics dry out and develop high
series resistances which some had when new. Miniature electrolytics are
the achilles tendon of solid state radios and computers. Indeed a few
years ago a whole year's worth of mother board from China were bad
because the bought 3 cent electrolytics that weren't good for 5 volts
because the back yard maker didn't know how to really make electrolytic
capacitors. Many exploded or caught fire when power was first applied.
73, Jerry, K0CQ
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