Actually the problem can be simpler than you think.
I discovered recently that the LPT-ports integrated in motherboards are
often not capable of supporting of base current of the transistor and their
voltage drops and thus the transistor opens and PTT releases. To check this
you only need a digital voltmeter. Check the voltages without the keying
circuit and with the keying circuit connected.
The simplest (=quick & dirty but works) solution is to put a kiloohm-range
resistor (I used 4.7k) paralel with the LPT's output to stabilize its level.
I finally replaced the transistors in my keying&PTT&2radio circuits with
FET's and the problem is gone.
73 de Zoli HA1AG
Ps: This of course, doesn't solve problem of the radio going to TX when the
PC is booting up.
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