Hi All!
At 10:43 AM 11/4/97 -0800, Tree N6TR <n6tr@teleport.com> wrote:
>I found some 2.2 or 2.7K resistors and pulled all of the signals
>coming from the computer up to 5 volts with them. This totally
>fixed the problem and ran for 1485 QSOs during the SS with no
>problems.
It is important to realize that the generic PC parallel port pin is designed to
drive a printer input impedance of typically >100 KOhms and <10 pF at data
rates of a few hundred KHz. We we must plan for a minimum of only 50 uA of
base current, which multiplied by the beta of the transistor must be greater
than the load current needed for the collector to pull the PTT line down to
ground, typically <5 mA.
We need a transistor with a guaranteed beta of 50 or better. This is very
doable, and is probably not the real problem.
Problems can be related to the Strobe signal, pin 1, where all the emitters are
tied, and all the base and collector currents converge. Current sink specs are
typically better than sourcing, but fairly limited too. Some else, (w2vjn
perhaps) said he adds an extra transistor pair at the Strobe pin, just to make
certain he can sink all the emitter current from the PTT and CW transistors,
and now has no problems. It seems to me that this would be a more robust
design, and less work too.
Don't use those RS cheapo grab bag transistors, I have learned this lesson the
hard way on Field Day where the tools and workplace were very primative; use
real, marked, 2N3904 / 2N3906 or equivalents, and you should be OK without the
pullup resistors.
73
AE0M, Tony Becker - becker@sprintmail.com - Silicon Valley, USA
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