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[Amps] Any Circuit guys out there

To: "Amps reflector" <amps@contesting.com>
Subject: [Amps] Any Circuit guys out there
From: "JayKay" <pcb4u@verizon.net>
Date: Wed, 11 Aug 2010 19:42:37 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Folks

I have  problem that looks simple to correct but the obvious isn't working 
or its the infamous "tree in the forest scenario".

NOTE: Logic is CMOS

The circuit is a PNP transistor switch being driven by a band switch. In 
parallel the +V from the band switch is also sent via  a diode (isolation) 
to an OR gate input.

Position 1 works fine.

Position 2 opens the +V to the PNP base and the OR gate input. The line is 
floating but is controlled as follows. The OR gate has a resistor on its 
input (to ground) to ensure its input remains at "0" and doesn't "dance 
around" driving the OR gate to an unknown output. Since the PNP input has a 
base resistor to ground the PNP turns ON as expected but it looks like the 
voltage at its base is finding its way to the OR gate input. I put in an 
additional steering diode in the PNP base circuit to halt the voltage from 
backing into the paralled OR gate input. It doesn't stop it.

I removed the PNP from the board and its the circuit causing the voltage to 
feedback to the OR gate even with the additional steering diode in the PNP 
base circuit. So it looks like there is no sneak path

If there is anyone out there that may see what I'm missing, I'll make a pdf 
file and send it to you.

I have updated the circuit schematic with an additional inverting circuit 
which will do the job.

I would hate to scrap this R&D board (over 100 components) if a not so 
obvious fix can be "white wired" into the board.


Thanks
Jay
W2IJ 

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